/// pass_typing
///
/// Performs type inference and type checking. Type inference is implemented by
/// applying constraints on (sub)trees of types. During this process the
/// resolver takes the `ParserType` structs (the representation of the types
/// written by the programmer), converts them to `InferenceType` structs (the
/// temporary data structure used during type inference) and attempts to arrive
/// at `ConcreteType` structs (the representation of a fully checked and
/// validated type).
///
/// The resolver will visit every statement and expression relevant to the
/// procedure and insert and determine its initial type based on context (e.g. a
/// return statement's expression must match the function's return type, an
/// if statement's test expression must evaluate to a boolean). When all are
/// visited we attempt to make progress in evaluating the types. Whenever a type
/// is progressed we queue the related expressions for further type progression.
/// Once no more expressions are in the queue the algorithm is finished. At this
/// point either all types are inferred (or can be trivially implicitly
/// determined), or we have incomplete types. In the latter case we return an
/// error.
///
/// TODO: Needs a thorough rewrite:
///  0. polymorph_progress is intentionally broken at the moment. Make it work
///     again and use a normal VecSomething.
///  1. The foundation for doing all of the work with predetermined indices
///     instead of with HashMaps is there, but it is not really used because of
///     time constraints. When time is available, rewrite the system such that
///     AST IDs are not needed, and only indices into arrays are used.
///  2. Remove the `msg` type?
///  3. Disallow certain types in certain operations (e.g. `Void`).

macro_rules! debug_log_enabled {
    () => { false };
}

macro_rules! debug_log {
    ($format:literal) => {
        enabled_debug_print!(false, "types", $format);
    };
    ($format:literal, $($args:expr),*) => {
        enabled_debug_print!(false, "types", $format, $($args),*);
    };
}

use std::collections::VecDeque;

use crate::collections::{ScopedBuffer, ScopedSection, DequeSet};
use crate::protocol::ast::*;
use crate::protocol::input_source::ParseError;
use crate::protocol::parser::ModuleCompilationPhase;
use crate::protocol::parser::type_table::*;
use crate::protocol::parser::token_parsing::*;
use super::visitor::{
    BUFFER_INIT_CAP_LARGE,
    BUFFER_INIT_CAP_SMALL,
    Ctx,
};

// -----------------------------------------------------------------------------
// Inference type
// -----------------------------------------------------------------------------

const VOID_TEMPLATE: [InferenceTypePart; 1] = [ InferenceTypePart::Void ];
const MESSAGE_TEMPLATE: [InferenceTypePart; 2] = [ InferenceTypePart::Message, InferenceTypePart::UInt8 ];
const BOOL_TEMPLATE: [InferenceTypePart; 1] = [ InferenceTypePart::Bool ];
const CHARACTER_TEMPLATE: [InferenceTypePart; 1] = [ InferenceTypePart::Character ];
const STRING_TEMPLATE: [InferenceTypePart; 2] = [ InferenceTypePart::String, InferenceTypePart::Character ];
const NUMBERLIKE_TEMPLATE: [InferenceTypePart; 1] = [ InferenceTypePart::NumberLike ];
const INTEGERLIKE_TEMPLATE: [InferenceTypePart; 1] = [ InferenceTypePart::IntegerLike ];
const ARRAY_TEMPLATE: [InferenceTypePart; 2] = [ InferenceTypePart::Array, InferenceTypePart::Unknown ];
const SLICE_TEMPLATE: [InferenceTypePart; 2] = [ InferenceTypePart::Slice, InferenceTypePart::Unknown ];
const ARRAYLIKE_TEMPLATE: [InferenceTypePart; 2] = [ InferenceTypePart::ArrayLike, InferenceTypePart::Unknown ];

/// TODO: @performance Turn into PartialOrd+Ord to simplify checks
#[derive(Debug, Clone, Eq, PartialEq)]
pub(crate) enum InferenceTypePart {
    // When we infer types of AST elements that support polymorphic arguments,
    // then we might have the case that multiple embedded types depend on the
    // polymorphic type (e.g. func bla(T a, T[] b) -> T[][]). If we can infer
    // the type in one place (e.g. argument a), then we may propagate this
    // information to other types (e.g. argument b and the return type). For
    // this reason we place markers in the `InferenceType` instances such that
    // we know which part of the type was originally a polymorphic argument.
    Marker(u32),
    // Completely unknown type, needs to be inferred
    Unknown,
    // Partially known type, may be inferred to to be the appropriate related 
    // type.
    // IndexLike,      // index into array/slice
    NumberLike,     // any kind of integer/float
    IntegerLike,    // any kind of integer
    ArrayLike,      // array or slice. Note that this must have a subtype
    PortLike,       // input or output port
    // Special types that cannot be instantiated by the user
    Void, // For builtin functions that do not return anything
    // Concrete types without subtypes
    Bool,
    UInt8,
    UInt16,
    UInt32,
    UInt64,
    SInt8,
    SInt16,
    SInt32,
    SInt64,
    Character,
    String,
    // One subtype
    Message,
    Array,
    Slice,
    Input,
    Output,
    // Tuple with any number of subtypes (for practical reasons 1 element is impossible)
    Tuple(u32),
    // A user-defined type with any number of subtypes
    Instance(DefinitionId, u32)
}

impl InferenceTypePart {
    fn is_marker(&self) -> bool {
        match self {
            InferenceTypePart::Marker(_) => true,
            _ => false,
        }
    }

    /// Checks if the type is concrete, markers are interpreted as concrete
    /// types.
    fn is_concrete(&self) -> bool {
        use InferenceTypePart as ITP;
        match self {
            ITP::Unknown | ITP::NumberLike |
            ITP::IntegerLike | ITP::ArrayLike | ITP::PortLike => false,
            _ => true
        }
    }

    fn is_concrete_number(&self) -> bool {
        use InferenceTypePart as ITP;
        match self {
            ITP::UInt8 | ITP::UInt16 | ITP::UInt32 | ITP::UInt64 |
            ITP::SInt8 | ITP::SInt16 | ITP::SInt32 | ITP::SInt64 => true,
            _ => false,
        }
    }

    fn is_concrete_integer(&self) -> bool {
        use InferenceTypePart as ITP;
        match self {
            ITP::UInt8 | ITP::UInt16 | ITP::UInt32 | ITP::UInt64 |
            ITP::SInt8 | ITP::SInt16 | ITP::SInt32 | ITP::SInt64 => true,
            _ => false,
        }
    }

    fn is_concrete_arraylike(&self) -> bool {
        use InferenceTypePart as ITP;
        match self {
            ITP::Array | ITP::Slice | ITP::String | ITP::Message => true,
            _ => false,
        }
    }

    fn is_concrete_port(&self) -> bool {
        use InferenceTypePart as ITP;
        match self {
            ITP::Input | ITP::Output => true,
            _ => false,
        }
    }

    /// Checks if a part is less specific than the argument. Only checks for 
    /// single-part inference (i.e. not the replacement of an `Unknown` variant 
    /// with the argument)
    fn may_be_inferred_from(&self, arg: &InferenceTypePart) -> bool {
        use InferenceTypePart as ITP;

        (*self == ITP::IntegerLike && arg.is_concrete_integer()) ||
        (*self == ITP::NumberLike && (arg.is_concrete_number() || *arg == ITP::IntegerLike)) ||
        (*self == ITP::ArrayLike && arg.is_concrete_arraylike()) ||
        (*self == ITP::PortLike && arg.is_concrete_port())
    }

    /// Checks if a part is more specific

    /// Returns the change in "iteration depth" when traversing this particular
    /// part. The iteration depth is used to traverse the tree in a linear 
    /// fashion. It is basically `number_of_subtypes - 1`
    fn depth_change(&self) -> i32 {
        use InferenceTypePart as ITP;
        match &self {
            ITP::Unknown | ITP::NumberLike | ITP::IntegerLike |
            ITP::Void | ITP::Bool |
            ITP::UInt8 | ITP::UInt16 | ITP::UInt32 | ITP::UInt64 |
            ITP::SInt8 | ITP::SInt16 | ITP::SInt32 | ITP::SInt64 |
            ITP::Character => {
                -1
            },
            ITP::Marker(_) |
            ITP::ArrayLike | ITP::Message | ITP::Array | ITP::Slice |
            ITP::PortLike | ITP::Input | ITP::Output | ITP::String => {
                // One subtype, so do not modify depth
                0
            },
            ITP::Tuple(num) | ITP::Instance(_, num) => {
                (*num as i32) - 1
            }
        }
    }
}

#[derive(Debug, Clone)]
struct InferenceType {
    has_marker: bool,
    is_done: bool,
    parts: Vec<InferenceTypePart>,
}

impl InferenceType {
    /// Generates a new InferenceType. The two boolean flags will be checked in
    /// debug mode.
    fn new(has_marker: bool, is_done: bool, parts: Vec<InferenceTypePart>) -> Self {
        dbg_code!({
            debug_assert!(!parts.is_empty());
            let parts_body_marker = parts.iter().any(|v| v.is_marker());
            debug_assert_eq!(has_marker, parts_body_marker);
            let parts_done = parts.iter().all(|v| v.is_concrete());
            debug_assert_eq!(is_done, parts_done, "{:?}", parts);
        });
        Self{ has_marker, is_done, parts }
    }

    /// Replaces a type subtree with the provided subtree. The caller must make
    /// sure the the replacement is a well formed type subtree.
    fn replace_subtree(&mut self, start_idx: usize, with: &[InferenceTypePart]) {
        let end_idx = Self::find_subtree_end_idx(&self.parts, start_idx);
        debug_assert_eq!(with.len(), Self::find_subtree_end_idx(with, 0));
        self.parts.splice(start_idx..end_idx, with.iter().cloned());
        self.recompute_is_done();
    }

    // TODO: @performance, might all be done inline in the type inference methods
    fn recompute_is_done(&mut self) {
        self.is_done = self.parts.iter().all(|v| v.is_concrete());
    }

    /// Seeks a body marker starting at the specified position. If a marker is
    /// found then its value and the index of the type subtree that follows it
    /// is returned.
    fn find_marker(&self, mut start_idx: usize) -> Option<(u32, usize)> {
        while start_idx < self.parts.len() {
            if let InferenceTypePart::Marker(marker) = &self.parts[start_idx] {
                return Some((*marker, start_idx + 1))
            }

            start_idx += 1;
        }

        None
    }

    /// Returns an iterator over all body markers and the partial type tree that
    /// follows those markers. If it is a problem that `InferenceType` is 
    /// borrowed by the iterator, then use `find_body_marker`.
    fn marker_iter(&self) -> InferenceTypeMarkerIter {
        InferenceTypeMarkerIter::new(&self.parts)
    }

    /// Given that the `parts` are a depth-first serialized tree of types, this
    /// function finds the subtree anchored at a specific node. The returned 
    /// index is exclusive.
    fn find_subtree_end_idx(parts: &[InferenceTypePart], start_idx: usize) -> usize {
        let mut depth = 1;
        let mut idx = start_idx;

        while idx < parts.len() {
            depth += parts[idx].depth_change();
            if depth == 0 {
                return idx + 1;
            }
            idx += 1;
        }

        // If here, then the inference type is malformed
        unreachable!("Malformed type: {:?}", parts);
    }

    /// Call that attempts to infer the part at `to_infer.parts[to_infer_idx]` 
    /// using the subtree at `template.parts[template_idx]`. Will return 
    /// `Some(depth_change_due_to_traversal)` if type inference has been 
    /// applied. In this case the indices will also be modified to point to the 
    /// next part in both templates. If type inference has not (or: could not) 
    /// be applied then `None` will be returned. Note that this might mean that 
    /// the types are incompatible.
    ///
    /// As this is a helper functions, some assumptions: the parts are not 
    /// exactly equal, and neither of them contains a marker. Also: only the
    /// `to_infer` parts are checked for inference. It might be that this 
    /// function returns `None`, but that that `template` is still compatible
    /// with `to_infer`, e.g. when `template` has an `Unknown` part.
    fn infer_part_for_single_type(
        to_infer: &mut InferenceType, to_infer_idx: &mut usize,
        template_parts: &[InferenceTypePart], template_idx: &mut usize,
    ) -> Option<i32> {
        use InferenceTypePart as ITP;

        let to_infer_part = &to_infer.parts[*to_infer_idx];
        let template_part = &template_parts[*template_idx];

        // Check for programmer mistakes
        debug_assert_ne!(to_infer_part, template_part);
        debug_assert!(!to_infer_part.is_marker(), "marker encountered in 'infer part'");
        debug_assert!(!template_part.is_marker(), "marker encountered in 'template part'");

        // Inference of a somewhat-specified type
        if to_infer_part.may_be_inferred_from(template_part) {
            let depth_change = to_infer_part.depth_change();
            debug_assert_eq!(depth_change, template_part.depth_change());

            to_infer.parts[*to_infer_idx] = template_part.clone();

            *to_infer_idx += 1;
            *template_idx += 1;
            return Some(depth_change);
        }

        // Inference of a completely unknown type
        if *to_infer_part == ITP::Unknown {
            // template part is different, so cannot be unknown, hence copy the
            // entire subtree. Make sure not to copy markers.
            let template_end_idx = Self::find_subtree_end_idx(template_parts, *template_idx);
            to_infer.parts[*to_infer_idx] = template_parts[*template_idx].clone(); // first element

            *to_infer_idx += 1;
            for template_idx in *template_idx + 1..template_end_idx {
                let template_part = &template_parts[template_idx];
                if !template_part.is_marker() {
                    to_infer.parts.insert(*to_infer_idx, template_part.clone());
                    *to_infer_idx += 1;
                }
            }
            *template_idx = template_end_idx;

            // Note: by definition the LHS was Unknown and the RHS traversed a 
            // full subtree.
            return Some(-1);
        }

        None
    }

    /// Call that checks if the `to_check` part is compatible with the `infer`
    /// part. This is essentially a copy of `infer_part_for_single_type`, but
    /// without actually copying the type parts.
    fn check_part_for_single_type(
        to_check_parts: &[InferenceTypePart], to_check_idx: &mut usize,
        template_parts: &[InferenceTypePart], template_idx: &mut usize
    ) -> Option<i32> {
        use InferenceTypePart as ITP;

        let to_check_part = &to_check_parts[*to_check_idx];
        let template_part = &template_parts[*template_idx];

        // Checking programmer errors
        debug_assert_ne!(to_check_part, template_part);
        debug_assert!(!to_check_part.is_marker(), "marker encountered in 'to_check part'");
        debug_assert!(!template_part.is_marker(), "marker encountered in 'template part'");

        if to_check_part.may_be_inferred_from(template_part) {
            let depth_change = to_check_part.depth_change();
            debug_assert_eq!(depth_change, template_part.depth_change());
            *to_check_idx += 1;
            *template_idx += 1;
            return Some(depth_change);
        }

        if *to_check_part == ITP::Unknown {
            *to_check_idx += 1;
            *template_idx = Self::find_subtree_end_idx(template_parts, *template_idx);

            // By definition LHS and RHS had depth change of -1
            return Some(-1);
        }

        None
    }

    /// Attempts to infer types between two `InferenceType` instances. This 
    /// function is unsafe as it accepts pointers to work around Rust's 
    /// borrowing rules. The caller must ensure that the pointers are distinct.
    unsafe fn infer_subtrees_for_both_types(
        type_a: *mut InferenceType, start_idx_a: usize,
        type_b: *mut InferenceType, start_idx_b: usize
    ) -> DualInferenceResult {
        debug_assert!(!std::ptr::eq(type_a, type_b), "encountered pointers to the same inference type");
        let type_a = &mut *type_a;
        let type_b = &mut *type_b;

        let mut modified_a = false;
        let mut modified_b = false;
        let mut idx_a = start_idx_a;
        let mut idx_b = start_idx_b;
        let mut depth = 1;

        while depth > 0 {
            // Advance indices if we encounter markers or equal parts
            let part_a = &type_a.parts[idx_a];
            let part_b = &type_b.parts[idx_b];
            
            if part_a == part_b {
                let depth_change = part_a.depth_change();
                depth += depth_change;
                debug_assert_eq!(depth_change, part_b.depth_change());
                idx_a += 1;
                idx_b += 1;
                continue;
            }
            if part_a.is_marker() { idx_a += 1; continue; }
            if part_b.is_marker() { idx_b += 1; continue; }

            // Types are not equal and are both not markers
            if let Some(depth_change) = Self::infer_part_for_single_type(type_a, &mut idx_a, &type_b.parts, &mut idx_b) {
                depth += depth_change;
                modified_a = true;
                continue;
            }
            if let Some(depth_change) = Self::infer_part_for_single_type(type_b, &mut idx_b, &type_a.parts, &mut idx_a) {
                depth += depth_change;
                modified_b = true;
                continue;
            }

            // Types can not be inferred in any way: types must be incompatible
            return DualInferenceResult::Incompatible;
        }

        if modified_a { type_a.recompute_is_done(); }
        if modified_b { type_b.recompute_is_done(); }

        // If here then we completely inferred the subtrees.
        match (modified_a, modified_b) {
            (false, false) => DualInferenceResult::Neither,
            (false, true) => DualInferenceResult::Second,
            (true, false) => DualInferenceResult::First,
            (true, true) => DualInferenceResult::Both
        }
    }

    /// Attempts to infer the first subtree based on the template. Like
    /// `infer_subtrees_for_both_types`, but now only applying inference to
    /// `to_infer` based on the type information in `template`.
    ///
    /// The `forced_template` flag controls whether `to_infer` is considered
    /// valid if it is more specific then the template. When `forced_template`
    /// is false, then as long as the `to_infer` and `template` types are
    /// compatible the inference will succeed. If `forced_template` is true,
    /// then `to_infer` MUST be less specific than `template` (e.g.
    /// `IntegerLike` is less specific than `UInt32`)
    fn infer_subtree_for_single_type(
        to_infer: &mut InferenceType, mut to_infer_idx: usize,
        template: &[InferenceTypePart], mut template_idx: usize,
        forced_template: bool,
    ) -> SingleInferenceResult {
        let mut modified = false;
        let mut depth = 1;

        while depth > 0 {
            let to_infer_part = &to_infer.parts[to_infer_idx];
            let template_part = &template[template_idx];

            if to_infer_part == template_part {
                let depth_change = to_infer_part.depth_change();
                depth += depth_change;
                debug_assert_eq!(depth_change, template_part.depth_change());
                to_infer_idx += 1;
                template_idx += 1;
                continue;
            }
            if to_infer_part.is_marker() { to_infer_idx += 1; continue; }
            if template_part.is_marker() { template_idx += 1; continue; }

            // Types are not equal and not markers. So check if we can infer 
            // anything
            if let Some(depth_change) = Self::infer_part_for_single_type(
                to_infer, &mut to_infer_idx, template, &mut template_idx
            ) {
                depth += depth_change;
                modified = true;
                continue;
            }

            if !forced_template {
                // We cannot infer anything, but the template may still be
                // compatible with the type we're inferring
                if let Some(depth_change) = Self::check_part_for_single_type(
                    template, &mut template_idx, &to_infer.parts, &mut to_infer_idx
                ) {
                    depth += depth_change;
                    continue;
                }
            }

            return SingleInferenceResult::Incompatible
        }

        if modified {
            to_infer.recompute_is_done();
            return SingleInferenceResult::Modified;
        } else {
            return SingleInferenceResult::Unmodified;
        }
    }

    /// Checks if both types are compatible, doesn't perform any inference
    fn check_subtrees(
        type_parts_a: &[InferenceTypePart], start_idx_a: usize,
        type_parts_b: &[InferenceTypePart], start_idx_b: usize
    ) -> bool {
        let mut depth = 1;
        let mut idx_a = start_idx_a;
        let mut idx_b = start_idx_b;

        while depth > 0 {
            let part_a = &type_parts_a[idx_a];
            let part_b = &type_parts_b[idx_b];

            if part_a == part_b {
                let depth_change = part_a.depth_change();
                depth += depth_change;
                debug_assert_eq!(depth_change, part_b.depth_change());
                idx_a += 1;
                idx_b += 1;
                continue;
            }
            
            if part_a.is_marker() { idx_a += 1; continue; }
            if part_b.is_marker() { idx_b += 1; continue; }

            if let Some(depth_change) = Self::check_part_for_single_type(
                type_parts_a, &mut idx_a, type_parts_b, &mut idx_b
            ) {
                depth += depth_change;
                continue;
            }
            if let Some(depth_change) = Self::check_part_for_single_type(
                type_parts_b, &mut idx_b, type_parts_a, &mut idx_a
            ) {
                depth += depth_change;
                continue;
            }

            return false;
        }

        true
    }

    /// Performs the conversion of the inference type into a concrete type.
    /// By calling this function you must make sure that no unspecified types
    /// (e.g. Unknown or IntegerLike) exist in the type. Will not clear or check
    /// if the supplied `ConcreteType` is empty, will simply append to the parts
    /// vector.
    fn write_concrete_type(&self, concrete_type: &mut ConcreteType) {
        use InferenceTypePart as ITP;
        use ConcreteTypePart as CTP;

        // Make sure inference type is specified but concrete type is not yet specified
        debug_assert!(!self.parts.is_empty());
        concrete_type.parts.reserve(self.parts.len());

        let mut idx = 0;
        while idx < self.parts.len() {
            let part = &self.parts[idx];
            let converted_part = match part {
                ITP::Marker(_) => {
                    // Markers are removed when writing to the concrete type.
                    idx += 1;
                    continue;
                },
                ITP::Unknown | ITP::NumberLike |
                ITP::IntegerLike | ITP::ArrayLike | ITP::PortLike => {
                    // Should not happen if type inferencing works correctly: we
                    // should have returned a programmer-readable error or have
                    // inferred all types.
                    unreachable!("attempted to convert inference type part {:?} into concrete type", part);
                },
                ITP::Void => CTP::Void,
                ITP::Message => CTP::Message,
                ITP::Bool => CTP::Bool,
                ITP::UInt8 => CTP::UInt8,
                ITP::UInt16 => CTP::UInt16,
                ITP::UInt32 => CTP::UInt32,
                ITP::UInt64 => CTP::UInt64,
                ITP::SInt8 => CTP::SInt8,
                ITP::SInt16 => CTP::SInt16,
                ITP::SInt32 => CTP::SInt32,
                ITP::SInt64 => CTP::SInt64,
                ITP::Character => CTP::Character,
                ITP::String => {
                    // Inferred type has a 'char' subtype to simplify array
                    // checking, we remove it here.
                    debug_assert_eq!(self.parts[idx + 1], InferenceTypePart::Character);
                    idx += 1;
                    CTP::String
                },
                ITP::Array => CTP::Array,
                ITP::Slice => CTP::Slice,
                ITP::Input => CTP::Input,
                ITP::Output => CTP::Output,
                ITP::Tuple(num) => CTP::Tuple(*num),
                ITP::Instance(id, num) => CTP::Instance(*id, *num),
            };

            concrete_type.parts.push(converted_part);
            idx += 1;
        }
    }

    /// Writes a human-readable version of the type to a string. This is used
    /// to display error messages
    fn write_display_name(
        buffer: &mut String, heap: &Heap, parts: &[InferenceTypePart], mut idx: usize
    ) -> usize {
        use InferenceTypePart as ITP;

        match &parts[idx] {
            ITP::Marker(_marker_idx) => {
                if debug_log_enabled!() {
                    buffer.push_str(&format!("{{Marker:{}}}", *_marker_idx));
                }
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
            },
            ITP::Unknown => buffer.push_str("?"),
            ITP::NumberLike => buffer.push_str("numberlike"),
            ITP::IntegerLike => buffer.push_str("integerlike"),
            ITP::ArrayLike => {
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push_str("[?]");
            },
            ITP::PortLike => {
                buffer.push_str("portlike<");
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push('>');
            }
            ITP::Void => buffer.push_str("void"),
            ITP::Bool => buffer.push_str(KW_TYPE_BOOL_STR),
            ITP::UInt8 => buffer.push_str(KW_TYPE_UINT8_STR),
            ITP::UInt16 => buffer.push_str(KW_TYPE_UINT16_STR),
            ITP::UInt32 => buffer.push_str(KW_TYPE_UINT32_STR),
            ITP::UInt64 => buffer.push_str(KW_TYPE_UINT64_STR),
            ITP::SInt8 => buffer.push_str(KW_TYPE_SINT8_STR),
            ITP::SInt16 => buffer.push_str(KW_TYPE_SINT16_STR),
            ITP::SInt32 => buffer.push_str(KW_TYPE_SINT32_STR),
            ITP::SInt64 => buffer.push_str(KW_TYPE_SINT64_STR),
            ITP::Character => buffer.push_str(KW_TYPE_CHAR_STR),
            ITP::String => {
                buffer.push_str(KW_TYPE_STRING_STR);
                idx += 1; // skip the 'char' subtype
            },
            ITP::Message => {
                buffer.push_str(KW_TYPE_MESSAGE_STR);
                buffer.push('<');
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push('>');
            },
            ITP::Array => {
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push_str("[]");
            },
            ITP::Slice => {
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push_str("[..]");
            },
            ITP::Input => {
                buffer.push_str(KW_TYPE_IN_PORT_STR);
                buffer.push('<');
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push('>');
            },
            ITP::Output => {
                buffer.push_str(KW_TYPE_OUT_PORT_STR);
                buffer.push('<');
                idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                buffer.push('>');
            },
            ITP::Tuple(num_sub) => {
                buffer.push('(');
                if *num_sub > 0 {
                    idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                    for _sub_idx in 1..*num_sub {
                        buffer.push_str(", ");
                        idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                    }
                }
                buffer.push(')');
            }
            ITP::Instance(definition_id, num_sub) => {
                let definition = &heap[*definition_id];
                buffer.push_str(definition.identifier().value.as_str());
                if *num_sub > 0 {
                    buffer.push('<');
                    idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                    for _sub_idx in 1..*num_sub {
                        buffer.push_str(", ");
                        idx = Self::write_display_name(buffer, heap, parts, idx + 1);
                    }
                    buffer.push('>');
                }
            },
        }

        idx
    }

    /// Returns the display name of a (part of) the type tree. Will allocate a
    /// string.
    fn partial_display_name(heap: &Heap, parts: &[InferenceTypePart]) -> String {
        let mut buffer = String::with_capacity(parts.len() * 6);
        Self::write_display_name(&mut buffer, heap, parts, 0);
        buffer
    }

    /// Returns the display name of the full type tree. Will allocate a string.
    fn display_name(&self, heap: &Heap) -> String {
        Self::partial_display_name(heap, &self.parts)
    }
}

impl Default for InferenceType {
    fn default() -> Self {
        Self{
            has_marker: false,
            is_done: false,
            parts: Vec::new(),
        }
    }
}

/// Iterator over the subtrees that follow a marker in an `InferenceType`
/// instance. Returns immutable slices over the internal parts
struct InferenceTypeMarkerIter<'a> {
    parts: &'a [InferenceTypePart],
    idx: usize,
}

impl<'a> InferenceTypeMarkerIter<'a> {
    fn new(parts: &'a [InferenceTypePart]) -> Self {
        Self{ parts, idx: 0 }
    }
}

impl<'a> Iterator for InferenceTypeMarkerIter<'a> {
    type Item = (u32, &'a [InferenceTypePart]);

    fn next(&mut self) -> Option<Self::Item> {
        // Iterate until we find a marker
        while self.idx < self.parts.len() {
            if let InferenceTypePart::Marker(marker) = self.parts[self.idx] {
                // Found a marker, find the subtree end
                let start_idx = self.idx + 1;
                let end_idx = InferenceType::find_subtree_end_idx(self.parts, start_idx);

                // Modify internal index, then return items
                self.idx = end_idx;
                return Some((marker, &self.parts[start_idx..end_idx]));
            }

            self.idx += 1;
        }

        None
    }
}

#[derive(Debug, PartialEq, Eq)]
enum DualInferenceResult {
    Neither,        // neither argument is clarified
    First,          // first argument is clarified using the second one
    Second,         // second argument is clarified using the first one
    Both,           // both arguments are clarified
    Incompatible,   // types are incompatible: programmer error
}

impl DualInferenceResult {
    fn modified_lhs(&self) -> bool {
        match self {
            DualInferenceResult::First | DualInferenceResult::Both => true,
            _ => false
        }
    }
    fn modified_rhs(&self) -> bool {
        match self {
            DualInferenceResult::Second | DualInferenceResult::Both => true,
            _ => false
        }
    }
}

#[derive(Debug, PartialEq, Eq)]
enum SingleInferenceResult {
    Unmodified,
    Modified,
    Incompatible
}

// -----------------------------------------------------------------------------
// PassTyping - Public Interface
// -----------------------------------------------------------------------------

type InferNodeIndex = usize;
type PolyDataIndex = isize;
type VarDataIndex = usize;

pub(crate) struct ResolveQueueElement {
    // Note that using the `definition_id` and the `monomorph_idx` one may
    // query the type table for the full procedure type, thereby retrieving
    // the polymorphic arguments to the procedure.
    pub(crate) root_id: RootId,
    pub(crate) definition_id: DefinitionId,
    pub(crate) reserved_type_id: TypeId,
    pub(crate) reserved_monomorph_index: u32,
}

pub(crate) type ResolveQueue = VecDeque<ResolveQueueElement>;

struct InferenceNode {
    // filled in during type inference
    expr_type: InferenceType,               // result type from expression
    expr_id: ExpressionId,                  // expression that is evaluated
    inference_rule: InferenceRule,          // rule used to infer node type
    parent_index: Option<InferNodeIndex>,   // parent of inference node
    field_index: i32,                       // index of struct field or tuple member
    poly_data_index: PolyDataIndex,         // index to inference data for polymorphic types
    // filled in once type inference is done
    info_type_id: TypeId,
    info_variant: ExpressionInfoVariant,
}

impl InferenceNode {
    #[inline]
    fn as_expression_info(&self) -> ExpressionInfo {
        return ExpressionInfo {
            type_id: self.info_type_id,
            variant: self.info_variant
        }
    }
}

/// Inferencing rule to apply. Some of these are reasonably generic. Other ones
/// require so much custom logic that we'll not try to come up with an
/// abstraction.
enum InferenceRule {
    Noop,
    MonoTemplate(InferenceRuleTemplate),
    BiEqual(InferenceRuleBiEqual),
    TriEqualArgs(InferenceRuleTriEqualArgs),
    TriEqualAll(InferenceRuleTriEqualAll),
    Concatenate(InferenceRuleTwoArgs),
    IndexingExpr(InferenceRuleIndexingExpr),
    SlicingExpr(InferenceRuleSlicingExpr),
    SelectStructField(InferenceRuleSelectStructField),
    SelectTupleMember(InferenceRuleSelectTupleMember),
    LiteralStruct(InferenceRuleLiteralStruct),
    LiteralEnum,
    LiteralUnion(InferenceRuleLiteralUnion),
    LiteralArray(InferenceRuleLiteralArray),
    LiteralTuple(InferenceRuleLiteralTuple),
    CastExpr(InferenceRuleCastExpr),
    CallExpr(InferenceRuleCallExpr),
    VariableExpr(InferenceRuleVariableExpr),
}

impl InferenceRule {
    union_cast_to_ref_method_impl!(as_mono_template, InferenceRuleTemplate, InferenceRule::MonoTemplate);
    union_cast_to_ref_method_impl!(as_bi_equal, InferenceRuleBiEqual, InferenceRule::BiEqual);
    union_cast_to_ref_method_impl!(as_tri_equal_args, InferenceRuleTriEqualArgs, InferenceRule::TriEqualArgs);
    union_cast_to_ref_method_impl!(as_tri_equal_all, InferenceRuleTriEqualAll, InferenceRule::TriEqualAll);
    union_cast_to_ref_method_impl!(as_concatenate, InferenceRuleTwoArgs, InferenceRule::Concatenate);
    union_cast_to_ref_method_impl!(as_indexing_expr, InferenceRuleIndexingExpr, InferenceRule::IndexingExpr);
    union_cast_to_ref_method_impl!(as_slicing_expr, InferenceRuleSlicingExpr, InferenceRule::SlicingExpr);
    union_cast_to_ref_method_impl!(as_select_struct_field, InferenceRuleSelectStructField, InferenceRule::SelectStructField);
    union_cast_to_ref_method_impl!(as_select_tuple_member, InferenceRuleSelectTupleMember, InferenceRule::SelectTupleMember);
    union_cast_to_ref_method_impl!(as_literal_struct, InferenceRuleLiteralStruct, InferenceRule::LiteralStruct);
    union_cast_to_ref_method_impl!(as_literal_union, InferenceRuleLiteralUnion, InferenceRule::LiteralUnion);
    union_cast_to_ref_method_impl!(as_literal_array, InferenceRuleLiteralArray, InferenceRule::LiteralArray);
    union_cast_to_ref_method_impl!(as_literal_tuple, InferenceRuleLiteralTuple, InferenceRule::LiteralTuple);
    union_cast_to_ref_method_impl!(as_cast_expr, InferenceRuleCastExpr, InferenceRule::CastExpr);
    union_cast_to_ref_method_impl!(as_call_expr, InferenceRuleCallExpr, InferenceRule::CallExpr);
    union_cast_to_ref_method_impl!(as_variable_expr, InferenceRuleVariableExpr, InferenceRule::VariableExpr);
}

// Note: InferenceRuleTemplate is `Copy`, so don't add dynamically allocated
// members in the future (or review places where this struct is copied)
#[derive(Clone, Copy)]
struct InferenceRuleTemplate {
    template: &'static [InferenceTypePart],
    application: InferenceRuleTemplateApplication,
}

impl InferenceRuleTemplate {
    fn new_none() -> Self {
        return Self{
            template: &[],
            application: InferenceRuleTemplateApplication::None,
        };
    }

    fn new_forced(template: &'static [InferenceTypePart]) -> Self {
        return Self{
            template,
            application: InferenceRuleTemplateApplication::Forced,
        };
    }

    fn new_template(template: &'static [InferenceTypePart]) -> Self {
        return Self{
            template,
            application: InferenceRuleTemplateApplication::Template,
        }
    }
}

#[derive(Clone, Copy)]
enum InferenceRuleTemplateApplication {
    None, // do not apply template, silly, but saves some bytes
    Forced,
    Template,
}

/// Type equality applied to 'self' and the argument. An optional template will
/// be applied to 'self' first. Example: "bitwise not"
struct InferenceRuleBiEqual {
    template: InferenceRuleTemplate,
    argument_index: InferNodeIndex,
}

/// Type equality applied to two arguments. Template can be applied to 'self'
/// (generally forced, since this rule does not apply a type equality constraint
/// to 'self') and the two arguments. Example: "equality operator"
struct InferenceRuleTriEqualArgs {
    argument_template: InferenceRuleTemplate,
    result_template: InferenceRuleTemplate,
    argument1_index: InferNodeIndex,
    argument2_index: InferNodeIndex,
}

/// Type equality applied to 'self' and two arguments. Template may be
/// optionally applied to 'self'. Example: "addition operator"
struct InferenceRuleTriEqualAll {
    template: InferenceRuleTemplate,
    argument1_index: InferNodeIndex,
    argument2_index: InferNodeIndex,
}

/// Information for an inference rule that is applied to 'self' and two
/// arguments, see `InferenceRule` for its meaning.
struct InferenceRuleTwoArgs {
    argument1_index: InferNodeIndex,
    argument2_index: InferNodeIndex,
}

struct InferenceRuleIndexingExpr {
    subject_index: InferNodeIndex,
    index_index: InferNodeIndex,
}

struct InferenceRuleSlicingExpr {
    subject_index: InferNodeIndex,
    from_index: InferNodeIndex,
    to_index: InferNodeIndex,
}

struct InferenceRuleSelectStructField {
    subject_index: InferNodeIndex,
    selected_field: Identifier,
}

struct InferenceRuleSelectTupleMember {
    subject_index: InferNodeIndex,
    selected_index: u64,
}

struct InferenceRuleLiteralStruct {
    element_indices: Vec<InferNodeIndex>,
}

struct InferenceRuleLiteralUnion {
    element_indices: Vec<InferNodeIndex>
}

struct InferenceRuleLiteralArray {
    element_indices: Vec<InferNodeIndex>
}

struct InferenceRuleLiteralTuple {
    element_indices: Vec<InferNodeIndex>
}

struct InferenceRuleCastExpr {
    subject_index: InferNodeIndex,
}

struct InferenceRuleCallExpr {
    argument_indices: Vec<InferNodeIndex>
}

/// Data associated with a variable expression: an expression that reads the
/// value from a variable.
struct InferenceRuleVariableExpr {
    var_data_index: VarDataIndex, // shared variable information
}

/// This particular visitor will recurse depth-first into the AST and ensures
/// that all expressions have the appropriate types.
pub(crate) struct PassTyping {
    // Current definition we're typechecking.
    reserved_type_id: TypeId,
    reserved_monomorph_index: u32,
    procedure_id: ProcedureDefinitionId,
    procedure_kind: ProcedureKind,
    poly_vars: Vec<ConcreteType>,
    // Temporary variables during construction of inference rulesr
    parent_index: Option<InferNodeIndex>,
    // Buffers for iteration over various types
    var_buffer: ScopedBuffer<VariableId>,
    expr_buffer: ScopedBuffer<ExpressionId>,
    stmt_buffer: ScopedBuffer<StatementId>,
    bool_buffer: ScopedBuffer<bool>,
    index_buffer: ScopedBuffer<usize>,
    definition_buffer: ScopedBuffer<DefinitionId>,
    poly_progress_buffer: ScopedBuffer<u32>,
    // Mapping from parser type to inferred type. We attempt to continue to
    // specify these types until we're stuck or we've fully determined the type.
    infer_nodes: Vec<InferenceNode>,                     // will be transferred to type table at end
    poly_data: Vec<PolyData>,       // data for polymorph inference
    var_data: Vec<VarData>,
    // Keeping track of which expressions need to be reinferred because the
    // expressions they're linked to made progression on an associated type
    node_queued: DequeSet<InferNodeIndex>,
}

/// Generic struct that is used to store inferred types associated with
/// polymorphic types.
struct PolyData {
    first_rule_application: bool,
    definition_id: DefinitionId, // the definition, only used for user feedback
    /// Inferred types of the polymorphic variables as they are written down
    /// at the type's definition.
    poly_vars: Vec<InferenceType>,
    expr_types: PolyDataTypes,
}

// silly structure, just so we can use `PolyDataTypeIndex` ergonomically while
// making sure we're still capable of borrowing from `poly_vars`.
struct PolyDataTypes {
    /// Inferred types of associated types (e.g. struct fields, tuple members,
    /// function arguments). These types may depend on the polymorphic variables
    /// defined above.
    associated: Vec<InferenceType>,
    /// Inferred "returned" type (e.g. if a struct field is selected, then this
    /// contains the type of the selected field, for a function call it contains
    /// the return type). May depend on the polymorphic variables defined above.
    returned: InferenceType,
}

#[derive(Clone, Copy)]
enum PolyDataTypeIndex {
    Associated(usize), // indexes into `PolyData.associated`
    Returned,
}

impl PolyDataTypes {
    fn get_type(&self, index: PolyDataTypeIndex) -> &InferenceType {
        match index {
            PolyDataTypeIndex::Associated(index) => return &self.associated[index],
            PolyDataTypeIndex::Returned => return &self.returned,
        }
    }

    fn get_type_mut(&mut self, index: PolyDataTypeIndex) -> &mut InferenceType {
        match index {
            PolyDataTypeIndex::Associated(index) => return &mut self.associated[index],
            PolyDataTypeIndex::Returned => return &mut self.returned,
        }
    }
}

struct VarData {
    var_id: VariableId,
    var_type: InferenceType,
    used_at: Vec<InferNodeIndex>, // of variable expressions
    linked_var: Option<VarDataIndex>,
}

impl PassTyping {
    pub(crate) fn new() -> Self {
        PassTyping {
            reserved_type_id: TypeId::new_invalid(),
            reserved_monomorph_index: u32::MAX,
            procedure_id: ProcedureDefinitionId::new_invalid(),
            procedure_kind: ProcedureKind::Function,
            poly_vars: Vec::new(),
            parent_index: None,
            var_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_LARGE),
            expr_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_LARGE),
            stmt_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_LARGE),
            bool_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_SMALL),
            index_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_SMALL),
            definition_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_LARGE),
            poly_progress_buffer: ScopedBuffer::with_capacity(BUFFER_INIT_CAP_SMALL),
            infer_nodes: Vec::with_capacity(BUFFER_INIT_CAP_LARGE),
            poly_data: Vec::with_capacity(BUFFER_INIT_CAP_SMALL),
            var_data: Vec::with_capacity(BUFFER_INIT_CAP_SMALL),
            node_queued: DequeSet::new(),
        }
    }

    pub(crate) fn queue_module_definitions(&mut self, ctx: &mut Ctx, queue: &mut ResolveQueue) {
        debug_assert_eq!(ctx.module().phase, ModuleCompilationPhase::ValidatedAndLinked);
        let root_id = ctx.module().root_id;
        let root = &ctx.heap.protocol_descriptions[root_id];
        let definitions_section = self.definition_buffer.start_section_initialized(&root.definitions);

        for definition_id in definitions_section.iter_copied() {
            let definition = &ctx.heap[definition_id];

            let first_concrete_part_and_procedure_id = match definition {
                Definition::Procedure(definition) => {
                    if definition.poly_vars.is_empty() {
                        if definition.kind == ProcedureKind::Function {
                            Some((ConcreteTypePart::Function(definition.this, 0), definition.this))
                        } else {
                            Some((ConcreteTypePart::Component(definition.this, 0), definition.this))
                        }
                    } else {
                        None
                    }
                }
                Definition::Enum(_) | Definition::Struct(_) | Definition::Union(_) => None,
            };

            if let Some((first_concrete_part, procedure_id)) = first_concrete_part_and_procedure_id {
                let procedure = &mut ctx.heap[procedure_id];
                let monomorph_index = procedure.monomorphs.len() as u32;
                procedure.monomorphs.push(ProcedureDefinitionMonomorph::new_invalid());

                let concrete_type = ConcreteType{ parts: vec![first_concrete_part] };
                let type_id = ctx.types.reserve_procedure_monomorph_type_id(&definition_id, concrete_type, monomorph_index);
                queue.push_back(ResolveQueueElement{
                    root_id,
                    definition_id,
                    reserved_type_id: type_id,
                    reserved_monomorph_index: monomorph_index,
                })
            }
        }

        definitions_section.forget();
    }

    pub(crate) fn handle_module_definition(
        &mut self, ctx: &mut Ctx, queue: &mut ResolveQueue, element: ResolveQueueElement
    ) -> VisitorResult {
        self.reset();
        debug_assert_eq!(ctx.module().root_id, element.root_id);
        debug_assert!(self.poly_vars.is_empty());

        // Prepare for visiting the definition
        self.reserved_type_id = element.reserved_type_id;
        self.reserved_monomorph_index = element.reserved_monomorph_index;

        let proc_base = ctx.types.get_base_definition(&element.definition_id).unwrap();
        if proc_base.is_polymorph {
            let monomorph = ctx.types.get_monomorph(element.reserved_type_id);
            for poly_arg in monomorph.concrete_type.embedded_iter(0) {
                self.poly_vars.push(ConcreteType{ parts: Vec::from(poly_arg) });
            }
        }

        // Visit the definition, setting up the type resolving process, then
        // (attempt to) resolve all types
        self.visit_definition(ctx, element.definition_id)?;
        self.resolve_types(ctx, queue)?;
        Ok(())
    }

    fn reset(&mut self) {
        self.reserved_type_id = TypeId::new_invalid();
        self.procedure_id = ProcedureDefinitionId::new_invalid();
        self.procedure_kind = ProcedureKind::Function;
        self.poly_vars.clear();
        self.parent_index = None;

        self.infer_nodes.clear();
        self.poly_data.clear();
        self.var_data.clear();
        self.node_queued.clear();
    }
}

// -----------------------------------------------------------------------------
// PassTyping - Visitor-like implementation
// -----------------------------------------------------------------------------

type VisitorResult = Result<(), ParseError>;
type VisitExprResult = Result<InferNodeIndex, ParseError>;

impl PassTyping {
    // Definitions

    fn visit_definition(&mut self, ctx: &mut Ctx, id: DefinitionId) -> VisitorResult {
        return visitor_recursive_definition_impl!(self, &ctx.heap[id], ctx);
    }

    fn visit_enum_definition(&mut self, _: &mut Ctx, _: EnumDefinitionId) -> VisitorResult { return Ok(()) }
    fn visit_struct_definition(&mut self, _: &mut Ctx, _: StructDefinitionId) -> VisitorResult { return Ok(()) }
    fn visit_union_definition(&mut self, _: &mut Ctx, _: UnionDefinitionId) -> VisitorResult { return Ok(()) }

    fn visit_procedure_definition(&mut self, ctx: &mut Ctx, id: ProcedureDefinitionId) -> VisitorResult {
        let procedure_def = &ctx.heap[id];

        self.procedure_id = id;
        self.procedure_kind = procedure_def.kind;
        let body_id = procedure_def.body;
        let procedure_is_builtin = procedure_def.source.is_builtin();

        debug_log!("{}", "-".repeat(50));
        debug_log!("Visiting procedure: '{}' (id: {}, kind: {:?})", procedure_def.identifier.value.as_str(), id.0.index, procedure_def.kind);
        debug_log!("{}", "-".repeat(50));

        // Visit parameters
        let section = self.var_buffer.start_section_initialized(procedure_def.parameters.as_slice());
        for param_id in section.iter_copied() {
            let param = &ctx.heap[param_id];
            let var_type = self.determine_inference_type_from_parser_type_elements(&param.parser_type.elements, true);
            debug_assert!(var_type.is_done, "expected function arguments to be concrete types");
            self.var_data.push(VarData{
                var_id: param_id,
                var_type,
                used_at: Vec::new(),
                linked_var: None
            })
        }
        section.forget();

        // Visit all of the expressions within the body
        self.parent_index = None;
        if !procedure_is_builtin {
            return self.visit_block_stmt(ctx, body_id);
        } else {
            return Ok(());
        }
    }

    // Statements

    fn visit_stmt(&mut self, ctx: &mut Ctx, id: StatementId) -> VisitorResult {
        return visitor_recursive_statement_impl!(self, &ctx.heap[id], ctx, Ok(()));
    }

    fn visit_block_stmt(&mut self, ctx: &mut Ctx, id: BlockStatementId) -> VisitorResult {
        // Transfer statements for traversal
        let block = &ctx.heap[id];

        let section = self.stmt_buffer.start_section_initialized(block.statements.as_slice());
        for stmt_id in section.iter_copied() {
            self.visit_stmt(ctx, stmt_id)?;
        }
        section.forget();

        Ok(())
    }

    fn visit_local_stmt(&mut self, ctx: &mut Ctx, id: LocalStatementId) -> VisitorResult {
        return visitor_recursive_local_impl!(self, &ctx.heap[id], ctx);
    }

    fn visit_local_memory_stmt(&mut self, ctx: &mut Ctx, id: MemoryStatementId) -> VisitorResult {
        let memory_stmt = &ctx.heap[id];
        let initial_expr_id = memory_stmt.initial_expr;

        let local = &ctx.heap[memory_stmt.variable];
        let var_type = self.determine_inference_type_from_parser_type_elements(&local.parser_type.elements, true);
        self.var_data.push(VarData{
            var_id: memory_stmt.variable,
            var_type,
            used_at: Vec::new(),
            linked_var: None,
        });

        // Process the initial value
        self.visit_assignment_expr(ctx, initial_expr_id)?;

        Ok(())
    }

    fn visit_local_channel_stmt(&mut self, ctx: &mut Ctx, id: ChannelStatementId) -> VisitorResult {
        let channel_stmt = &ctx.heap[id];

        let from_var_index = self.var_data.len() as VarDataIndex;
        let to_var_index = from_var_index + 1;

        let from_local = &ctx.heap[channel_stmt.from];
        let from_var_type = self.determine_inference_type_from_parser_type_elements(&from_local.parser_type.elements, true);
        self.var_data.push(VarData{
            var_id: channel_stmt.from,
            var_type: from_var_type,
            used_at: Vec::new(),
            linked_var: Some(to_var_index),
        });

        let to_local = &ctx.heap[channel_stmt.to];
        let to_var_type = self.determine_inference_type_from_parser_type_elements(&to_local.parser_type.elements, true);
        self.var_data.push(VarData{
            var_id: channel_stmt.to,
            var_type: to_var_type,
            used_at: Vec::new(),
            linked_var: Some(from_var_index),
        });

        Ok(())
    }

    fn visit_labeled_stmt(&mut self, ctx: &mut Ctx, id: LabeledStatementId) -> VisitorResult {
        let labeled_stmt = &ctx.heap[id];
        let substmt_id = labeled_stmt.body;
        self.visit_stmt(ctx, substmt_id)
    }

    fn visit_if_stmt(&mut self, ctx: &mut Ctx, id: IfStatementId) -> VisitorResult {
        let if_stmt = &ctx.heap[id];

        let true_body_case = if_stmt.true_case;
        let false_body_case = if_stmt.false_case;
        let test_expr_id = if_stmt.test;

        self.visit_expr(ctx, test_expr_id)?;
        self.visit_stmt(ctx, true_body_case.body)?;
        if let Some(false_body_case) = false_body_case {
            self.visit_stmt(ctx, false_body_case.body)?;
        }

        Ok(())
    }

    fn visit_while_stmt(&mut self, ctx: &mut Ctx, id: WhileStatementId) -> VisitorResult {
        let while_stmt = &ctx.heap[id];

        let body_id = while_stmt.body;
        let test_expr_id = while_stmt.test;

        self.visit_expr(ctx, test_expr_id)?;
        self.visit_stmt(ctx, body_id)?;

        Ok(())
    }

    fn visit_break_stmt(&mut self, _: &mut Ctx, _: BreakStatementId) -> VisitorResult { return Ok(()) }
    fn visit_continue_stmt(&mut self, _: &mut Ctx, _: ContinueStatementId) -> VisitorResult { return Ok(()) }

    fn visit_synchronous_stmt(&mut self, ctx: &mut Ctx, id: SynchronousStatementId) -> VisitorResult {
        let sync_stmt = &ctx.heap[id];
        let body_id = sync_stmt.body;

        self.visit_stmt(ctx, body_id)
    }

    fn visit_fork_stmt(&mut self, ctx: &mut Ctx, id: ForkStatementId) -> VisitorResult {
        let fork_stmt = &ctx.heap[id];
        let left_body_id = fork_stmt.left_body;
        let right_body_id = fork_stmt.right_body;

        self.visit_stmt(ctx, left_body_id)?;
        if let Some(right_body_id) = right_body_id {
            self.visit_stmt(ctx, right_body_id)?;
        }

        Ok(())
    }

    fn visit_select_stmt(&mut self, ctx: &mut Ctx, id: SelectStatementId) -> VisitorResult {
        let select_stmt = &ctx.heap[id];

        let mut section = self.stmt_buffer.start_section();
        let num_cases = select_stmt.cases.len();

        for case in &select_stmt.cases {
            section.push(case.guard);
            section.push(case.body);
        }

        for case_index in 0..num_cases {
            let base_index = 2 * case_index;
            let guard_stmt_id = section[base_index    ];
            let block_stmt_id = section[base_index + 1];

            self.visit_stmt(ctx, guard_stmt_id)?;
            self.visit_stmt(ctx, block_stmt_id)?;
        }
        section.forget();

        Ok(())
    }

    fn visit_return_stmt(&mut self, ctx: &mut Ctx, id: ReturnStatementId) -> VisitorResult {
        let return_stmt = &ctx.heap[id];
        debug_assert_eq!(return_stmt.expressions.len(), 1);
        let expr_id = return_stmt.expressions[0];

        self.visit_expr(ctx, expr_id)?;
        return Ok(());
    }

    fn visit_goto_stmt(&mut self, _: &mut Ctx, _: GotoStatementId) -> VisitorResult { return Ok(()) }

    fn visit_new_stmt(&mut self, ctx: &mut Ctx, id: NewStatementId) -> VisitorResult {
        let new_stmt = &ctx.heap[id];
        let call_expr_id = new_stmt.expression;

        self.visit_call_expr(ctx, call_expr_id)?;
        return Ok(());
    }

    fn visit_expr_stmt(&mut self, ctx: &mut Ctx, id: ExpressionStatementId) -> VisitorResult {
        let expr_stmt = &ctx.heap[id];
        let subexpr_id = expr_stmt.expression;

        self.visit_expr(ctx, subexpr_id)?;
        return Ok(());
    }

    // Expressions

    fn visit_expr(&mut self, ctx: &mut Ctx, id: ExpressionId) -> VisitExprResult {
        return visitor_recursive_expression_impl!(self, &ctx.heap[id], ctx);
    }

    fn visit_assignment_expr(&mut self, ctx: &mut Ctx, id: AssignmentExpressionId) -> VisitExprResult {
        use AssignmentOperator as AO;

        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let assign_expr = &ctx.heap[id];
        let assign_op = assign_expr.operation;
        let left_expr_id = assign_expr.left;
        let right_expr_id = assign_expr.right;

        let old_parent = self.parent_index.replace(self_index);
        let left_index = self.visit_expr(ctx, left_expr_id)?;
        let right_index = self.visit_expr(ctx, right_expr_id)?;

        let node = &mut self.infer_nodes[self_index];
        let argument_template = match assign_op {
            AO::Set =>
                InferenceRuleTemplate::new_none(),
            AO::Concatenated =>
                InferenceRuleTemplate::new_template(&ARRAYLIKE_TEMPLATE),
            AO::Multiplied | AO::Divided | AO::Added | AO::Subtracted =>
                InferenceRuleTemplate::new_template(&NUMBERLIKE_TEMPLATE),
            AO::Remained | AO::ShiftedLeft | AO::ShiftedRight |
            AO::BitwiseAnded | AO::BitwiseXored | AO::BitwiseOred =>
                InferenceRuleTemplate::new_template(&INTEGERLIKE_TEMPLATE),
        };

        node.inference_rule = InferenceRule::TriEqualArgs(InferenceRuleTriEqualArgs{
            argument_template,
            result_template: InferenceRuleTemplate::new_forced(&VOID_TEMPLATE),
            argument1_index: left_index,
            argument2_index: right_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_tri_equal_args(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_binding_expr(&mut self, ctx: &mut Ctx, id: BindingExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let binding_expr = &ctx.heap[id];
        let bound_to_id = binding_expr.bound_to;
        let bound_from_id = binding_expr.bound_from;

        let old_parent = self.parent_index.replace(self_index);
        let arg_to_index = self.visit_expr(ctx, bound_to_id)?;
        let arg_from_index = self.visit_expr(ctx, bound_from_id)?;

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::TriEqualArgs(InferenceRuleTriEqualArgs{
            argument_template: InferenceRuleTemplate::new_none(),
            result_template: InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE),
            argument1_index: arg_to_index,
            argument2_index: arg_from_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_tri_equal_args(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_conditional_expr(&mut self, ctx: &mut Ctx, id: ConditionalExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let conditional_expr = &ctx.heap[id];
        let test_expr_id = conditional_expr.test;
        let true_expr_id = conditional_expr.true_expression;
        let false_expr_id = conditional_expr.false_expression;

        let old_parent = self.parent_index.replace(self_index);
        self.visit_expr(ctx, test_expr_id)?;
        let true_index = self.visit_expr(ctx, true_expr_id)?;
        let false_index = self.visit_expr(ctx, false_expr_id)?;

        // Note: the test to the conditional expression has already been forced
        // to the boolean type. So the only thing we need to do while progressing
        // is to apply an equal3 constraint to the arguments and the result of
        // the expression.
        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::TriEqualAll(InferenceRuleTriEqualAll{
            template: InferenceRuleTemplate::new_none(),
            argument1_index: true_index,
            argument2_index: false_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_tri_equal_all(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_binary_expr(&mut self, ctx: &mut Ctx, id: BinaryExpressionId) -> VisitExprResult {
        use BinaryOperator as BO;

        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let binary_expr = &ctx.heap[id];
        let binary_op = binary_expr.operation;
        let lhs_expr_id = binary_expr.left;
        let rhs_expr_id = binary_expr.right;

        let old_parent = self.parent_index.replace(self_index);
        let left_index = self.visit_expr(ctx, lhs_expr_id)?;
        let right_index = self.visit_expr(ctx, rhs_expr_id)?;

        let inference_rule = match binary_op {
            BO::Concatenate =>
                InferenceRule::Concatenate(InferenceRuleTwoArgs{
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
            BO::LogicalAnd | BO::LogicalOr =>
                InferenceRule::TriEqualAll(InferenceRuleTriEqualAll{
                    template: InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE),
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
            BO::BitwiseOr | BO::BitwiseXor | BO::BitwiseAnd | BO::Remainder | BO::ShiftLeft | BO::ShiftRight =>
                InferenceRule::TriEqualAll(InferenceRuleTriEqualAll{
                    template: InferenceRuleTemplate::new_template(&INTEGERLIKE_TEMPLATE),
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
            BO::Equality | BO::Inequality =>
                InferenceRule::TriEqualArgs(InferenceRuleTriEqualArgs{
                    argument_template: InferenceRuleTemplate::new_none(),
                    result_template: InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE),
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
            BO::LessThan | BO::GreaterThan | BO::LessThanEqual | BO::GreaterThanEqual =>
                InferenceRule::TriEqualArgs(InferenceRuleTriEqualArgs{
                    argument_template: InferenceRuleTemplate::new_template(&NUMBERLIKE_TEMPLATE),
                    result_template: InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE),
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
            BO::Add | BO::Subtract | BO::Multiply | BO::Divide =>
                InferenceRule::TriEqualAll(InferenceRuleTriEqualAll{
                    template: InferenceRuleTemplate::new_template(&NUMBERLIKE_TEMPLATE),
                    argument1_index: left_index,
                    argument2_index: right_index,
                }),
        };

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = inference_rule;

        self.parent_index = old_parent;
        self.progress_inference_rule(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_unary_expr(&mut self, ctx: &mut Ctx, id: UnaryExpressionId) -> VisitExprResult {
        use UnaryOperator as UO;

        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let unary_expr = &ctx.heap[id];
        let operation = unary_expr.operation;
        let arg_expr_id = unary_expr.expression;

        let old_parent = self.parent_index.replace(self_index);
        let argument_index = self.visit_expr(ctx, arg_expr_id)?;

        let template = match operation {
            UO::Positive | UO::Negative =>
                InferenceRuleTemplate::new_template(&NUMBERLIKE_TEMPLATE),
            UO::BitwiseNot =>
                InferenceRuleTemplate::new_template(&INTEGERLIKE_TEMPLATE),
            UO::LogicalNot =>
                InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE),
        };

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::BiEqual(InferenceRuleBiEqual{
            template, argument_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_bi_equal(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_indexing_expr(&mut self, ctx: &mut Ctx, id: IndexingExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let indexing_expr = &ctx.heap[id];
        let subject_expr_id = indexing_expr.subject;
        let index_expr_id = indexing_expr.index;

        let old_parent = self.parent_index.replace(self_index);
        let subject_index = self.visit_expr(ctx, subject_expr_id)?;
        let index_index = self.visit_expr(ctx, index_expr_id)?; // cool name, bro

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::IndexingExpr(InferenceRuleIndexingExpr{
            subject_index, index_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_indexing_expr(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_slicing_expr(&mut self, ctx: &mut Ctx, id: SlicingExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let slicing_expr = &ctx.heap[id];
        let subject_expr_id = slicing_expr.subject;
        let from_expr_id = slicing_expr.from_index;
        let to_expr_id = slicing_expr.to_index;

        let old_parent = self.parent_index.replace(self_index);
        let subject_index = self.visit_expr(ctx, subject_expr_id)?;
        let from_index = self.visit_expr(ctx, from_expr_id)?;
        let to_index = self.visit_expr(ctx, to_expr_id)?;

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::SlicingExpr(InferenceRuleSlicingExpr{
            subject_index, from_index, to_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_slicing_expr(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_select_expr(&mut self, ctx: &mut Ctx, id: SelectExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let select_expr = &ctx.heap[id];
        let subject_expr_id = select_expr.subject;

        let old_parent = self.parent_index.replace(self_index);
        let subject_index = self.visit_expr(ctx, subject_expr_id)?;

        let node = &mut self.infer_nodes[self_index];
        let inference_rule = match &ctx.heap[id].kind {
            SelectKind::StructField(field_identifier) =>
                InferenceRule::SelectStructField(InferenceRuleSelectStructField{
                    subject_index,
                    selected_field: field_identifier.clone(),
                }),
            SelectKind::TupleMember(member_index) =>
                InferenceRule::SelectTupleMember(InferenceRuleSelectTupleMember{
                    subject_index,
                    selected_index: *member_index,
                }),
        };
        node.inference_rule = inference_rule;

        self.parent_index = old_parent;
        self.progress_inference_rule(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_literal_expr(&mut self, ctx: &mut Ctx, id: LiteralExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let old_parent = self.parent_index.replace(self_index);

        let literal_expr = &ctx.heap[id];
        match &literal_expr.value {
            Literal::Null => {
                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::MonoTemplate(InferenceRuleTemplate::new_template(&MESSAGE_TEMPLATE));
            },
            Literal::Integer(_) => {
                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::MonoTemplate(InferenceRuleTemplate::new_template(&INTEGERLIKE_TEMPLATE));
            },
            Literal::True | Literal::False => {
                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::MonoTemplate(InferenceRuleTemplate::new_forced(&BOOL_TEMPLATE));
            },
            Literal::Character(_) => {
                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::MonoTemplate(InferenceRuleTemplate::new_forced(&CHARACTER_TEMPLATE));
            },
            Literal::String(_) => {
                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::MonoTemplate(InferenceRuleTemplate::new_forced(&STRING_TEMPLATE));
            },
            Literal::Struct(literal) => {
                // Visit field expressions
                let mut expr_ids = self.expr_buffer.start_section();
                for field in &literal.fields {
                    expr_ids.push(field.value);
                }

                let mut expr_indices = self.index_buffer.start_section();
                for expr_id in expr_ids.iter_copied() {
                    let expr_index = self.visit_expr(ctx, expr_id)?;
                    expr_indices.push(expr_index);
                }
                expr_ids.forget();
                let element_indices = expr_indices.into_vec();

                // Assign rule and extra data index to inference node
                let poly_data_index = self.insert_initial_struct_polymorph_data(ctx, id);
                let node = &mut self.infer_nodes[self_index];
                node.poly_data_index = poly_data_index;
                node.inference_rule = InferenceRule::LiteralStruct(InferenceRuleLiteralStruct{
                    element_indices,
                });
            },
            Literal::Enum(_) => {
                // Enumerations do not carry any subexpressions, but may still
                // have a user-defined polymorphic marker variable. For this 
                // reason we may still have to apply inference to this 
                // polymorphic variable
                let poly_data_index = self.insert_initial_enum_polymorph_data(ctx, id);
                let node = &mut self.infer_nodes[self_index];
                node.poly_data_index = poly_data_index;
                node.inference_rule = InferenceRule::LiteralEnum;
            },
            Literal::Union(literal) => {
                // May carry subexpressions and polymorphic arguments
                let expr_ids = self.expr_buffer.start_section_initialized(literal.values.as_slice());
                let poly_data_index = self.insert_initial_union_polymorph_data(ctx, id);

                let mut expr_indices = self.index_buffer.start_section();
                for expr_id in expr_ids.iter_copied() {
                    let expr_index = self.visit_expr(ctx, expr_id)?;
                    expr_indices.push(expr_index);
                }
                expr_ids.forget();
                let element_indices = expr_indices.into_vec();

                let node = &mut self.infer_nodes[self_index];
                node.poly_data_index = poly_data_index;
                node.inference_rule = InferenceRule::LiteralUnion(InferenceRuleLiteralUnion{
                    element_indices,
                });
            },
            Literal::Array(expressions) => {
                let expr_ids = self.expr_buffer.start_section_initialized(expressions.as_slice());

                let mut expr_indices = self.index_buffer.start_section();
                for expr_id in expr_ids.iter_copied() {
                    let expr_index = self.visit_expr(ctx, expr_id)?;
                    expr_indices.push(expr_index);
                }
                expr_ids.forget();
                let element_indices = expr_indices.into_vec();

                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::LiteralArray(InferenceRuleLiteralArray{
                    element_indices,
                });
            },
            Literal::Tuple(expressions) => {
                let expr_ids = self.expr_buffer.start_section_initialized(expressions.as_slice());

                let mut expr_indices = self.index_buffer.start_section();
                for expr_id in expr_ids.iter_copied() {
                    let expr_index = self.visit_expr(ctx, expr_id)?;
                    expr_indices.push(expr_index);
                }
                expr_ids.forget();
                let element_indices = expr_indices.into_vec();

                let node = &mut self.infer_nodes[self_index];
                node.inference_rule = InferenceRule::LiteralTuple(InferenceRuleLiteralTuple{
                    element_indices,
                })
            }
        }

        self.parent_index = old_parent;
        self.progress_inference_rule(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_cast_expr(&mut self, ctx: &mut Ctx, id: CastExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let cast_expr = &ctx.heap[id];
        let subject_expr_id = cast_expr.subject;

        let old_parent = self.parent_index.replace(self_index);
        let subject_index = self.visit_expr(ctx, subject_expr_id)?;

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::CastExpr(InferenceRuleCastExpr{
            subject_index,
        });

        self.parent_index = old_parent;

        // The cast expression is a bit special at this point: the progression
        // function simply makes sure input/output types are compatible. But if
        // the programmer explicitly specified the output type, then we can
        // already perform that inference rule here.
        {
            let cast_expr = &ctx.heap[id];
            let specified_type = self.determine_inference_type_from_parser_type_elements(&cast_expr.to_type.elements, true);
            let _progress = self.apply_template_constraint(ctx, self_index, &specified_type.parts)?;
        }

        self.progress_inference_rule_cast_expr(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_call_expr(&mut self, ctx: &mut Ctx, id: CallExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;
        let extra_index = self.insert_initial_call_polymorph_data(ctx, id);

        // By default we set the polymorph idx for calls to 0. If the call
        // refers to a non-polymorphic function, then it will be "monomorphed"
        // once, hence we end up pointing to the correct instance.
        self.infer_nodes[self_index].field_index = 0;

        // Visit all arguments
        let old_parent = self.parent_index.replace(self_index);

        let call_expr = &ctx.heap[id];
        let expr_ids = self.expr_buffer.start_section_initialized(call_expr.arguments.as_slice());
        let mut expr_indices = self.index_buffer.start_section();

        for arg_expr_id in expr_ids.iter_copied() {
            let expr_index = self.visit_expr(ctx, arg_expr_id)?;
            expr_indices.push(expr_index);
        }
        expr_ids.forget();
        let argument_indices = expr_indices.into_vec();

        let node = &mut self.infer_nodes[self_index];
        node.poly_data_index = extra_index;
        node.inference_rule = InferenceRule::CallExpr(InferenceRuleCallExpr{
            argument_indices,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_call_expr(ctx, self_index)?;
        return Ok(self_index);
    }

    fn visit_variable_expr(&mut self, ctx: &mut Ctx, id: VariableExpressionId) -> VisitExprResult {
        let upcast_id = id.upcast();
        let self_index = self.insert_initial_inference_node(ctx, upcast_id)?;

        let var_expr = &ctx.heap[id];
        debug_assert!(var_expr.declaration.is_some());
        let old_parent = self.parent_index.replace(self_index);

        let declaration = &ctx.heap[var_expr.declaration.unwrap()];
        let mut var_data_index = None;
        for (index, var_data) in self.var_data.iter().enumerate() {
            if var_data.var_id == declaration.this {
                var_data_index = Some(index);
                break;
            }
        }

        let var_data_index = if let Some(var_data_index) = var_data_index {
            let var_data = &mut self.var_data[var_data_index];
            var_data.used_at.push(self_index);

            var_data_index
        } else {
            // If we're in a binding expression then it might the first time we
            // encounter the variable, so add a `VarData` entry.
            debug_assert_eq!(declaration.kind, VariableKind::Binding);
            let var_type = self.determine_inference_type_from_parser_type_elements(
                &declaration.parser_type.elements, true
            );
            let var_data_index = self.var_data.len();
            self.var_data.push(VarData{
                var_id: declaration.this,
                var_type,
                used_at: vec![self_index],
                linked_var: None,
            });

            var_data_index
        };

        let node = &mut self.infer_nodes[self_index];
        node.inference_rule = InferenceRule::VariableExpr(InferenceRuleVariableExpr{
            var_data_index,
        });

        self.parent_index = old_parent;
        self.progress_inference_rule_variable_expr(ctx, self_index)?;
        return Ok(self_index);
    }
}

// -----------------------------------------------------------------------------
// PassTyping - Type-inference progression
// -----------------------------------------------------------------------------

impl PassTyping {
    #[allow(dead_code)] // used when debug flag at the top of this file is true.
    fn debug_get_display_name(&self, ctx: &Ctx, node_index: InferNodeIndex) -> String {
        let expr_type = &self.infer_nodes[node_index].expr_type;
        expr_type.display_name(&ctx.heap)
    }

    fn resolve_types(&mut self, ctx: &mut Ctx, queue: &mut ResolveQueue) -> Result<(), ParseError> {
        // Keep inferring until we can no longer make any progress
        while !self.node_queued.is_empty() {
            while !self.node_queued.is_empty() {
                let node_index = self.node_queued.pop_front().unwrap();
                self.progress_inference_rule(ctx, node_index)?;
            }

            // Nothing is queued anymore. However we might have integer literals
            // whose type cannot be inferred. For convenience's sake we'll
            // infer these to be s32.
            for (infer_node_index, infer_node) in self.infer_nodes.iter_mut().enumerate() {
                let expr_type = &mut infer_node.expr_type;
                if !expr_type.is_done && expr_type.parts.len() == 1 && expr_type.parts[0] == InferenceTypePart::IntegerLike {
                    // Force integer type to s32
                    expr_type.parts[0] = InferenceTypePart::SInt32;
                    expr_type.is_done = true;

                    // Requeue expression (and its parent, if it exists)
                    self.node_queued.push_back(infer_node_index);
                    if let Some(node_parent_index) = infer_node.parent_index {
                        self.node_queued.push_back(node_parent_index);
                    }
                }
            }
        }

        // Helper for transferring polymorphic variables to concrete types and
        // checking if they're completely specified
        fn poly_data_type_to_concrete_type(
            ctx: &Ctx, expr_id: ExpressionId, inference_poly_args: &Vec<InferenceType>,
            first_concrete_part: ConcreteTypePart,
        ) -> Result<ConcreteType, ParseError> {
            // Prepare storage vector
            let mut num_inference_parts = 0;
            for inference_type in inference_poly_args {
                num_inference_parts += inference_type.parts.len();
            }

            let mut concrete_type = ConcreteType{
                parts: Vec::with_capacity(1 + num_inference_parts),
            };
            concrete_type.parts.push(first_concrete_part);

            // Go through all polymorphic arguments and add them to the concrete
            // types.
            for (poly_idx, poly_type) in inference_poly_args.iter().enumerate() {
                if !poly_type.is_done {
                    let expr = &ctx.heap[expr_id];
                    let definition = match expr {
                        Expression::Call(expr) => expr.procedure.upcast(),
                        Expression::Literal(expr) => match &expr.value {
                            Literal::Enum(lit) => lit.definition,
                            Literal::Union(lit) => lit.definition,
                            Literal::Struct(lit) => lit.definition,
                            _ => unreachable!()
                        },
                        _ => unreachable!(),
                    };
                    let poly_vars = ctx.heap[definition].poly_vars();
                    return Err(ParseError::new_error_at_span(
                        &ctx.module().source, expr.operation_span(), format!(
                            "could not fully infer the type of polymorphic variable '{}' of this expression (got '{}')",
                            poly_vars[poly_idx].value.as_str(), poly_type.display_name(&ctx.heap)
                        )
                    ));
                }

                poly_type.write_concrete_type(&mut concrete_type);
            }

            Ok(concrete_type)
        }

        // Every expression checked, and new monomorphs are queued. Transfer the
        // expression information to the AST. If this is the first time we're
        // visiting this procedure then we assign expression indices as well.
        let procedure = &ctx.heap[self.procedure_id];
        let num_infer_nodes = self.infer_nodes.len();
        let mut monomorph = ProcedureDefinitionMonomorph{
            argument_types: Vec::with_capacity(procedure.parameters.len()),
            expr_info: Vec::with_capacity(num_infer_nodes),
        };

        // For all of the expressions look up the TypeId (or create a new one).
        // For function calls and component instantiations figure out if they
        // need to be typechecked
        for infer_node in self.infer_nodes.iter_mut() {
            // Determine type ID
            let expr = &ctx.heap[infer_node.expr_id];

            // TODO: Maybe optimize? Split insertion up into lookup, then clone
            //  if needed?
            let mut concrete_type = ConcreteType::default();
            infer_node.expr_type.write_concrete_type(&mut concrete_type);
            let info_type_id = ctx.types.add_monomorphed_type(ctx.modules, ctx.heap, ctx.arch, concrete_type)?;

            // Determine procedure type ID, i.e. a called/instantiated
            // procedure's signature.
            let info_variant = if let Expression::Call(expr) = expr {
                // Construct full function type. If not yet typechecked then
                // queue it for typechecking.
                let poly_data = &self.poly_data[infer_node.poly_data_index as usize];
                debug_assert!(expr.method.is_user_defined() || expr.method.is_public_builtin());
                let procedure_id = expr.procedure;
                let num_poly_vars = poly_data.poly_vars.len() as u32;

                let first_part = match expr.method {
                    Method::UserFunction => ConcreteTypePart::Function(procedure_id, num_poly_vars),
                    Method::UserComponent => ConcreteTypePart::Component(procedure_id, num_poly_vars),
                    _ => ConcreteTypePart::Function(procedure_id, num_poly_vars),
                };


                let definition_id = procedure_id.upcast();
                let signature_type = poly_data_type_to_concrete_type(
                    ctx, infer_node.expr_id, &poly_data.poly_vars, first_part
                )?;

                let (type_id, monomorph_index) = if let Some(type_id) = ctx.types.get_procedure_monomorph_type_id(&definition_id, &signature_type.parts) {
                    // Procedure is already typechecked
                    let monomorph_index = ctx.types.get_monomorph(type_id).variant.as_procedure().monomorph_index;
                    (type_id, monomorph_index)
                } else {
                    // Procedure is not yet typechecked, reserve a TypeID and a monomorph index
                    let procedure_to_check = &mut ctx.heap[procedure_id];
                    let monomorph_index = procedure_to_check.monomorphs.len() as u32;
                    procedure_to_check.monomorphs.push(ProcedureDefinitionMonomorph::new_invalid());
                    let type_id = ctx.types.reserve_procedure_monomorph_type_id(&definition_id, signature_type, monomorph_index);

                    if !procedure_to_check.source.is_builtin() {
                        // Only perform typechecking on the user-defined
                        // procedures
                        queue.push_back(ResolveQueueElement{
                            root_id: ctx.heap[definition_id].defined_in(),
                            definition_id,
                            reserved_type_id: type_id,
                            reserved_monomorph_index: monomorph_index,
                        });
                    }

                    (type_id, monomorph_index)
                };

                ExpressionInfoVariant::Procedure(type_id, monomorph_index)
            } else if let Expression::Select(_expr) = expr {
                ExpressionInfoVariant::Select(infer_node.field_index)
            } else {
                ExpressionInfoVariant::Generic
            };

            infer_node.info_type_id = info_type_id;
            infer_node.info_variant = info_variant;
        }

        // Write the types of the arguments
        let procedure = &ctx.heap[self.procedure_id];
        for parameter_id in procedure.parameters.iter().copied() {
            let mut concrete = ConcreteType::default();
            let var_data = self.var_data.iter().find(|v| v.var_id == parameter_id).unwrap();
            var_data.var_type.write_concrete_type(&mut concrete);
            let type_id = ctx.types.add_monomorphed_type(ctx.modules, ctx.heap, ctx.arch, concrete)?;
            monomorph.argument_types.push(type_id)
        }

        // Determine if we have already assigned type indices to the expressions
        // before (the indices that, for a monomorph, can retrieve the type of
        // the expression).
        let has_type_indices = self.reserved_monomorph_index > 0;
        if has_type_indices {
            // already have indices, so resize and then index into it
            debug_assert!(monomorph.expr_info.is_empty());
            monomorph.expr_info.resize(num_infer_nodes, ExpressionInfo::new_invalid());
            for infer_node in self.infer_nodes.iter() {
                let type_index = ctx.heap[infer_node.expr_id].type_index();
                monomorph.expr_info[type_index as usize] = infer_node.as_expression_info();
            }
        } else {
            // no indices yet, need to be assigned in AST
            for infer_node in self.infer_nodes.iter() {
                let type_index = monomorph.expr_info.len();
                monomorph.expr_info.push(infer_node.as_expression_info());
                *ctx.heap[infer_node.expr_id].type_index_mut() = type_index as i32;
            }
        }

        // Push the information into the AST
        let procedure = &mut ctx.heap[self.procedure_id];
        procedure.monomorphs[self.reserved_monomorph_index as usize] = monomorph;

        Ok(())
    }

    fn progress_inference_rule(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        use InferenceRule as IR;

        let node = &self.infer_nodes[node_index];
        match &node.inference_rule {
            IR::Noop =>
                unreachable!(),
            IR::MonoTemplate(_) =>
                self.progress_inference_rule_mono_template(ctx, node_index),
            IR::BiEqual(_) =>
                self.progress_inference_rule_bi_equal(ctx, node_index),
            IR::TriEqualArgs(_) =>
                self.progress_inference_rule_tri_equal_args(ctx, node_index),
            IR::TriEqualAll(_) =>
                self.progress_inference_rule_tri_equal_all(ctx, node_index),
            IR::Concatenate(_) =>
                self.progress_inference_rule_concatenate(ctx, node_index),
            IR::IndexingExpr(_) =>
                self.progress_inference_rule_indexing_expr(ctx, node_index),
            IR::SlicingExpr(_) =>
                self.progress_inference_rule_slicing_expr(ctx, node_index),
            IR::SelectStructField(_) =>
                self.progress_inference_rule_select_struct_field(ctx, node_index),
            IR::SelectTupleMember(_) =>
                self.progress_inference_rule_select_tuple_member(ctx, node_index),
            IR::LiteralStruct(_) =>
                self.progress_inference_rule_literal_struct(ctx, node_index),
            IR::LiteralEnum =>
                self.progress_inference_rule_literal_enum(ctx, node_index),
            IR::LiteralUnion(_) =>
                self.progress_inference_rule_literal_union(ctx, node_index),
            IR::LiteralArray(_) =>
                self.progress_inference_rule_literal_array(ctx, node_index),
            IR::LiteralTuple(_) =>
                self.progress_inference_rule_literal_tuple(ctx, node_index),
            IR::CastExpr(_) =>
                self.progress_inference_rule_cast_expr(ctx, node_index),
            IR::CallExpr(_) =>
                self.progress_inference_rule_call_expr(ctx, node_index),
            IR::VariableExpr(_) =>
                self.progress_inference_rule_variable_expr(ctx, node_index),
        }
    }

    fn progress_inference_rule_mono_template(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = *node.inference_rule.as_mono_template();

        let progress = self.progress_template(ctx, node_index, rule.application, rule.template)?;
        if progress { self.queue_node_parent(node_index); }

        return Ok(());
    }

    fn progress_inference_rule_bi_equal(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_bi_equal();
        let template = rule.template;
        let arg_index = rule.argument_index;

        let base_progress = self.progress_template(ctx, node_index, template.application, template.template)?;
        let (node_progress, arg_progress) = self.apply_equal2_constraint(ctx, node_index, node_index, 0, arg_index, 0)?;

        if base_progress || node_progress { self.queue_node_parent(node_index); }
        if arg_progress { self.queue_node(arg_index); }

        return Ok(())
    }

    fn progress_inference_rule_tri_equal_args(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_tri_equal_args();

        let result_template = rule.result_template;
        let argument_template = rule.argument_template;
        let arg1_index = rule.argument1_index;
        let arg2_index = rule.argument2_index;

        let self_template_progress = self.progress_template(ctx, node_index, result_template.application, result_template.template)?;
        let arg1_template_progress = self.progress_template(ctx, arg1_index, argument_template.application, argument_template.template)?;
        let (arg1_progress, arg2_progress) = self.apply_equal2_constraint(ctx, node_index, arg1_index, 0, arg2_index, 0)?;

        if self_template_progress { self.queue_node_parent(node_index); }
        if arg1_template_progress || arg1_progress { self.queue_node(arg1_index); }
        if arg2_progress { self.queue_node(arg2_index); }

        return Ok(());
    }

    fn progress_inference_rule_tri_equal_all(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_tri_equal_all();

        let template = rule.template;
        let arg1_index = rule.argument1_index;
        let arg2_index = rule.argument2_index;

        let template_progress = self.progress_template(ctx, node_index, template.application, template.template)?;
        let (node_progress, arg1_progress, arg2_progress) =
            self.apply_equal3_constraint(ctx, node_index, arg1_index, arg2_index, 0)?;

        if template_progress || node_progress { self.queue_node_parent(node_index); }
        if arg1_progress { self.queue_node(arg1_index); }
        if arg2_progress { self.queue_node(arg2_index); }

        return Ok(());
    }

    fn progress_inference_rule_concatenate(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_concatenate();
        let arg1_index = rule.argument1_index;
        let arg2_index = rule.argument2_index;

        // Two cases: one of the arguments is a string (then all must be), or
        // one of the arguments is an array (and all must be arrays).
        let (expr_is_str, expr_is_not_str) = self.type_is_certainly_or_certainly_not_string(node_index);
        let (arg1_is_str, arg1_is_not_str) = self.type_is_certainly_or_certainly_not_string(arg1_index);
        let (arg2_is_str, arg2_is_not_str) = self.type_is_certainly_or_certainly_not_string(arg2_index);

        let someone_is_str = expr_is_str || arg1_is_str || arg2_is_str;
        let someone_is_not_str = expr_is_not_str || arg1_is_not_str || arg2_is_not_str;
        // Note: this statement is an expression returning the progression bools
        let (node_progress, arg1_progress, arg2_progress) = if someone_is_str {
            // One of the arguments is a string, then all must be strings
            self.apply_equal3_constraint(ctx, node_index, arg1_index, arg2_index, 0)?
        } else {
            let progress_expr = if someone_is_not_str {
                // Output must be a normal array
                self.apply_template_constraint(ctx, node_index, &ARRAY_TEMPLATE)?
            } else {
                // Output may still be anything
                self.apply_template_constraint(ctx, node_index, &ARRAYLIKE_TEMPLATE)?
            };

            let progress_arg1 = self.apply_template_constraint(ctx, arg1_index, &ARRAYLIKE_TEMPLATE)?;
            let progress_arg2 = self.apply_template_constraint(ctx, arg2_index, &ARRAYLIKE_TEMPLATE)?;

            // If they're all arraylike, then we want the subtype to match
            let (subtype_expr, subtype_arg1, subtype_arg2) =
                self.apply_equal3_constraint(ctx, node_index, arg1_index, arg2_index, 1)?;

            (progress_expr || subtype_expr, progress_arg1 || subtype_arg1, progress_arg2 || subtype_arg2)
        };

        if node_progress { self.queue_node_parent(node_index); }
        if arg1_progress { self.queue_node(arg1_index); }
        if arg2_progress { self.queue_node(arg2_index); }

        return Ok(())
    }

    fn progress_inference_rule_indexing_expr(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_indexing_expr();
        let subject_index = rule.subject_index;
        let index_index = rule.index_index; // which one?

        // Subject is arraylike, index in integerlike
        let subject_template_progress = self.apply_template_constraint(ctx, subject_index, &ARRAYLIKE_TEMPLATE)?;
        let index_template_progress = self.apply_template_constraint(ctx, index_index, &INTEGERLIKE_TEMPLATE)?;

        // If subject is type `Array<T>`, then expr type is `T`
        let (node_progress, subject_progress) =
            self.apply_equal2_constraint(ctx, node_index, node_index, 0, subject_index, 1)?;

        if node_progress { self.queue_node_parent(node_index); }
        if subject_template_progress || subject_progress { self.queue_node(subject_index); }
        if index_template_progress { self.queue_node(index_index); }

        return Ok(());
    }

    fn progress_inference_rule_slicing_expr(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_slicing_expr();
        let subject_index = rule.subject_index;
        let from_index_index = rule.from_index;
        let to_index_index = rule.to_index;

        debug_log!("Rule slicing [node: {}, expr: {}]", node_index, node.expr_id.index);

        // Subject is arraylike, indices are integerlike
        let subject_template_progress = self.apply_template_constraint(ctx, subject_index, &ARRAYLIKE_TEMPLATE)?;
        let from_template_progress = self.apply_template_constraint(ctx, from_index_index, &INTEGERLIKE_TEMPLATE)?;
        let to_template_progress = self.apply_template_constraint(ctx, to_index_index, &INTEGERLIKE_TEMPLATE)?;
        let (from_index_progress, to_index_progress) =
            self.apply_equal2_constraint(ctx, node_index, from_index_index, 0, to_index_index, 0)?;

        // Same as array indexing: result depends on whether subject is string
        // or array
        let (is_string, is_not_string) = self.type_is_certainly_or_certainly_not_string(node_index);
        let (node_progress, subject_progress) = if is_string {
            // Certainly a string
            (
                self.apply_forced_constraint(ctx, node_index, &STRING_TEMPLATE)?,
                false
            )
        } else if is_not_string {
            // Certainly not a string, apply template constraint. Then make sure
            // that if we have an `Array<T>`, that the slice produces `Slice<T>`
            let node_template_progress = self.apply_template_constraint(ctx, node_index, &SLICE_TEMPLATE)?;
            let (node_progress, subject_progress) =
                self.apply_equal2_constraint(ctx, node_index, node_index, 1, subject_index, 1)?;

            (
                node_template_progress || node_progress,
                subject_progress
            )
        } else {
            // Not sure yet
            let node_template_progress = self.apply_template_constraint(ctx, node_index, &ARRAYLIKE_TEMPLATE)?;
            let (node_progress, subject_progress) =
                self.apply_equal2_constraint(ctx, node_index, node_index, 1, subject_index, 1)?;

            (
                node_template_progress || node_progress,
                subject_progress
            )
        };

        if node_progress { self.queue_node_parent(node_index); }
        if subject_template_progress || subject_progress { self.queue_node(subject_index); }
        if from_template_progress || from_index_progress { self.queue_node(from_index_index); }
        if to_template_progress || to_index_progress { self.queue_node(to_index_index); }

        return Ok(());
    }

    fn progress_inference_rule_select_struct_field(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_select_struct_field();

        let subject_index = rule.subject_index;
        let selected_field = rule.selected_field.clone();

        fn get_definition_id_from_inference_type(inference_type: &InferenceType) -> Result<Option<DefinitionId>, ()> {
            for part in inference_type.parts.iter() {
                if part.is_marker() { continue; }
                if !part.is_concrete() { break; }

                if let InferenceTypePart::Instance(definition_id, _) = part {
                    return Ok(Some(*definition_id));
                } else {
                    return Err(())
                }
            }

            // Nothing is known yet
            return Ok(None);
        }

        if node.field_index < 0 {
            // Don't know the subject definition, hence the field yet. Try to
            // determine it.
            let subject_node = &self.infer_nodes[subject_index];
            match get_definition_id_from_inference_type(&subject_node.expr_type) {
                Ok(Some(definition_id)) => {
                    // Determined definition of subject for the first time.
                    let base_definition = ctx.types.get_base_definition(&definition_id).unwrap();
                    let struct_definition = if let DefinedTypeVariant::Struct(struct_definition) = &base_definition.definition {
                        struct_definition
                    } else {
                        return Err(ParseError::new_error_at_span(
                            &ctx.module().source, selected_field.span, format!(
                                "Can only apply field access to structs, got a subject of type '{}'",
                                subject_node.expr_type.display_name(&ctx.heap)
                            )
                        ));
                    };

                    // Seek the field that is referenced by the select
                    // expression
                    let mut field_found = false;
                    for (field_index, field) in struct_definition.fields.iter().enumerate() {
                        if field.identifier.value == selected_field.value {
                            // Found the field of interest
                            field_found = true;
                            let node = &mut self.infer_nodes[node_index];
                            node.field_index = field_index as i32;
                            break;
                        }
                    }

                    if !field_found {
                        let struct_definition = ctx.heap[definition_id].as_struct();
                        return Err(ParseError::new_error_at_span(
                            &ctx.module().source, selected_field.span, format!(
                                "this field does not exist on the struct '{}'",
                                struct_definition.identifier.value.as_str()
                            )
                        ));
                    }

                    // Insert the initial data needed to infer polymorphic
                    // fields
                    let extra_index = self.insert_initial_select_polymorph_data(ctx, node_index, definition_id);
                    let node = &mut self.infer_nodes[node_index];
                    node.poly_data_index = extra_index;
                },
                Ok(None) => {
                    // We don't know what to do yet, because we don't know the
                    // subject type yet.
                    return Ok(())
                },
                Err(()) => {
                    return Err(ParseError::new_error_at_span(
                        &ctx.module().source, rule.selected_field.span, format!(
                            "Can only apply field access to structs, got a subject of type '{}'",
                            subject_node.expr_type.display_name(&ctx.heap)
                        )
                    ));
                },
            }
        }

        // If here then the field index is known, hence we can start inferring
        // the type of the selected field
        let field_expr_id = self.infer_nodes[node_index].expr_id;
        let subject_expr_id = self.infer_nodes[subject_index].expr_id;
        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        let (_, progress_subject_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, subject_expr_id, "selected struct's",
            PolyDataTypeIndex::Associated(0), 0, subject_index, 0, &mut poly_progress_section
        )?;
        let (_, progress_field_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, field_expr_id, "selected field's",
            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section
        )?;

        // Maybe make progress on types due to inferred polymorphic variables
        let progress_subject_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Associated(0), subject_index, &poly_progress_section
        );
        let progress_field_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Returned, node_index, &poly_progress_section
        );

        if progress_subject_1 || progress_subject_2 { self.queue_node(subject_index); }
        if progress_field_1 || progress_field_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();
        self.finish_polydata_constraint(node_index);
        return Ok(())
    }

    fn progress_inference_rule_select_tuple_member(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_select_tuple_member();
        let subject_index = rule.subject_index;
        let tuple_member_index = rule.selected_index;

        if node.field_index < 0 {
            let subject_type = &self.infer_nodes[subject_index].expr_type;
            let tuple_size = get_tuple_size_from_inference_type(subject_type);
            let tuple_size = match tuple_size {
                Ok(Some(tuple_size)) => {
                    tuple_size
                },
                Ok(None) => {
                    // We can't infer anything yet
                    return Ok(())
                },
                Err(()) => {
                    let select_expr_span = ctx.heap[node.expr_id].full_span();
                    return Err(ParseError::new_error_at_span(
                        &ctx.module().source, select_expr_span, format!(
                            "tuple element select cannot be applied to a subject of type '{}'",
                            subject_type.display_name(&ctx.heap)
                        )
                    ));
                }
            };

            // If here then we at least have the tuple size. Now check if the
            // index doesn't exceed that size.
            if tuple_member_index >= tuple_size as u64 {
                let select_expr_span = ctx.heap[node.expr_id].full_span();
                return Err(ParseError::new_error_at_span(
                    &ctx.module().source, select_expr_span, format!(
                        "element index {} is out of bounds, tuple has {} elements",
                        tuple_member_index, tuple_size
                    )
                ));
            }

            // Within bounds, set index on the type inference node
            let node = &mut self.infer_nodes[node_index];
            node.field_index = tuple_member_index as i32;
        }

        // If here then we know we can use `tuple_member_index`. We need to keep
        // computing the offset to the subtype, as its value changes during
        // inference
        let subject_type = &self.infer_nodes[subject_index].expr_type;
        let mut selected_member_start_index = 1; // start just after the InferenceTypeElement::Tuple
        for _ in 0..tuple_member_index {
            selected_member_start_index = InferenceType::find_subtree_end_idx(&subject_type.parts, selected_member_start_index);
        }

        let (progress_member, progress_subject) = self.apply_equal2_constraint(
            ctx, node_index, node_index, 0, subject_index, selected_member_start_index
        )?;

        if progress_member { self.queue_node_parent(node_index); }
        if progress_subject { self.queue_node(subject_index); }

        return Ok(());
    }

    fn progress_inference_rule_literal_struct(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let node_expr_id = node.expr_id;
        let rule = node.inference_rule.as_literal_struct();

        // For each of the fields in the literal struct, apply the type equality
        // constraint. If the literal is polymorphic, then we try to progress
        // their types during this process
        let element_indices_section = self.index_buffer.start_section_initialized(&rule.element_indices);
        let mut poly_progress_section = self.poly_progress_buffer.start_section();
        for (field_index, field_node_index) in element_indices_section.iter_copied().enumerate() {
            let field_expr_id = self.infer_nodes[field_node_index].expr_id;
            let (_, progress_field) = self.apply_polydata_equal2_constraint(
                ctx, node_index, field_expr_id, "struct field's",
                PolyDataTypeIndex::Associated(field_index), 0,
                field_node_index, 0, &mut poly_progress_section
            )?;

            if progress_field { self.queue_node(field_node_index); }
        }

        // Now we do the same thing for the struct literal expression (the type
        // of the struct itself).
        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, node_expr_id, "struct literal's",
            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section
        )?;

        // And the other way around: if any of our polymorphic variables are
        // more specific then they were before, then we forward that information
        // back to our struct/fields.
        for (field_index, field_node_index) in element_indices_section.iter_copied().enumerate() {
            let progress_field = self.apply_polydata_polyvar_constraint(
                ctx, node_index, PolyDataTypeIndex::Associated(field_index),
                field_node_index, &poly_progress_section
            );

            if progress_field { self.queue_node(field_node_index); }
        }

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Returned,
            node_index, &poly_progress_section
        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();
        element_indices_section.forget();

        self.finish_polydata_constraint(node_index);
        return Ok(())
    }

    fn progress_inference_rule_literal_enum(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let node_expr_id = node.expr_id;
        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        // An enum literal type is simply, well, the enum's type. However, it
        // might still have polymorphic variables, hence the use of `PolyData`.
        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, node_expr_id, "enum literal's",
            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section
        )?;

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Returned, node_index, &poly_progress_section
        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();
        self.finish_polydata_constraint(node_index);
        return Ok(());
    }

    fn progress_inference_rule_literal_union(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let node_expr_id = node.expr_id;
        let rule = node.inference_rule.as_literal_union();

        // Infer type of any embedded values in the union variant. At the same
        // time progress the polymorphic variables associated with the union.
        let element_indices_section = self.index_buffer.start_section_initialized(&rule.element_indices);
        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        for (embedded_index, embedded_node_index) in element_indices_section.iter_copied().enumerate() {
            let embedded_node_expr_id = self.infer_nodes[embedded_node_index].expr_id;
            let (_, progress_embedded) = self.apply_polydata_equal2_constraint(
                ctx, node_index, embedded_node_expr_id, "embedded value's",
                PolyDataTypeIndex::Associated(embedded_index), 0,
                embedded_node_index, 0, &mut poly_progress_section
            )?;

            if progress_embedded { self.queue_node(embedded_node_index); }
        }

        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, node_expr_id, "union's",
            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section
        )?;

        // Propagate progress in the polymorphic variables to the expressions
        // that constitute the union literal.
        for (embedded_index, embedded_node_index) in element_indices_section.iter_copied().enumerate() {
            let progress_embedded = self.apply_polydata_polyvar_constraint(
                ctx, node_index, PolyDataTypeIndex::Associated(embedded_index),
                embedded_node_index, &poly_progress_section
            );

            if progress_embedded { self.queue_node(embedded_node_index); }
        }

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Returned, node_index, &poly_progress_section
        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();
        self.finish_polydata_constraint(node_index);
        return Ok(());
    }

    fn progress_inference_rule_literal_array(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_literal_array();

        // Apply equality rule to all of the elements that form the array
        let argument_node_indices = self.index_buffer.start_section_initialized(&rule.element_indices);
        let mut argument_progress_section = self.bool_buffer.start_section();
        self.apply_equal_n_constraint(ctx, node_index, &argument_node_indices, &mut argument_progress_section)?;

        debug_assert_eq!(argument_node_indices.len(), argument_progress_section.len());
        for argument_index in 0..argument_node_indices.len() {
            let argument_node_index = argument_node_indices[argument_index];
            let progress = argument_progress_section[argument_index];

            if progress { self.queue_node(argument_node_index); }
        }

        // If elements are of type `T`, then the array is of type `Array<T>`, so:
        let mut progress_literal = self.apply_template_constraint(ctx, node_index, &ARRAY_TEMPLATE)?;
        if argument_node_indices.len() != 0 {
            let argument_node_index = argument_node_indices[0];
            let (progress_literal_inner, progress_argument) = self.apply_equal2_constraint(
                ctx, node_index, node_index, 1, argument_node_index, 0
            )?;

            progress_literal = progress_literal || progress_literal_inner;

            // It is possible that the `Array<T>` has a more progress `T` then
            // the arguments. So in the case we progress our argument type we
            // simply queue this rule again
            if progress_argument { self.queue_node(node_index); }
        }

        argument_node_indices.forget();
        argument_progress_section.forget();

        if progress_literal { self.queue_node_parent(node_index); }
        return Ok(());
    }

    fn progress_inference_rule_literal_tuple(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_literal_tuple();

        let element_indices = self.index_buffer.start_section_initialized(&rule.element_indices);

        // Check if we need to apply the initial tuple template type. Note that
        // this is a hacky check.
        let num_tuple_elements = rule.element_indices.len();
        let mut template_type = Vec::with_capacity(num_tuple_elements + 1); // TODO: @performance
        template_type.push(InferenceTypePart::Tuple(num_tuple_elements as u32));
        for _ in 0..num_tuple_elements {
            template_type.push(InferenceTypePart::Unknown);
        }

        let mut progress_literal = self.apply_template_constraint(ctx, node_index, &template_type)?;

        // Because of the (early returning error) check above, we're certain
        // that the tuple has the correct number of elements. Now match each
        // element expression type to the tuple subtype.
        let mut element_subtree_start_index = 1; // first element is InferenceTypePart::Tuple
        for element_node_index in element_indices.iter_copied() {
            let (progress_literal_element, progress_element) = self.apply_equal2_constraint(
                ctx, node_index, node_index, element_subtree_start_index, element_node_index, 0
            )?;

            progress_literal = progress_literal || progress_literal_element;
            if progress_element {
                self.queue_node(element_node_index);
            }

            // Prepare for next element
            let node = &self.infer_nodes[node_index];
            let subtree_end_index = InferenceType::find_subtree_end_idx(&node.expr_type.parts, element_subtree_start_index);
            element_subtree_start_index = subtree_end_index;
        }
        debug_assert_eq!(element_subtree_start_index, self.infer_nodes[node_index].expr_type.parts.len());

        if progress_literal { self.queue_node_parent(node_index); }

        element_indices.forget();
        return Ok(());
    }

    fn progress_inference_rule_cast_expr(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_cast_expr();
        let subject_index = rule.subject_index;
        let subject = &self.infer_nodes[subject_index];

        // Make sure that both types are completely done. Note: a cast
        // expression cannot really infer anything between the subject and the
        // output type, we can only make sure that, at the end, the cast is
        // correct.
        if !node.expr_type.is_done || !subject.expr_type.is_done {
            return Ok(());
        }

        // Both types are known, currently the only valid casts are bool,
        // integer and character casts.
        fn is_bool_int_or_char(parts: &[InferenceTypePart]) -> bool {
            let mut index = 0;
            while index < parts.len() {
                let part = &parts[index];
                if !part.is_marker() { break; }
                index += 1;
            }

            debug_assert!(index != parts.len());
            let part = &parts[index];
            if *part == InferenceTypePart::Bool || *part == InferenceTypePart::Character || part.is_concrete_integer() {
                debug_assert!(index + 1 == parts.len()); // type is done, first part does not have children -> must be at end
                return true;
            } else {
                return false;
            }
        }

        let is_valid = if is_bool_int_or_char(&node.expr_type.parts) && is_bool_int_or_char(&subject.expr_type.parts) {
            true
        } else if InferenceType::check_subtrees(&node.expr_type.parts, 0, &subject.expr_type.parts, 0) {
            // again: check_subtrees is sufficient since both types are done
            true
        } else {
            false
        };

        if !is_valid {
            let cast_expr = &ctx.heap[node.expr_id];
            let subject_expr = &ctx.heap[subject.expr_id];
            return Err(ParseError::new_error_str_at_span(
                &ctx.module().source, cast_expr.full_span(), "invalid casting operation"
            ).with_info_at_span(
                &ctx.module().source, subject_expr.full_span(), format!(
                    "cannot cast the argument type '{}' to the type '{}'",
                    subject.expr_type.display_name(&ctx.heap),
                    node.expr_type.display_name(&ctx.heap)
                )
            ));
        }

        return Ok(())
    }

    fn progress_inference_rule_call_expr(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &self.infer_nodes[node_index];
        let node_expr_id = node.expr_id;
        let rule = node.inference_rule.as_call_expr();

        let mut poly_progress_section = self.poly_progress_buffer.start_section();
        let argument_node_indices = self.index_buffer.start_section_initialized(&rule.argument_indices);

        // Perform inference on arguments to function, while trying to figure
        // out the polymorphic variables
        for (argument_index, argument_node_index) in argument_node_indices.iter_copied().enumerate() {
            let argument_expr_id = self.infer_nodes[argument_node_index].expr_id;
            let (_, progress_argument) = self.apply_polydata_equal2_constraint(
                ctx, node_index, argument_expr_id, "argument's",
                PolyDataTypeIndex::Associated(argument_index), 0,
                argument_node_index, 0, &mut poly_progress_section
            )?;

            if progress_argument { self.queue_node(argument_node_index); }
        }

        // Same for the return type.
        let (_, progress_call_1) = self.apply_polydata_equal2_constraint(
            ctx, node_index, node_expr_id, "return",
            PolyDataTypeIndex::Returned, 0,
            node_index, 0, &mut poly_progress_section
        )?;

        // We will now apply any progression in the polymorphic variable type
        // back to the arguments.
        for (argument_index, argument_node_index) in argument_node_indices.iter_copied().enumerate() {
            let progress_argument = self.apply_polydata_polyvar_constraint(
                ctx, node_index, PolyDataTypeIndex::Associated(argument_index),
                argument_node_index, &poly_progress_section
            );

            if progress_argument { self.queue_node(argument_node_index); }
        }

        // And back to the return type.
        let progress_call_2 = self.apply_polydata_polyvar_constraint(
            ctx, node_index, PolyDataTypeIndex::Returned,
            node_index, &poly_progress_section
        );

        if progress_call_1 || progress_call_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();
        argument_node_indices.forget();

        self.finish_polydata_constraint(node_index);
        return Ok(())
    }

    fn progress_inference_rule_variable_expr(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {
        let node = &mut self.infer_nodes[node_index];
        let rule = node.inference_rule.as_variable_expr();
        let var_data_index = rule.var_data_index;

        let var_data = &mut self.var_data[var_data_index];
        // Apply inference to the shared variable type and the expression type
        let shared_type: *mut _ = &mut var_data.var_type;
        let expr_type: *mut _ = &mut node.expr_type;

        let inference_result = unsafe {
            // safety: vectors exist in different storage vectors, so cannot alias
            InferenceType::infer_subtrees_for_both_types(shared_type, 0, expr_type, 0)
        };

        if inference_result == DualInferenceResult::Incompatible {
            return Err(self.construct_variable_type_error(ctx, node_index));
        }

        let progress_var_data = inference_result.modified_lhs();
        let progress_expr = inference_result.modified_rhs();

        if progress_var_data {
            // We progressed the type of the shared variable, so propagate this
            // to all associated variable expressions (and relatived variables).
            for other_node_index in var_data.used_at.iter().copied() {
                if other_node_index != node_index {
                    self.node_queued.push_back(other_node_index);
                }
            }

            if let Some(linked_var_data_index) = var_data.linked_var {
                // Only perform one-way inference, progressing the linked
                // variable.
                // note: because this "linking" is used only for channels, we
                // will start inference one level below the top-level in the
                // type tree (i.e. ensure `T` in `in<T>` and `out<T>` is equal).
                debug_assert!(
                    var_data.var_type.parts[0] == InferenceTypePart::Input ||
                    var_data.var_type.parts[0] == InferenceTypePart::Output
                );
                let this_var_type: *const _ = &var_data.var_type;
                let linked_var_data = &mut self.var_data[linked_var_data_index];
                debug_assert!(
                    linked_var_data.var_type.parts[0] == InferenceTypePart::Input ||
                    linked_var_data.var_type.parts[0] == InferenceTypePart::Output
                );

                // safety: by construction var_data_index and linked_var_data_index cannot be the
                // same, hence we're not aliasing here.
                let inference_result = InferenceType::infer_subtree_for_single_type(
                    &mut linked_var_data.var_type, 1,
                    unsafe{ &(*this_var_type).parts }, 1, false
                );
                match inference_result {
                    SingleInferenceResult::Modified => {
                        for used_at in linked_var_data.used_at.iter().copied() {
                            self.node_queued.push_back(used_at);
                        }
                    },
                    SingleInferenceResult::Unmodified => {},
                    SingleInferenceResult::Incompatible => {
                        let var_data_this = &self.var_data[var_data_index];
                        let var_decl_this = &ctx.heap[var_data_this.var_id];
                        let var_data_linked = &self.var_data[linked_var_data_index];
                        let var_decl_linked = &ctx.heap[var_data_linked.var_id];

                        return Err(ParseError::new_error_at_span(
                            &ctx.module().source, var_decl_this.identifier.span, format!(
                                "conflicting types for this channel, this port has type '{}'",
                                var_data_this.var_type.display_name(&ctx.heap)
                            )
                        ).with_info_at_span(
                            &ctx.module().source, var_decl_linked.identifier.span, format!(
                                "while this port has type '{}'",
                                var_data_linked.var_type.display_name(&ctx.heap)
                            )
                        ));
                    }
                }
            }
        }

        if progress_expr { self.queue_node_parent(node_index); }

        return Ok(());
    }

    fn progress_template(&mut self, ctx: &Ctx, node_index: InferNodeIndex, application: InferenceRuleTemplateApplication, template: &[InferenceTypePart]) -> Result<bool, ParseError> {
        use InferenceRuleTemplateApplication as TA;

        match application {
            TA::None => Ok(false),
            TA::Template => self.apply_template_constraint(ctx, node_index, template),
            TA::Forced => self.apply_forced_constraint(ctx, node_index, template),
        }
    }

    fn queue_node_parent(&mut self, node_index: InferNodeIndex) {
        let node = &self.infer_nodes[node_index];
        if let Some(parent_node_index) = node.parent_index {
            self.node_queued.push_back(parent_node_index);
        }
    }

    #[inline]
    fn queue_node(&mut self, node_index: InferNodeIndex) {
        self.node_queued.push_back(node_index);
    }

    /// Returns whether the type is certainly a string (true, false), certainly
    /// not a string (false, true), or still unknown (false, false).
    fn type_is_certainly_or_certainly_not_string(&self, node_index: InferNodeIndex) -> (bool, bool) {
        let expr_type = &self.infer_nodes[node_index].expr_type;
        let mut part_index = 0;
        while part_index < expr_type.parts.len() {
            let part = &expr_type.parts[part_index];

            if part.is_marker() {
                part_index += 1;
                continue;
            }
            if !part.is_concrete() { break; }

            if *part == InferenceTypePart::String {
                // First part is a string
                return (true, false);
            } else {
                return (false, true);
            }
        }

        // If here then first non-marker type is not concrete
        if part_index == expr_type.parts.len() {
            // nothing known at all
            return (false, false);
        }

        // Special case: array-like where its argument is not a character
        if part_index + 1 < expr_type.parts.len() {
            if expr_type.parts[part_index] == InferenceTypePart::ArrayLike && expr_type.parts[part_index + 1] != InferenceTypePart::Character {
                return (false, true);
            }
        }


        (false, false)
    }

    /// Applies a template type constraint: the type associated with the
    /// supplied expression will be molded into the provided `template`. But
    /// will be considered valid if the template could've been molded into the
    /// expression type as well. Hence the template may be fully specified (e.g.
    /// a bool) or contain "inference" variables (e.g. an array of T)
    fn apply_template_constraint(
        &mut self, ctx: &Ctx, node_index: InferNodeIndex, template: &[InferenceTypePart]
    ) -> Result<bool, ParseError> {
        let expr_type = &mut self.infer_nodes[node_index].expr_type;
        match InferenceType::infer_subtree_for_single_type(expr_type, 0, template, 0, false) {
            SingleInferenceResult::Modified => Ok(true),
            SingleInferenceResult::Unmodified => Ok(false),
            SingleInferenceResult::Incompatible => Err(
                self.construct_template_type_error(ctx, node_index, template)
            )
        }
    }

    /// Applies a forced constraint: the supplied expression's type MUST be
    /// inferred from the template, the other way around is considered invalid.
    fn apply_forced_constraint(
        &mut self, ctx: &Ctx, node_index: InferNodeIndex, template: &[InferenceTypePart]
    ) -> Result<bool, ParseError> {
        let expr_type = &mut self.infer_nodes[node_index].expr_type;

        match InferenceType::infer_subtree_for_single_type(expr_type, 0, template, 0, true) {
            SingleInferenceResult::Modified => Ok(true),
            SingleInferenceResult::Unmodified => Ok(false),
            SingleInferenceResult::Incompatible => Err(
                self.construct_template_type_error(ctx, node_index, template)
            )
        }
    }

    /// Applies a type constraint that expects the two provided types to be
    /// equal. We attempt to make progress in inferring the types. If the call
    /// is successful then the composition of all types are made equal.
    /// The "parent" `expr_id` is provided to construct errors.
    fn apply_equal2_constraint(
        &mut self, ctx: &Ctx, node_index: InferNodeIndex,
        arg1_index: InferNodeIndex, arg1_start_idx: usize,
        arg2_index: InferNodeIndex, arg2_start_idx: usize
    ) -> Result<(bool, bool), ParseError> {
        let arg1_type: *mut _ = &mut self.infer_nodes[arg1_index].expr_type;
        let arg2_type: *mut _ = &mut self.infer_nodes[arg2_index].expr_type;

        let infer_res = unsafe{ InferenceType::infer_subtrees_for_both_types(
            arg1_type, arg1_start_idx,
            arg2_type, arg2_start_idx
        ) };
        if infer_res == DualInferenceResult::Incompatible {
            return Err(self.construct_arg_type_error(ctx, node_index, arg1_index, arg2_index));
        }

        Ok((infer_res.modified_lhs(), infer_res.modified_rhs()))
    }

    /// Applies an equal2 constraint between a member of the `PolyData` struct,
    /// and another inferred type. If any progress is made in the `PolyData`
    /// struct then the affected polymorphic variables are updated as well.
    ///
    /// Because a lot of types/expressions are involved in polymorphic typFe
    /// inference, some explanation: "outer_node" refers to the main expression
    /// that is the root cause of type inference (e.g. a struct literal
    /// expression, or a tuple member select expression). Associated with that
    /// outer node is `PolyData`, so that is what the "poly_data" variables
    /// are referring to. We are applying equality between a "poly_data" type
    /// and an associated expression (not necessarily the "outer_node", e.g.
    /// the expression that constructs the value of a struct field). Hence the
    /// "associated" variables.
    ///
    /// Finally, when an error occurs we'll first show the outer node's
    /// location. As info, the `error_location_expr_id` span is shown,
    /// indicating that the "`error_type_name` type has been resolved to
    /// `outer_node_type`, but this expression has been resolved to
    /// `associated_node_type`".
    fn apply_polydata_equal2_constraint(
        &mut self, ctx: &Ctx,
        outer_node_index: InferNodeIndex, error_location_expr_id: ExpressionId, error_type_name: &str,
        poly_data_type_index: PolyDataTypeIndex, poly_data_start_index: usize,
        associated_node_index: InferNodeIndex, associated_node_start_index: usize,
        poly_progress_section: &mut ScopedSection<u32>,
    ) -> Result<(bool, bool), ParseError> {
        let poly_data_index = self.infer_nodes[outer_node_index].poly_data_index;
        let poly_data = &mut self.poly_data[poly_data_index as usize];
        let poly_data_type = poly_data.expr_types.get_type_mut(poly_data_type_index);
        let associated_type: *mut _ = &mut self.infer_nodes[associated_node_index].expr_type;

        let inference_result = unsafe{
            // Safety: pointers originate from different vectors, so cannot
            // alias.
            let poly_data_type: *mut _ = poly_data_type;
            InferenceType::infer_subtrees_for_both_types(
                poly_data_type, poly_data_start_index,
                associated_type, associated_node_start_index
            )
        };

        let modified_poly_data = inference_result.modified_lhs();
        let modified_associated = inference_result.modified_rhs();
        if inference_result == DualInferenceResult::Incompatible {
            let outer_node_expr_id = self.infer_nodes[outer_node_index].expr_id;
            let outer_node_span = ctx.heap[outer_node_expr_id].full_span();
            let detailed_span = ctx.heap[error_location_expr_id].full_span();

            let outer_node_type = poly_data_type.display_name(&ctx.heap);
            let associated_type = self.infer_nodes[associated_node_index].expr_type.display_name(&ctx.heap);

            let source = &ctx.module().source;
            return Err(ParseError::new_error_str_at_span(
                source, outer_node_span, "failed to resolve the types of this expression"
            ).with_info_str_at_span(
                source, detailed_span, &format!(
                    "because the {} type has been resolved to '{}', but this expression has been resolved to '{}'",
                    error_type_name, outer_node_type, associated_type
                )
            ));
        }

        if modified_poly_data {
            debug_assert!(poly_data_type.has_marker);

            // Go through markers for polymorphic variables and use the
            // (hopefully) more specific types to update their representation
            // in the PolyData struct
            for (poly_var_index, poly_var_section) in poly_data_type.marker_iter() {
                let poly_var_type = &mut poly_data.poly_vars[poly_var_index as usize];
                match InferenceType::infer_subtree_for_single_type(poly_var_type, 0, poly_var_section, 0, false) {
                    SingleInferenceResult::Modified => {
                        poly_progress_section.push_unique(poly_var_index);
                    },
                    SingleInferenceResult::Unmodified => {
                        // nothing to do
                    },
                    SingleInferenceResult::Incompatible => {
                        return Err(Self::construct_poly_arg_error(
                            ctx, &self.poly_data[poly_data_index as usize],
                            self.infer_nodes[outer_node_index].expr_id
                        ));
                    }
                }
            }
        }

        return Ok((modified_poly_data, modified_associated));
    }

    /// After calling `apply_polydata_equal2_constraint` on several expressions
    /// that are associated with some kind of polymorphic expression, several of
    /// the polymorphic variables might have been inferred to more specific
    /// types than before.
    ///
    /// At this point one should call this function to apply the progress in
    /// these polymorphic variables back onto the types that are functions of
    /// these polymorphic variables.
    ///
    /// An example: a struct literal with a polymorphic variable `T` may have
    /// two fields `foo` and `bar` each with different types that are a function
    /// of the polymorhic variable `T`. If the expressions constructing the
    /// value for the field `foo` causes the type `T` to progress, then we can
    /// also progress the type of the expression that constructs `bar`.
    ///
    /// And so we have `outer_node_index` + `poly_data_type_index` pointing to
    /// the appropriate type in the `PolyData` struct. Which will be updated
    /// first using the polymorphic variables. If we happen to have updated that
    /// type, then we should also progress the associated expression, hence the
    /// `associated_node_index`.
    fn apply_polydata_polyvar_constraint(
        &mut self, _ctx: &Ctx,
        outer_node_index: InferNodeIndex, poly_data_type_index: PolyDataTypeIndex,
        associated_node_index: InferNodeIndex, poly_progress_section: &ScopedSection<u32>
    ) -> bool {
        let poly_data_index = self.infer_nodes[outer_node_index].poly_data_index;
        let poly_data = &mut self.poly_data[poly_data_index as usize];

        // Early exit, most common case (literals or functions calls which are
        // actually not polymorphic)
        if !poly_data.first_rule_application && poly_progress_section.len() == 0 {
            return false;
        }

        // safety: we're borrowing from two distinct fields, so should be fine
        let poly_data_type = poly_data.expr_types.get_type_mut(poly_data_type_index);
        let mut last_start_index = 0;
        let mut modified_poly_type = false;

        while let Some((poly_var_index, poly_var_start_index)) = poly_data_type.find_marker(last_start_index) {
            let poly_var_end_index = InferenceType::find_subtree_end_idx(&poly_data_type.parts, poly_var_start_index);

            if poly_data.first_rule_application || poly_progress_section.contains(&poly_var_index) {
                // We have updated this polymorphic variable, so try updating it
                // in the PolyData type
                let modified_in_poly_data = match InferenceType::infer_subtree_for_single_type(
                    poly_data_type, poly_var_start_index, &poly_data.poly_vars[poly_var_index as usize].parts, 0, false
                ) {
                    SingleInferenceResult::Modified => true,
                    SingleInferenceResult::Unmodified => false,
                    SingleInferenceResult::Incompatible => {
                        // practically impossible: before calling this function we gather all the
                        // data on the polymorphic variables from the associated expressions. So if
                        // the polymorphic variables in those expressions were not mutually
                        // compatible, we must have encountered that error already.
                        unreachable!()
                    },
                };

                modified_poly_type = modified_poly_type || modified_in_poly_data;
            }

            last_start_index = poly_var_end_index;
        }

        if modified_poly_type {
            let associated_type = &mut self.infer_nodes[associated_node_index].expr_type;
            match InferenceType::infer_subtree_for_single_type(
                associated_type, 0, &poly_data_type.parts, 0, true
            ) {
                SingleInferenceResult::Modified => return true,
                SingleInferenceResult::Unmodified => return false,
                SingleInferenceResult::Incompatible => unreachable!(), // same as above
            }
        } else {
            // Did not update associated type
            return false;
        }
    }

    /// Should be called after completing one full round of applying polydata
    /// constraints.
    fn finish_polydata_constraint(&mut self, outer_node_index: InferNodeIndex) {
        let poly_data_index = self.infer_nodes[outer_node_index].poly_data_index;
        let poly_data = &mut self.poly_data[poly_data_index as usize];
        poly_data.first_rule_application = false;
    }

    /// Applies a type constraint that expects all three provided types to be
    /// equal. In case we can make progress in inferring the types then we
    /// attempt to do so. If the call is successful then the composition of all
    /// types is made equal.
    fn apply_equal3_constraint(
        &mut self, ctx: &Ctx, node_index: InferNodeIndex,
        arg1_index: InferNodeIndex, arg2_index: InferNodeIndex,
        start_idx: usize
    ) -> Result<(bool, bool, bool), ParseError> {
        // Safety: all indices are unique
        //         containers may not be modified
        let expr_type: *mut _ = &mut self.infer_nodes[node_index].expr_type;
        let arg1_type: *mut _ = &mut self.infer_nodes[arg1_index].expr_type;
        let arg2_type: *mut _ = &mut self.infer_nodes[arg2_index].expr_type;

        let expr_res = unsafe{
            InferenceType::infer_subtrees_for_both_types(expr_type, start_idx, arg1_type, start_idx)
        };
        if expr_res == DualInferenceResult::Incompatible {
            return Err(self.construct_expr_type_error(ctx, node_index, arg1_index));
        }

        let args_res = unsafe{
            InferenceType::infer_subtrees_for_both_types(arg1_type, start_idx, arg2_type, start_idx) };
        if args_res == DualInferenceResult::Incompatible {
            return Err(self.construct_arg_type_error(ctx, node_index, arg1_index, arg2_index));
        }

        // If all types are compatible, but the second call caused the arg1_type
        // to be expanded, then we must also assign this to expr_type.
        let mut progress_expr = expr_res.modified_lhs();
        let mut progress_arg1 = expr_res.modified_rhs();
        let progress_arg2 = args_res.modified_rhs();

        if args_res.modified_lhs() { 
            unsafe {
                let end_idx = InferenceType::find_subtree_end_idx(&(*arg2_type).parts, start_idx);
                let subtree = &((*arg2_type).parts[start_idx..end_idx]);
                (*expr_type).replace_subtree(start_idx, subtree);
            }
            progress_expr = true;
            progress_arg1 = true;
        }

        Ok((progress_expr, progress_arg1, progress_arg2))
    }

    /// Applies equal constraint to N consecutive expressions. The returned
    /// `progress` vec will contain which expressions were progressed and will
    /// have length N.
    fn apply_equal_n_constraint(
        &mut self, ctx: &Ctx, outer_node_index: InferNodeIndex,
        arguments: &ScopedSection<InferNodeIndex>, progress: &mut ScopedSection<bool>
    ) -> Result<(), ParseError> {
        // Depending on the argument perform an early exit. This simplifies
        // later logic
        debug_assert_eq!(progress.len(), 0);
        match arguments.len() {
            0 => {
                // nothing to progress
                return Ok(())
            },
            1 => {
                // only one type, so nothing to infer
                progress.push(false);
                return Ok(())
            },
            n => {
                for _ in 0..n {
                    progress.push(false);
                }
            }
        }

        // We'll start doing pairwise inference for all of the inference nodes
        // (node[0] with node[1], then node[1] with node[2], then node[2] ...,
        // etc.), so when we're at the end we have `node[N-1]` as the most
        // progressed type.
        let mut last_index_requiring_inference = 0;

        for prev_argument_index in 0..arguments.len() - 1 {
            let next_argument_index = prev_argument_index + 1;

            let prev_node_index = arguments[prev_argument_index];
            let next_node_index = arguments[next_argument_index];
            let (prev_progress, next_progress) = self.apply_equal2_constraint(
                ctx, outer_node_index, prev_node_index, 0, next_node_index, 0
            )?;

            if prev_progress {
                // Previous node is progress, so every type in front of it needs
                // to be reinferred.
                progress[prev_argument_index] = true;
                last_index_requiring_inference = prev_argument_index;
            }
            progress[next_argument_index] = next_progress;
        }

        // Apply inference using the most progressed type (the last one) to the
        // ones that did not obtain this information during the inference
        // process.
        let last_argument_node_index = arguments[arguments.len() - 1];
        let last_argument_type: *mut _ = &mut self.infer_nodes[last_argument_node_index].expr_type;

        for argument_index in 0..last_index_requiring_inference {
            // We can cheat, we know the LHS is less specific than the right
            // hand side, so:
            let argument_node_index = arguments[argument_index];
            let argument_type = &mut self.infer_nodes[argument_node_index].expr_type;
            unsafe {
                // safety: we're dealing with different vectors, so cannot alias
                argument_type.replace_subtree(0, &(*last_argument_type).parts);
            }
            progress[argument_index] = true;
        }

        return Ok(());
    }

    /// Determines the `InferenceType` for the expression based on the
    /// expression parent (this is not done if the parent is a regular 'ol
    /// expression). Expects `parent_index` to be set to the parent of the
    /// inference node that is created here.
    fn insert_initial_inference_node(
        &mut self, ctx: &mut Ctx, expr_id: ExpressionId
    ) -> Result<InferNodeIndex, ParseError> {
        use ExpressionParent as EP;
        use InferenceTypePart as ITP;

        // Set the initial inference type based on the expression parent.
        let expr = &ctx.heap[expr_id];
        let inference_type = match expr.parent() {
            EP::None =>
                // Should have been set by linker
                unreachable!(),
            EP::Memory(_) | EP::ExpressionStmt(_) =>
                // Determined during type inference
                InferenceType::new(false, false, vec![ITP::Unknown]),
            EP::Expression(parent_id, idx_in_parent) => {
                // If we are the test expression of a conditional expression,
                // then we must resolve to a boolean
                let is_conditional = if let Expression::Conditional(_) = &ctx.heap[*parent_id] {
                    true
                } else {
                    false
                };

                if is_conditional && *idx_in_parent == 0 {
                    InferenceType::new(false, true, vec![ITP::Bool])
                } else {
                    InferenceType::new(false, false, vec![ITP::Unknown])
                }
            },
            EP::If(_) | EP::While(_) =>
                // Must be a boolean
                InferenceType::new(false, true, vec![ITP::Bool]),
            EP::Return(_) => {
                // Must match the return type of the function
                debug_assert_eq!(self.procedure_kind, ProcedureKind::Function);
                let returned = &ctx.heap[self.procedure_id].return_type.as_ref().unwrap();
                self.determine_inference_type_from_parser_type_elements(&returned.elements, true)
            },
            EP::New(_) =>
                // Must be a component call, which we assign a "Void" return
                // type
                InferenceType::new(false, true, vec![ITP::Void]),
        };

        let infer_index = self.infer_nodes.len() as InferNodeIndex;
        self.infer_nodes.push(InferenceNode {
            expr_type: inference_type,
            expr_id,
            inference_rule: InferenceRule::Noop,
            parent_index: self.parent_index,
            field_index: -1,
            poly_data_index: -1,
            info_type_id: TypeId::new_invalid(),
            info_variant: ExpressionInfoVariant::Generic,
        });

        return Ok(infer_index);
    }

    fn insert_initial_call_polymorph_data(
        &mut self, ctx: &mut Ctx, call_id: CallExpressionId
    ) -> PolyDataIndex {
        // Note: the polymorph variables may be partially specified and may
        // contain references to the wrapping definition's (i.e. the proctype
        // we are currently visiting) polymorphic arguments.
        //
        // The arguments of the call may refer to polymorphic variables in the
        // definition of the function we're calling, not of the wrapping
        // definition. We insert markers in these inferred types to be able to
        // map them back and forth to the polymorphic arguments of the function
        // we are calling.
        let call = &ctx.heap[call_id];

        // Handle the polymorphic arguments (if there are any)
        let num_poly_args = call.parser_type.elements[0].variant.num_embedded();
        let mut poly_args = Vec::with_capacity(num_poly_args);
        for embedded_elements in call.parser_type.iter_embedded(0) {
            poly_args.push(self.determine_inference_type_from_parser_type_elements(embedded_elements, true));
        }

        // Handle the arguments and return types
        let definition = &ctx.heap[call.procedure];
        debug_assert_eq!(poly_args.len(), definition.poly_vars.len());

        let mut parameter_types = Vec::with_capacity(definition.parameters.len());
        let parameter_section = self.var_buffer.start_section_initialized(&definition.parameters);
        for parameter_id in parameter_section.iter_copied() {
            let param = &ctx.heap[parameter_id];
            parameter_types.push(self.determine_inference_type_from_parser_type_elements(&param.parser_type.elements, false));
        }
        parameter_section.forget();

        let return_type = match &definition.return_type {
            None => {
                // Component, so returns a "Void"
                debug_assert_ne!(definition.kind, ProcedureKind::Function);
                InferenceType::new(false, true, vec![InferenceTypePart::Void])
            },
            Some(returned) => {
                debug_assert_eq!(definition.kind, ProcedureKind::Function);
                self.determine_inference_type_from_parser_type_elements(&returned.elements, false)
            }
        };

        let extra_data_idx = self.poly_data.len() as PolyDataIndex;
        self.poly_data.push(PolyData {
            first_rule_application: true,
            definition_id: call.procedure.upcast(),
            poly_vars: poly_args,
            expr_types: PolyDataTypes {
                associated: parameter_types,
                returned: return_type
            }
        });
        return extra_data_idx
    }

    fn insert_initial_struct_polymorph_data(
        &mut self, ctx: &mut Ctx, lit_id: LiteralExpressionId,
    ) -> PolyDataIndex {
        use InferenceTypePart as ITP;
        let literal = ctx.heap[lit_id].value.as_struct();

        // Handle polymorphic arguments
        let num_embedded = literal.parser_type.elements[0].variant.num_embedded();
        let mut total_num_poly_parts = 0;
        let mut poly_args = Vec::with_capacity(num_embedded);

        for embedded_elements in literal.parser_type.iter_embedded(0) {
            let poly_type = self.determine_inference_type_from_parser_type_elements(embedded_elements, true);
            total_num_poly_parts += poly_type.parts.len();
            poly_args.push(poly_type);
        }

        // Handle parser types on struct definition
        let defined_type = ctx.types.get_base_definition(&literal.definition).unwrap();
        let struct_type = defined_type.definition.as_struct();
        debug_assert_eq!(poly_args.len(), defined_type.poly_vars.len());

        // Note: programmer is capable of specifying fields in a struct literal
        // in a different order than on the definition. We take the literal-
        // specified order to be leading.
        let mut embedded_types = Vec::with_capacity(struct_type.fields.len());
        for lit_field in literal.fields.iter() {
            let def_field = &struct_type.fields[lit_field.field_idx];
            let inference_type = self.determine_inference_type_from_parser_type_elements(&def_field.parser_type.elements, false);
            embedded_types.push(inference_type);
        }

        // Return type is the struct type itself, with the appropriate 
        // polymorphic variables. So:
        // - 1 part for definition
        // - N_poly_arg marker parts for each polymorphic argument
        // - all the parts for the currently known polymorphic arguments 
        let parts_reserved = 1 + poly_args.len() + total_num_poly_parts;
        let mut parts = Vec::with_capacity(parts_reserved);
        parts.push(ITP::Instance(literal.definition, poly_args.len() as u32));
        let mut return_type_done = true;
        for (poly_var_idx, poly_var) in poly_args.iter().enumerate() {
            if !poly_var.is_done { return_type_done = false; }

            parts.push(ITP::Marker(poly_var_idx as u32));
            parts.extend(poly_var.parts.iter().cloned());
        }

        debug_assert_eq!(parts.len(), parts_reserved);
        let return_type = InferenceType::new(!poly_args.is_empty(), return_type_done, parts);

        let extra_data_index = self.poly_data.len() as PolyDataIndex;
        self.poly_data.push(PolyData {
            first_rule_application: true,
            definition_id: literal.definition,
            poly_vars: poly_args,
            expr_types: PolyDataTypes {
                associated: embedded_types,
                returned: return_type,
            },
        });

        return extra_data_index
    }

    /// Inserts the extra polymorphic data struct for enum expressions. These
    /// can never be determined from the enum itself, but may be inferred from
    /// the use of the enum.
    fn insert_initial_enum_polymorph_data(
        &mut self, ctx: &Ctx, lit_id: LiteralExpressionId
    ) -> PolyDataIndex {
        use InferenceTypePart as ITP;
        let literal = ctx.heap[lit_id].value.as_enum();

        // Handle polymorphic arguments to the enum
        let num_poly_args = literal.parser_type.elements[0].variant.num_embedded();
        let mut total_num_poly_parts = 0;
        let mut poly_args = Vec::with_capacity(num_poly_args);

        for embedded_elements in literal.parser_type.iter_embedded(0) {
            let poly_type = self.determine_inference_type_from_parser_type_elements(embedded_elements, true);
            total_num_poly_parts += poly_type.parts.len();
            poly_args.push(poly_type);
        }

        // Handle enum type itself
        let parts_reserved = 1 + poly_args.len() + total_num_poly_parts;
        let mut parts = Vec::with_capacity(parts_reserved);
        parts.push(ITP::Instance(literal.definition, poly_args.len() as u32));
        let mut enum_type_done = true;
        for (poly_var_idx, poly_var) in poly_args.iter().enumerate() {
            if !poly_var.is_done { enum_type_done = false; }

            parts.push(ITP::Marker(poly_var_idx as u32));
            parts.extend(poly_var.parts.iter().cloned());
        }

        debug_assert_eq!(parts.len(), parts_reserved);
        let enum_type = InferenceType::new(!poly_args.is_empty(), enum_type_done, parts);

        let extra_data_index = self.poly_data.len() as PolyDataIndex;
        self.poly_data.push(PolyData {
            first_rule_application: true,
            definition_id: literal.definition,
            poly_vars: poly_args,
            expr_types: PolyDataTypes {
                associated: Vec::new(),
                returned: enum_type,
            },
        });

        return extra_data_index;
    }

    /// Inserts the extra polymorphic data struct for unions. The polymorphic
    /// arguments may be partially determined from embedded values in the union.
    fn insert_initial_union_polymorph_data(
        &mut self, ctx: &Ctx, lit_id: LiteralExpressionId
    ) -> PolyDataIndex {
        use InferenceTypePart as ITP;
        let literal = ctx.heap[lit_id].value.as_union();

        // Construct the polymorphic variables
        let num_poly_args = literal.parser_type.elements[0].variant.num_embedded();
        let mut total_num_poly_parts = 0;
        let mut poly_args = Vec::with_capacity(num_poly_args);

        for embedded_elements in literal.parser_type.iter_embedded(0) {
            let poly_type = self.determine_inference_type_from_parser_type_elements(embedded_elements, true);
            total_num_poly_parts += poly_type.parts.len();
            poly_args.push(poly_type);
        }

        // Handle any of the embedded values in the variant, if specified
        let definition_id = literal.definition;
        let type_definition = ctx.types.get_base_definition(&definition_id).unwrap();
        let union_definition = type_definition.definition.as_union();
        debug_assert_eq!(poly_args.len(), type_definition.poly_vars.len());

        let variant_definition = &union_definition.variants[literal.variant_idx];
        debug_assert_eq!(variant_definition.embedded.len(), literal.values.len());

        let mut embedded = Vec::with_capacity(variant_definition.embedded.len());
        for embedded_parser_type in &variant_definition.embedded {
            let inference_type = self.determine_inference_type_from_parser_type_elements(&embedded_parser_type.elements, false);
            embedded.push(inference_type);
        }

        // Handle the type of the union itself
        let parts_reserved = 1 + poly_args.len() + total_num_poly_parts;
        let mut parts = Vec::with_capacity(parts_reserved);
        parts.push(ITP::Instance(definition_id, poly_args.len() as u32));
        let mut union_type_done = true;
        for (poly_var_idx, poly_var) in poly_args.iter().enumerate() {
            if !poly_var.is_done { union_type_done = false; }

            parts.push(ITP::Marker(poly_var_idx as u32));
            parts.extend(poly_var.parts.iter().cloned());
        }

        debug_assert_eq!(parts_reserved, parts.len());
        let union_type = InferenceType::new(!poly_args.is_empty(), union_type_done, parts);

        let extra_data_index = self.poly_data.len() as isize;
        self.poly_data.push(PolyData {
            first_rule_application: true,
            definition_id: literal.definition,
            poly_vars: poly_args,
            expr_types: PolyDataTypes {
                associated: embedded,
                returned: union_type,
            },
        });

        return extra_data_index;
    }

    /// Inserts the extra polymorphic data struct. Assumes that the select
    /// expression's referenced (definition_id, field_idx) has been resolved.
    fn insert_initial_select_polymorph_data(
        &mut self, ctx: &Ctx, node_index: InferNodeIndex, struct_def_id: DefinitionId
    ) -> PolyDataIndex {
        use InferenceTypePart as ITP;

        let definition = ctx.heap[struct_def_id].as_struct();
        let node = &self.infer_nodes[node_index];
        let field_index = node.field_index as usize;

        // Generate initial polyvar types and struct type
        // TODO: @Performance: we can immediately set the polyvars of the subject's struct type
        let num_poly_vars = definition.poly_vars.len();
        let mut poly_vars = Vec::with_capacity(num_poly_vars);
        let struct_parts_reserved = 1 + 2 * num_poly_vars;
        let mut struct_parts = Vec::with_capacity(struct_parts_reserved);
        struct_parts.push(ITP::Instance(struct_def_id, num_poly_vars as u32));

        for poly_idx in 0..num_poly_vars {
            poly_vars.push(InferenceType::new(true, false, vec![
                ITP::Marker(poly_idx as u32), ITP::Unknown,
            ]));
            struct_parts.push(ITP::Marker(poly_idx as u32));
            struct_parts.push(ITP::Unknown);
        }
        debug_assert_eq!(struct_parts.len(), struct_parts_reserved);

        // Generate initial field type
        let field_type = self.determine_inference_type_from_parser_type_elements(&definition.fields[field_index].parser_type.elements, false);

        let extra_data_index = self.poly_data.len() as PolyDataIndex;
        self.poly_data.push(PolyData {
            first_rule_application: true,
            definition_id: struct_def_id,
            poly_vars,
            expr_types: PolyDataTypes {
                associated: vec![InferenceType::new(num_poly_vars != 0, num_poly_vars == 0, struct_parts)],
                returned: field_type,
            },
        });

        return extra_data_index;
    }

    /// Determines the initial InferenceType from the provided ParserType. This
    /// may be called with two kinds of intentions:
    /// 1. To resolve a ParserType within the body of a function, or on
    ///     polymorphic arguments to calls/instantiations within that body. This
    ///     means that the polymorphic variables are known and can be replaced
    ///     with the monomorph we're instantiating.
    /// 2. To resolve a ParserType on a called function's definition or on
    ///     an instantiated datatype's members. This means that the polymorphic
    ///     arguments inside those ParserTypes refer to the polymorphic
    ///     variables in the called/instantiated type's definition.
    /// In the second case we place InferenceTypePart::Marker instances such
    /// that we can perform type inference on the polymorphic variables.
    fn determine_inference_type_from_parser_type_elements(
        &mut self, elements: &[ParserTypeElement],
        use_definitions_known_poly_args: bool
    ) -> InferenceType {
        use ParserTypeVariant as PTV;
        use InferenceTypePart as ITP;

        let mut infer_type = Vec::with_capacity(elements.len());
        let mut has_inferred = false;
        let mut has_markers = false;

        for element in elements {
            match &element.variant {
                // Compiler-only types
                PTV::Void => { infer_type.push(ITP::Void); },
                PTV::InputOrOutput => { infer_type.push(ITP::PortLike); has_inferred = true },
                PTV::ArrayLike => { infer_type.push(ITP::ArrayLike); has_inferred = true },
                PTV::IntegerLike => { infer_type.push(ITP::IntegerLike); has_inferred = true },
                // Builtins
                PTV::Message => {
                    // TODO: @types Remove the Message -> Byte hack at some point...
                    infer_type.push(ITP::Message);
                    infer_type.push(ITP::UInt8);
                },
                PTV::Bool => { infer_type.push(ITP::Bool); },
                PTV::UInt8 => { infer_type.push(ITP::UInt8); },
                PTV::UInt16 => { infer_type.push(ITP::UInt16); },
                PTV::UInt32 => { infer_type.push(ITP::UInt32); },
                PTV::UInt64 => { infer_type.push(ITP::UInt64); },
                PTV::SInt8 => { infer_type.push(ITP::SInt8); },
                PTV::SInt16 => { infer_type.push(ITP::SInt16); },
                PTV::SInt32 => { infer_type.push(ITP::SInt32); },
                PTV::SInt64 => { infer_type.push(ITP::SInt64); },
                PTV::Character => { infer_type.push(ITP::Character); },
                PTV::String => {
                    infer_type.push(ITP::String);
                    infer_type.push(ITP::Character);
                },
                // Special markers
                PTV::IntegerLiteral => { unreachable!("integer literal type on variable type"); },
                PTV::Inferred => {
                    infer_type.push(ITP::Unknown);
                    has_inferred = true;
                },
                // With nested types
                PTV::Array => { infer_type.push(ITP::Array); },
                PTV::Input => { infer_type.push(ITP::Input); },
                PTV::Output => { infer_type.push(ITP::Output); },
                PTV::Tuple(num_embedded) => { infer_type.push(ITP::Tuple(*num_embedded)); },
                PTV::PolymorphicArgument(belongs_to_definition, poly_arg_idx) => {
                    let poly_arg_idx = *poly_arg_idx;
                    if use_definitions_known_poly_args {
                        // Refers to polymorphic argument on procedure we're currently processing.
                        // This argument is already known.
                        debug_assert_eq!(*belongs_to_definition, self.procedure_id.upcast());
                        debug_assert!((poly_arg_idx as usize) < self.poly_vars.len());

                        Self::determine_inference_type_from_concrete_type(
                            &mut infer_type, &self.poly_vars[poly_arg_idx as usize].parts
                        );
                    } else {
                        // Polymorphic argument has to be inferred
                        has_markers = true;
                        has_inferred = true;
                        infer_type.push(ITP::Marker(poly_arg_idx));
                        infer_type.push(ITP::Unknown)
                    }
                },
                PTV::Definition(definition_id, num_embedded) => {
                    infer_type.push(ITP::Instance(*definition_id, *num_embedded));
                }
            }
        }

        InferenceType::new(has_markers, !has_inferred, infer_type)
    }

    /// Determines the inference type from an already concrete type. Applies the
    /// various type "hacks" inside the type inferencer.
    fn determine_inference_type_from_concrete_type(parser_type: &mut Vec<InferenceTypePart>, concrete_type: &[ConcreteTypePart]) {
        use InferenceTypePart as ITP;
        use ConcreteTypePart as CTP;

        for concrete_part in concrete_type {
            match concrete_part {
                CTP::Void => parser_type.push(ITP::Void),
                CTP::Message => {
                    parser_type.push(ITP::Message);
                    parser_type.push(ITP::UInt8)
                },
                CTP::Bool => parser_type.push(ITP::Bool),
                CTP::UInt8 => parser_type.push(ITP::UInt8),
                CTP::UInt16 => parser_type.push(ITP::UInt16),
                CTP::UInt32 => parser_type.push(ITP::UInt32),
                CTP::UInt64 => parser_type.push(ITP::UInt64),
                CTP::SInt8 => parser_type.push(ITP::SInt8),
                CTP::SInt16 => parser_type.push(ITP::SInt16),
                CTP::SInt32 => parser_type.push(ITP::SInt32),
                CTP::SInt64 => parser_type.push(ITP::SInt64),
                CTP::Character => parser_type.push(ITP::Character),
                CTP::String => {
                    parser_type.push(ITP::String);
                    parser_type.push(ITP::Character)
                },
                CTP::Array => parser_type.push(ITP::Array),
                CTP::Slice => parser_type.push(ITP::Slice),
                CTP::Input => parser_type.push(ITP::Input),
                CTP::Output => parser_type.push(ITP::Output),
                CTP::Pointer => unreachable!("pointer type during concrete to inference type conversion"),
                CTP::Tuple(num) => parser_type.push(ITP::Tuple(*num)),
                CTP::Instance(id, num) => parser_type.push(ITP::Instance(*id, *num)),
                CTP::Function(_, _) => unreachable!("function type during concrete to inference type conversion"),
                CTP::Component(_, _) => unreachable!("component type during concrete to inference type conversion"),
            }
        }
    }

    /// Construct an error when an expression's type does not match. This
    /// happens if we infer the expression type from its arguments (e.g. the
    /// expression type of an addition operator is the type of the arguments)
    /// But the expression type was already set due to our parent (e.g. an
    /// "if statement" or a "logical not" always expecting a boolean)
    fn construct_expr_type_error(
        &self, ctx: &Ctx, expr_index: InferNodeIndex, arg_index: InferNodeIndex
    ) -> ParseError {
        // TODO: Expand and provide more meaningful information for humans
        let expr_node = &self.infer_nodes[expr_index];
        let arg_node = &self.infer_nodes[arg_index];

        let expr = &ctx.heap[expr_node.expr_id];
        let arg = &ctx.heap[arg_node.expr_id];

        return ParseError::new_error_at_span(
            &ctx.module().source, expr.operation_span(), format!(
                "incompatible types: this expression expected a '{}'",
                expr_node.expr_type.display_name(&ctx.heap)
            )
        ).with_info_at_span(
            &ctx.module().source, arg.full_span(), format!(
                "but this expression yields a '{}'",
                arg_node.expr_type.display_name(&ctx.heap)
            )
        )
    }

    fn construct_arg_type_error(
        &self, ctx: &Ctx, expr_index: InferNodeIndex,
        arg1_index: InferNodeIndex, arg2_index: InferNodeIndex
    ) -> ParseError {
        let arg1_node = &self.infer_nodes[arg1_index];
        let arg2_node = &self.infer_nodes[arg2_index];

        let expr_id = self.infer_nodes[expr_index].expr_id;
        let expr = &ctx.heap[expr_id];
        let arg1 = &ctx.heap[arg1_node.expr_id];
        let arg2 = &ctx.heap[arg2_node.expr_id];

        return ParseError::new_error_str_at_span(
            &ctx.module().source, expr.operation_span(),
            "incompatible types: cannot apply this expression"
        ).with_info_at_span(
            &ctx.module().source, arg1.full_span(), format!(
                "Because this expression has type '{}'",
                arg1_node.expr_type.display_name(&ctx.heap)
            )
        ).with_info_at_span(
            &ctx.module().source, arg2.full_span(), format!(
                "But this expression has type '{}'",
                arg2_node.expr_type.display_name(&ctx.heap)
            )
        )
    }

    fn construct_template_type_error(
        &self, ctx: &Ctx, node_index: InferNodeIndex, template: &[InferenceTypePart]
    ) -> ParseError {
        let node = &self.infer_nodes[node_index];
        let expr = &ctx.heap[node.expr_id];
        let expr_type = &node.expr_type;

        return ParseError::new_error_at_span(
            &ctx.module().source, expr.full_span(), format!(
                "incompatible types: got a '{}' but expected a '{}'",
                expr_type.display_name(&ctx.heap), 
                InferenceType::partial_display_name(&ctx.heap, template)
            )
        )
    }

    fn construct_variable_type_error(
        &self, ctx: &Ctx, node_index: InferNodeIndex,
    ) -> ParseError {
        let node = &self.infer_nodes[node_index];
        let rule = node.inference_rule.as_variable_expr();

        let var_data = &self.var_data[rule.var_data_index];
        let var_decl = &ctx.heap[var_data.var_id];
        let var_expr = &ctx.heap[node.expr_id];

        return ParseError::new_error_at_span(
            &ctx.module().source, var_decl.identifier.span, format!(
                "conflicting types for this variable, previously assigned the type '{}'",
                var_data.var_type.display_name(&ctx.heap)
            )
        ).with_info_at_span(
            &ctx.module().source, var_expr.full_span(), format!(
                "but inferred to have incompatible type '{}' here",
                node.expr_type.display_name(&ctx.heap)
            )
        );
    }

    /// Constructs a human interpretable error in the case that type inference
    /// on a polymorphic variable to a function call or literal construction 
    /// failed. This may only be caused by a pair of inference types (which may 
    /// come from arguments or the return type) having two different inferred 
    /// values for that polymorphic variable.
    ///
    /// So we find this pair and construct the error using it.
    ///
    /// We assume that the expression is a function call or a struct literal,
    /// and that an actual error has occurred.
    fn construct_poly_arg_error(
        ctx: &Ctx, poly_data: &PolyData, expr_id: ExpressionId
    ) -> ParseError {
        // Helper function to check for polymorph mismatch between two inference
        // types.
        fn has_poly_mismatch<'a>(type_a: &'a InferenceType, type_b: &'a InferenceType) -> Option<(u32, &'a [InferenceTypePart], &'a [InferenceTypePart])> {
            if !type_a.has_marker || !type_b.has_marker {
                return None
            }

            for (marker_a, section_a) in type_a.marker_iter() {
                for (marker_b, section_b) in type_b.marker_iter() {
                    if marker_a != marker_b {
                        // Not the same polymorphic variable
                        continue;
                    }

                    if !InferenceType::check_subtrees(section_a, 0, section_b, 0) {
                        // Not compatible
                        return Some((marker_a, section_a, section_b))
                    }
                }
            }

            None
        }

        // Helper function to check for polymorph mismatch between an inference
        // type and the polymorphic variables in the poly_data struct.
        fn has_explicit_poly_mismatch<'a>(
            poly_vars: &'a [InferenceType], arg: &'a InferenceType
        ) -> Option<(u32, &'a [InferenceTypePart], &'a [InferenceTypePart])> {
            for (marker, section) in arg.marker_iter() {
                debug_assert!((marker as usize) < poly_vars.len());
                let poly_section = &poly_vars[marker as usize].parts;
                if !InferenceType::check_subtrees(poly_section, 0, section, 0) {
                    return Some((marker, poly_section, section))
                }
            }

            None
        }

        // Helpers function to retrieve polyvar name and definition name
        fn get_poly_var_and_definition_name<'a>(ctx: &'a Ctx, poly_var_idx: u32, definition_id: DefinitionId) -> (&'a str, &'a str) {
            let definition = &ctx.heap[definition_id];
            let poly_var = definition.poly_vars()[poly_var_idx as usize].value.as_str();
            let func_name = definition.identifier().value.as_str();

            (poly_var, func_name)
        }

        // Helper function to construct initial error
        fn construct_main_error(ctx: &Ctx, poly_data: &PolyData, poly_var_idx: u32, expr: &Expression) -> ParseError {
            match expr {
                Expression::Call(expr) => {
                    let (poly_var, func_name) = get_poly_var_and_definition_name(ctx, poly_var_idx, poly_data.definition_id);
                    return ParseError::new_error_at_span(
                        &ctx.module().source, expr.func_span, format!(
                            "Conflicting type for polymorphic variable '{}' of '{}'",
                            poly_var, func_name
                        )
                    )
                },
                Expression::Literal(expr) => {
                    let (poly_var, type_name) = get_poly_var_and_definition_name(ctx, poly_var_idx, poly_data.definition_id);
                    return ParseError::new_error_at_span(
                        &ctx.module().source, expr.span, format!(
                            "Conflicting type for polymorphic variable '{}' of instantiation of '{}'",
                            poly_var, type_name
                        )
                    );
                },
                Expression::Select(expr) => {
                    let (poly_var, struct_name) = get_poly_var_and_definition_name(ctx, poly_var_idx, poly_data.definition_id);
                    let field_name = match &expr.kind {
                        SelectKind::StructField(v) => v,
                        SelectKind::TupleMember(_) => unreachable!(), // because we're constructing a polymorph error, and tuple access does not deal with polymorphs
                    };
                    return ParseError::new_error_at_span(
                        &ctx.module().source, expr.full_span, format!(
                            "Conflicting type for polymorphic variable '{}' while accessing field '{}' of '{}'",
                            poly_var, field_name.value.as_str(), struct_name
                        )
                    )
                }
                _ => unreachable!("called construct_poly_arg_error without an expected expression, got: {:?}", expr)
            }
        }

        // Actual checking
        let expr = &ctx.heap[expr_id];
        let (expr_args, expr_return_name) = match expr {
            Expression::Call(expr) => 
                (
                    expr.arguments.clone(),
                    "return type"
                ),
            Expression::Literal(expr) => {
                let expressions = match &expr.value {
                    Literal::Struct(v) => v.fields.iter()
                        .map(|f| f.value)
                        .collect(),
                    Literal::Enum(_) => Vec::new(),
                    Literal::Union(v) => v.values.clone(),
                    _ => unreachable!()
                };

                ( expressions, "literal" )
            },
            Expression::Select(expr) =>
                // Select expression uses the polymorphic variables of the 
                // struct it is accessing, so get the subject expression.
                (
                    vec![expr.subject],
                    "selected field"
                ),
            _ => unreachable!(),
        };

        // - check return type with itself
        if let Some((poly_idx, section_a, section_b)) = has_poly_mismatch(
            &poly_data.expr_types.returned, &poly_data.expr_types.returned
        ) {
            return construct_main_error(ctx, poly_data, poly_idx, expr)
                .with_info_at_span(
                    &ctx.module().source, expr.full_span(), format!(
                        "The {} inferred the conflicting types '{}' and '{}'",
                        expr_return_name,
                        InferenceType::partial_display_name(&ctx.heap, section_a),
                        InferenceType::partial_display_name(&ctx.heap, section_b)
                    )
                );
        }

        // - check arguments with each other argument and with return type
        for (arg_a_idx, arg_a) in poly_data.expr_types.associated.iter().enumerate() {
            for (arg_b_idx, arg_b) in poly_data.expr_types.associated.iter().enumerate() {
                if arg_b_idx > arg_a_idx {
                    break;
                }

                if let Some((poly_idx, section_a, section_b)) = has_poly_mismatch(&arg_a, &arg_b) {
                    let error = construct_main_error(ctx, poly_data, poly_idx, expr);
                    if arg_a_idx == arg_b_idx {
                        // Same argument
                        let arg = &ctx.heap[expr_args[arg_a_idx]];
                        return error.with_info_at_span(
                            &ctx.module().source, arg.full_span(), format!(
                                "This argument inferred the conflicting types '{}' and '{}'",
                                InferenceType::partial_display_name(&ctx.heap, section_a),
                                InferenceType::partial_display_name(&ctx.heap, section_b)
                            )
                        );
                    } else {
                        let arg_a = &ctx.heap[expr_args[arg_a_idx]];
                        let arg_b = &ctx.heap[expr_args[arg_b_idx]];
                        return error.with_info_at_span(
                            &ctx.module().source, arg_a.full_span(), format!(
                                "This argument inferred it to '{}'",
                                InferenceType::partial_display_name(&ctx.heap, section_a)
                            )
                        ).with_info_at_span(
                            &ctx.module().source, arg_b.full_span(), format!(
                                "While this argument inferred it to '{}'",
                                InferenceType::partial_display_name(&ctx.heap, section_b)
                            )
                        )
                    }
                }
            }

            // Check with return type
            if let Some((poly_idx, section_arg, section_ret)) = has_poly_mismatch(arg_a, &poly_data.expr_types.returned) {
                let arg = &ctx.heap[expr_args[arg_a_idx]];
                return construct_main_error(ctx, poly_data, poly_idx, expr)
                    .with_info_at_span(
                        &ctx.module().source, arg.full_span(), format!(
                            "This argument inferred it to '{}'",
                            InferenceType::partial_display_name(&ctx.heap, section_arg)
                        )
                    )
                    .with_info_at_span(
                        &ctx.module().source, expr.full_span(), format!(
                            "While the {} inferred it to '{}'",
                            expr_return_name,
                            InferenceType::partial_display_name(&ctx.heap, section_ret)
                        )
                    );
            }
        }

        // Now check against the explicitly specified polymorphic variables (if
        // any).
        for (arg_idx, arg) in poly_data.expr_types.associated.iter().enumerate() {
            if let Some((poly_idx, poly_section, arg_section)) = has_explicit_poly_mismatch(&poly_data.poly_vars, arg) {
                let arg = &ctx.heap[expr_args[arg_idx]];
                return construct_main_error(ctx, poly_data, poly_idx, expr)
                    .with_info_at_span(
                        &ctx.module().source, arg.full_span(), format!(
                            "The polymorphic variable has type '{}' (which might have been partially inferred) while the argument inferred it to '{}'",
                            InferenceType::partial_display_name(&ctx.heap, poly_section),
                            InferenceType::partial_display_name(&ctx.heap, arg_section)
                        )
                    );
            }
        }

        if let Some((poly_idx, poly_section, ret_section)) = has_explicit_poly_mismatch(&poly_data.poly_vars, &poly_data.expr_types.returned) {
            return construct_main_error(ctx, poly_data, poly_idx, expr)
                .with_info_at_span(
                    &ctx.module().source, expr.full_span(), format!(
                        "The polymorphic variable has type '{}' (which might have been partially inferred) while the {} inferred it to '{}'",
                        InferenceType::partial_display_name(&ctx.heap, poly_section),
                        expr_return_name,
                        InferenceType::partial_display_name(&ctx.heap, ret_section)
                    )
                )
        }

        unreachable!("construct_poly_arg_error without actual error found?")
    }
}

fn get_tuple_size_from_inference_type(inference_type: &InferenceType) -> Result<Option<u32>, ()> {
    for part in &inference_type.parts {
        if part.is_marker() { continue; }
        if !part.is_concrete() { break; }

        if let InferenceTypePart::Tuple(size) = part {
            return Ok(Some(*size));
        } else {
            return Err(()); // not a tuple!
        }
    }

    return Ok(None);
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::protocol::arena::Id;
    use InferenceTypePart as ITP;
    use InferenceType as IT;

    #[test]
    fn test_single_part_inference() {
        // lhs argument inferred from rhs
        let pairs = [
            (ITP::NumberLike, ITP::UInt8),
            (ITP::IntegerLike, ITP::SInt32),
            (ITP::Unknown, ITP::UInt64),
            (ITP::Unknown, ITP::Bool)
        ];
        for (lhs, rhs) in pairs.iter() {
            // Using infer-both
            let mut lhs_type = IT::new(false, false, vec![lhs.clone()]);
            let mut rhs_type = IT::new(false, true, vec![rhs.clone()]);
            let result = unsafe{ IT::infer_subtrees_for_both_types(
                &mut lhs_type, 0, &mut rhs_type, 0
            ) };
            assert_eq!(DualInferenceResult::First, result);
            assert_eq!(lhs_type.parts, rhs_type.parts);

            // Using infer-single
            let mut lhs_type = IT::new(false, false, vec![lhs.clone()]);
            let rhs_type = IT::new(false, true, vec![rhs.clone()]);
            let result = IT::infer_subtree_for_single_type(
                &mut lhs_type, 0, &rhs_type.parts, 0, false
            );
            assert_eq!(SingleInferenceResult::Modified, result);
            assert_eq!(lhs_type.parts, rhs_type.parts);
        }
    }

    #[test]
    fn test_multi_part_inference() {
        let pairs = [
            (vec![ITP::ArrayLike, ITP::NumberLike], vec![ITP::Slice, ITP::SInt8]),
            (vec![ITP::Unknown], vec![ITP::Input, ITP::Array, ITP::String, ITP::Character]),
            (vec![ITP::PortLike, ITP::SInt32], vec![ITP::Input, ITP::SInt32]),
            (vec![ITP::Unknown], vec![ITP::Output, ITP::SInt32]),
            (
                vec![ITP::Instance(Id::new(0), 2), ITP::Input, ITP::Unknown, ITP::Output, ITP::Unknown],
                vec![ITP::Instance(Id::new(0), 2), ITP::Input, ITP::Array, ITP::SInt32, ITP::Output, ITP::SInt32]
            )
        ];

        for (lhs, rhs) in pairs.iter() {
            let mut lhs_type = IT::new(false, false, lhs.clone());
            let mut rhs_type = IT::new(false, true, rhs.clone());
            let result = unsafe{ IT::infer_subtrees_for_both_types(
                &mut lhs_type, 0, &mut rhs_type, 0
            ) };
            assert_eq!(DualInferenceResult::First, result);
            assert_eq!(lhs_type.parts, rhs_type.parts);

            let mut lhs_type = IT::new(false, false, lhs.clone());
            let rhs_type = IT::new(false, true, rhs.clone());
            let result = IT::infer_subtree_for_single_type(
                &mut lhs_type, 0, &rhs_type.parts, 0, false
            );
            assert_eq!(SingleInferenceResult::Modified, result);
            assert_eq!(lhs_type.parts, rhs_type.parts)
        }
    }
}