1 files changed, 723 insertions, 0 deletions

diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
new file mode 100644
index 000000000..1e9f4b208
--- /dev/null
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -0,0 +1,723 @@
+//! Type inference, i.e. the process of walking through the code and determining
+//! the type of each expression and pattern.
+//!
+//! For type inference, compare the implementations in rustc (the various
+//! check_* methods in librustc_typeck/check/mod.rs are a good entry point) and
+//! IntelliJ-Rust (org.rust.lang.core.types.infer). Our entry point for
+//! inference here is the `infer` function, which infers the types of all
+//! expressions in a given function.
+//!
+//! During inference, types (i.e. the `Ty` struct) can contain type 'variables'
+//! which represent currently unknown types; as we walk through the expressions,
+//! we might determine that certain variables need to be equal to each other, or
+//! to certain types. To record this, we use the union-find implementation from
+//! the `ena` crate, which is extracted from rustc.
+
+use std::borrow::Cow;
+use std::mem;
+use std::ops::Index;
+use std::sync::Arc;
+
+use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
+use rustc_hash::FxHashMap;
+
+use hir_def::{
+    body::Body,
+    data::{ConstData, FunctionData},
+    expr::{BindingAnnotation, ExprId, PatId},
+    path::{known, Path},
+    resolver::{HasResolver, Resolver, TypeNs},
+    type_ref::{Mutability, TypeRef},
+    AdtId, AssocItemId, DefWithBodyId, FunctionId, StructFieldId, TypeAliasId, VariantId,
+};
+use hir_expand::{diagnostics::DiagnosticSink, name};
+use ra_arena::map::ArenaMap;
+use ra_prof::profile;
+use test_utils::tested_by;
+
+use super::{
+    primitive::{FloatTy, IntTy},
+    traits::{Guidance, Obligation, ProjectionPredicate, Solution},
+    ApplicationTy, InEnvironment, ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
+    TypeWalk, Uncertain,
+};
+use crate::{db::HirDatabase, infer::diagnostics::InferenceDiagnostic};
+
+macro_rules! ty_app {
+    ($ctor:pat, $param:pat) => {
+        crate::Ty::Apply(crate::ApplicationTy { ctor: $ctor, parameters: $param })
+    };
+    ($ctor:pat) => {
+        ty_app!($ctor, _)
+    };
+}
+
+mod unify;
+mod path;
+mod expr;
+mod pat;
+mod coerce;
+
+/// The entry point of type inference.
+pub fn infer_query(db: &impl HirDatabase, def: DefWithBodyId) -> Arc<InferenceResult> {
+    let _p = profile("infer_query");
+    let resolver = def.resolver(db);
+    let mut ctx = InferenceContext::new(db, def, resolver);
+
+    match def {
+        DefWithBodyId::ConstId(c) => ctx.collect_const(&db.const_data(c)),
+        DefWithBodyId::FunctionId(f) => ctx.collect_fn(&db.function_data(f)),
+        DefWithBodyId::StaticId(s) => ctx.collect_const(&db.static_data(s)),
+    }
+
+    ctx.infer_body();
+
+    Arc::new(ctx.resolve_all())
+}
+
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+enum ExprOrPatId {
+    ExprId(ExprId),
+    PatId(PatId),
+}
+
+impl_froms!(ExprOrPatId: ExprId, PatId);
+
+/// Binding modes inferred for patterns.
+/// https://doc.rust-lang.org/reference/patterns.html#binding-modes
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum BindingMode {
+    Move,
+    Ref(Mutability),
+}
+
+impl BindingMode {
+    pub fn convert(annotation: BindingAnnotation) -> BindingMode {
+        match annotation {
+            BindingAnnotation::Unannotated | BindingAnnotation::Mutable => BindingMode::Move,
+            BindingAnnotation::Ref => BindingMode::Ref(Mutability::Shared),
+            BindingAnnotation::RefMut => BindingMode::Ref(Mutability::Mut),
+        }
+    }
+}
+
+impl Default for BindingMode {
+    fn default() -> Self {
+        BindingMode::Move
+    }
+}
+
+/// A mismatch between an expected and an inferred type.
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct TypeMismatch {
+    pub expected: Ty,
+    pub actual: Ty,
+}
+
+/// The result of type inference: A mapping from expressions and patterns to types.
+#[derive(Clone, PartialEq, Eq, Debug, Default)]
+pub struct InferenceResult {
+    /// For each method call expr, records the function it resolves to.
+    method_resolutions: FxHashMap<ExprId, FunctionId>,
+    /// For each field access expr, records the field it resolves to.
+    field_resolutions: FxHashMap<ExprId, StructFieldId>,
+    /// For each field in record literal, records the field it resolves to.
+    record_field_resolutions: FxHashMap<ExprId, StructFieldId>,
+    /// For each struct literal, records the variant it resolves to.
+    variant_resolutions: FxHashMap<ExprOrPatId, VariantId>,
+    /// For each associated item record what it resolves to
+    assoc_resolutions: FxHashMap<ExprOrPatId, AssocItemId>,
+    diagnostics: Vec<InferenceDiagnostic>,
+    pub type_of_expr: ArenaMap<ExprId, Ty>,
+    pub type_of_pat: ArenaMap<PatId, Ty>,
+    pub(super) type_mismatches: ArenaMap<ExprId, TypeMismatch>,
+}
+
+impl InferenceResult {
+    pub fn method_resolution(&self, expr: ExprId) -> Option<FunctionId> {
+        self.method_resolutions.get(&expr).copied()
+    }
+    pub fn field_resolution(&self, expr: ExprId) -> Option<StructFieldId> {
+        self.field_resolutions.get(&expr).copied()
+    }
+    pub fn record_field_resolution(&self, expr: ExprId) -> Option<StructFieldId> {
+        self.record_field_resolutions.get(&expr).copied()
+    }
+    pub fn variant_resolution_for_expr(&self, id: ExprId) -> Option<VariantId> {
+        self.variant_resolutions.get(&id.into()).copied()
+    }
+    pub fn variant_resolution_for_pat(&self, id: PatId) -> Option<VariantId> {
+        self.variant_resolutions.get(&id.into()).copied()
+    }
+    pub fn assoc_resolutions_for_expr(&self, id: ExprId) -> Option<AssocItemId> {
+        self.assoc_resolutions.get(&id.into()).copied()
+    }
+    pub fn assoc_resolutions_for_pat(&self, id: PatId) -> Option<AssocItemId> {
+        self.assoc_resolutions.get(&id.into()).copied()
+    }
+    pub fn type_mismatch_for_expr(&self, expr: ExprId) -> Option<&TypeMismatch> {
+        self.type_mismatches.get(expr)
+    }
+    pub fn add_diagnostics(
+        &self,
+        db: &impl HirDatabase,
+        owner: FunctionId,
+        sink: &mut DiagnosticSink,
+    ) {
+        self.diagnostics.iter().for_each(|it| it.add_to(db, owner, sink))
+    }
+}
+
+impl Index<ExprId> for InferenceResult {
+    type Output = Ty;
+
+    fn index(&self, expr: ExprId) -> &Ty {
+        self.type_of_expr.get(expr).unwrap_or(&Ty::Unknown)
+    }
+}
+
+impl Index<PatId> for InferenceResult {
+    type Output = Ty;
+
+    fn index(&self, pat: PatId) -> &Ty {
+        self.type_of_pat.get(pat).unwrap_or(&Ty::Unknown)
+    }
+}
+
+/// The inference context contains all information needed during type inference.
+#[derive(Clone, Debug)]
+struct InferenceContext<'a, D: HirDatabase> {
+    db: &'a D,
+    owner: DefWithBodyId,
+    body: Arc<Body>,
+    resolver: Resolver,
+    var_unification_table: InPlaceUnificationTable<TypeVarId>,
+    trait_env: Arc<TraitEnvironment>,
+    obligations: Vec<Obligation>,
+    result: InferenceResult,
+    /// The return type of the function being inferred.
+    return_ty: Ty,
+
+    /// Impls of `CoerceUnsized` used in coercion.
+    /// (from_ty_ctor, to_ty_ctor) => coerce_generic_index
+    // FIXME: Use trait solver for this.
+    // Chalk seems unable to work well with builtin impl of `Unsize` now.
+    coerce_unsized_map: FxHashMap<(TypeCtor, TypeCtor), usize>,
+}
+
+impl<'a, D: HirDatabase> InferenceContext<'a, D> {
+    fn new(db: &'a D, owner: DefWithBodyId, resolver: Resolver) -> Self {
+        InferenceContext {
+            result: InferenceResult::default(),
+            var_unification_table: InPlaceUnificationTable::new(),
+            obligations: Vec::default(),
+            return_ty: Ty::Unknown, // set in collect_fn_signature
+            trait_env: TraitEnvironment::lower(db, &resolver),
+            coerce_unsized_map: Self::init_coerce_unsized_map(db, &resolver),
+            db,
+            owner,
+            body: db.body(owner.into()),
+            resolver,
+        }
+    }
+
+    fn resolve_all(mut self) -> InferenceResult {
+        // FIXME resolve obligations as well (use Guidance if necessary)
+        let mut result = mem::replace(&mut self.result, InferenceResult::default());
+        let mut tv_stack = Vec::new();
+        for ty in result.type_of_expr.values_mut() {
+            let resolved = self.resolve_ty_completely(&mut tv_stack, mem::replace(ty, Ty::Unknown));
+            *ty = resolved;
+        }
+        for ty in result.type_of_pat.values_mut() {
+            let resolved = self.resolve_ty_completely(&mut tv_stack, mem::replace(ty, Ty::Unknown));
+            *ty = resolved;
+        }
+        result
+    }
+
+    fn write_expr_ty(&mut self, expr: ExprId, ty: Ty) {
+        self.result.type_of_expr.insert(expr, ty);
+    }
+
+    fn write_method_resolution(&mut self, expr: ExprId, func: FunctionId) {
+        self.result.method_resolutions.insert(expr, func);
+    }
+
+    fn write_field_resolution(&mut self, expr: ExprId, field: StructFieldId) {
+        self.result.field_resolutions.insert(expr, field);
+    }
+
+    fn write_variant_resolution(&mut self, id: ExprOrPatId, variant: VariantId) {
+        self.result.variant_resolutions.insert(id, variant);
+    }
+
+    fn write_assoc_resolution(&mut self, id: ExprOrPatId, item: AssocItemId) {
+        self.result.assoc_resolutions.insert(id, item.into());
+    }
+
+    fn write_pat_ty(&mut self, pat: PatId, ty: Ty) {
+        self.result.type_of_pat.insert(pat, ty);
+    }
+
+    fn push_diagnostic(&mut self, diagnostic: InferenceDiagnostic) {
+        self.result.diagnostics.push(diagnostic);
+    }
+
+    fn make_ty(&mut self, type_ref: &TypeRef) -> Ty {
+        let ty = Ty::from_hir(
+            self.db,
+            // FIXME use right resolver for block
+            &self.resolver,
+            type_ref,
+        );
+        let ty = self.insert_type_vars(ty);
+        self.normalize_associated_types_in(ty)
+    }
+
+    fn unify_substs(&mut self, substs1: &Substs, substs2: &Substs, depth: usize) -> bool {
+        substs1.0.iter().zip(substs2.0.iter()).all(|(t1, t2)| self.unify_inner(t1, t2, depth))
+    }
+
+    fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        self.unify_inner(ty1, ty2, 0)
+    }
+
+    fn unify_inner(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
+        if depth > 1000 {
+            // prevent stackoverflows
+            panic!("infinite recursion in unification");
+        }
+        if ty1 == ty2 {
+            return true;
+        }
+        // try to resolve type vars first
+        let ty1 = self.resolve_ty_shallow(ty1);
+        let ty2 = self.resolve_ty_shallow(ty2);
+        match (&*ty1, &*ty2) {
+            (Ty::Apply(a_ty1), Ty::Apply(a_ty2)) if a_ty1.ctor == a_ty2.ctor => {
+                self.unify_substs(&a_ty1.parameters, &a_ty2.parameters, depth + 1)
+            }
+            _ => self.unify_inner_trivial(&ty1, &ty2),
+        }
+    }
+
+    fn unify_inner_trivial(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        match (ty1, ty2) {
+            (Ty::Unknown, _) | (_, Ty::Unknown) => true,
+
+            (Ty::Infer(InferTy::TypeVar(tv1)), Ty::Infer(InferTy::TypeVar(tv2)))
+            | (Ty::Infer(InferTy::IntVar(tv1)), Ty::Infer(InferTy::IntVar(tv2)))
+            | (Ty::Infer(InferTy::FloatVar(tv1)), Ty::Infer(InferTy::FloatVar(tv2)))
+            | (
+                Ty::Infer(InferTy::MaybeNeverTypeVar(tv1)),
+                Ty::Infer(InferTy::MaybeNeverTypeVar(tv2)),
+            ) => {
+                // both type vars are unknown since we tried to resolve them
+                self.var_unification_table.union(*tv1, *tv2);
+                true
+            }
+
+            // The order of MaybeNeverTypeVar matters here.
+            // Unifying MaybeNeverTypeVar and TypeVar will let the latter become MaybeNeverTypeVar.
+            // Unifying MaybeNeverTypeVar and other concrete type will let the former become it.
+            (Ty::Infer(InferTy::TypeVar(tv)), other)
+            | (other, Ty::Infer(InferTy::TypeVar(tv)))
+            | (Ty::Infer(InferTy::MaybeNeverTypeVar(tv)), other)
+            | (other, Ty::Infer(InferTy::MaybeNeverTypeVar(tv)))
+            | (Ty::Infer(InferTy::IntVar(tv)), other @ ty_app!(TypeCtor::Int(_)))
+            | (other @ ty_app!(TypeCtor::Int(_)), Ty::Infer(InferTy::IntVar(tv)))
+            | (Ty::Infer(InferTy::FloatVar(tv)), other @ ty_app!(TypeCtor::Float(_)))
+            | (other @ ty_app!(TypeCtor::Float(_)), Ty::Infer(InferTy::FloatVar(tv))) => {
+                // the type var is unknown since we tried to resolve it
+                self.var_unification_table.union_value(*tv, TypeVarValue::Known(other.clone()));
+                true
+            }
+
+            _ => false,
+        }
+    }
+
+    fn new_type_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::TypeVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    fn new_integer_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::IntVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    fn new_float_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::FloatVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    fn new_maybe_never_type_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::MaybeNeverTypeVar(
+            self.var_unification_table.new_key(TypeVarValue::Unknown),
+        ))
+    }
+
+    /// Replaces Ty::Unknown by a new type var, so we can maybe still infer it.
+    fn insert_type_vars_shallow(&mut self, ty: Ty) -> Ty {
+        match ty {
+            Ty::Unknown => self.new_type_var(),
+            Ty::Apply(ApplicationTy { ctor: TypeCtor::Int(Uncertain::Unknown), .. }) => {
+                self.new_integer_var()
+            }
+            Ty::Apply(ApplicationTy { ctor: TypeCtor::Float(Uncertain::Unknown), .. }) => {
+                self.new_float_var()
+            }
+            _ => ty,
+        }
+    }
+
+    fn insert_type_vars(&mut self, ty: Ty) -> Ty {
+        ty.fold(&mut |ty| self.insert_type_vars_shallow(ty))
+    }
+
+    fn resolve_obligations_as_possible(&mut self) {
+        let obligations = mem::replace(&mut self.obligations, Vec::new());
+        for obligation in obligations {
+            let in_env = InEnvironment::new(self.trait_env.clone(), obligation.clone());
+            let canonicalized = self.canonicalizer().canonicalize_obligation(in_env);
+            let solution = self
+                .db
+                .trait_solve(self.resolver.krate().unwrap().into(), canonicalized.value.clone());
+
+            match solution {
+                Some(Solution::Unique(substs)) => {
+                    canonicalized.apply_solution(self, substs.0);
+                }
+                Some(Solution::Ambig(Guidance::Definite(substs))) => {
+                    canonicalized.apply_solution(self, substs.0);
+                    self.obligations.push(obligation);
+                }
+                Some(_) => {
+                    // FIXME use this when trying to resolve everything at the end
+                    self.obligations.push(obligation);
+                }
+                None => {
+                    // FIXME obligation cannot be fulfilled => diagnostic
+                }
+            };
+        }
+    }
+
+    /// Resolves the type as far as currently possible, replacing type variables
+    /// by their known types. All types returned by the infer_* functions should
+    /// be resolved as far as possible, i.e. contain no type variables with
+    /// known type.
+    fn resolve_ty_as_possible(&mut self, tv_stack: &mut Vec<TypeVarId>, ty: Ty) -> Ty {
+        self.resolve_obligations_as_possible();
+
+        ty.fold(&mut |ty| match ty {
+            Ty::Infer(tv) => {
+                let inner = tv.to_inner();
+                if tv_stack.contains(&inner) {
+                    tested_by!(type_var_cycles_resolve_as_possible);
+                    // recursive type
+                    return tv.fallback_value();
+                }
+                if let Some(known_ty) =
+                    self.var_unification_table.inlined_probe_value(inner).known()
+                {
+                    // known_ty may contain other variables that are known by now
+                    tv_stack.push(inner);
+                    let result = self.resolve_ty_as_possible(tv_stack, known_ty.clone());
+                    tv_stack.pop();
+                    result
+                } else {
+                    ty
+                }
+            }
+            _ => ty,
+        })
+    }
+
+    /// If `ty` is a type variable with known type, returns that type;
+    /// otherwise, return ty.
+    fn resolve_ty_shallow<'b>(&mut self, ty: &'b Ty) -> Cow<'b, Ty> {
+        let mut ty = Cow::Borrowed(ty);
+        // The type variable could resolve to a int/float variable. Hence try
+        // resolving up to three times; each type of variable shouldn't occur
+        // more than once
+        for i in 0..3 {
+            if i > 0 {
+                tested_by!(type_var_resolves_to_int_var);
+            }
+            match &*ty {
+                Ty::Infer(tv) => {
+                    let inner = tv.to_inner();
+                    match self.var_unification_table.inlined_probe_value(inner).known() {
+                        Some(known_ty) => {
+                            // The known_ty can't be a type var itself
+                            ty = Cow::Owned(known_ty.clone());
+                        }
+                        _ => return ty,
+                    }
+                }
+                _ => return ty,
+            }
+        }
+        log::error!("Inference variable still not resolved: {:?}", ty);
+        ty
+    }
+
+    /// Recurses through the given type, normalizing associated types mentioned
+    /// in it by replacing them by type variables and registering obligations to
+    /// resolve later. This should be done once for every type we get from some
+    /// type annotation (e.g. from a let type annotation, field type or function
+    /// call). `make_ty` handles this already, but e.g. for field types we need
+    /// to do it as well.
+    fn normalize_associated_types_in(&mut self, ty: Ty) -> Ty {
+        let ty = self.resolve_ty_as_possible(&mut vec![], ty);
+        ty.fold(&mut |ty| match ty {
+            Ty::Projection(proj_ty) => self.normalize_projection_ty(proj_ty),
+            _ => ty,
+        })
+    }
+
+    fn normalize_projection_ty(&mut self, proj_ty: ProjectionTy) -> Ty {
+        let var = self.new_type_var();
+        let predicate = ProjectionPredicate { projection_ty: proj_ty, ty: var.clone() };
+        let obligation = Obligation::Projection(predicate);
+        self.obligations.push(obligation);
+        var
+    }
+
+    /// Resolves the type completely; type variables without known type are
+    /// replaced by Ty::Unknown.
+    fn resolve_ty_completely(&mut self, tv_stack: &mut Vec<TypeVarId>, ty: Ty) -> Ty {
+        ty.fold(&mut |ty| match ty {
+            Ty::Infer(tv) => {
+                let inner = tv.to_inner();
+                if tv_stack.contains(&inner) {
+                    tested_by!(type_var_cycles_resolve_completely);
+                    // recursive type
+                    return tv.fallback_value();
+                }
+                if let Some(known_ty) =
+                    self.var_unification_table.inlined_probe_value(inner).known()
+                {
+                    // known_ty may contain other variables that are known by now
+                    tv_stack.push(inner);
+                    let result = self.resolve_ty_completely(tv_stack, known_ty.clone());
+                    tv_stack.pop();
+                    result
+                } else {
+                    tv.fallback_value()
+                }
+            }
+            _ => ty,
+        })
+    }
+
+    fn resolve_variant(&mut self, path: Option<&Path>) -> (Ty, Option<VariantId>) {
+        let path = match path {
+            Some(path) => path,
+            None => return (Ty::Unknown, None),
+        };
+        let resolver = &self.resolver;
+        // FIXME: this should resolve assoc items as well, see this example:
+        // https://play.rust-lang.org/?gist=087992e9e22495446c01c0d4e2d69521
+        match resolver.resolve_path_in_type_ns_fully(self.db, &path) {
+            Some(TypeNs::AdtId(AdtId::StructId(strukt))) => {
+                let substs = Ty::substs_from_path(self.db, resolver, path, strukt.into());
+                let ty = self.db.ty(strukt.into());
+                let ty = self.insert_type_vars(ty.apply_substs(substs));
+                (ty, Some(strukt.into()))
+            }
+            Some(TypeNs::EnumVariantId(var)) => {
+                let substs = Ty::substs_from_path(self.db, resolver, path, var.into());
+                let ty = self.db.ty(var.parent.into());
+                let ty = self.insert_type_vars(ty.apply_substs(substs));
+                (ty, Some(var.into()))
+            }
+            Some(_) | None => (Ty::Unknown, None),
+        }
+    }
+
+    fn collect_const(&mut self, data: &ConstData) {
+        self.return_ty = self.make_ty(&data.type_ref);
+    }
+
+    fn collect_fn(&mut self, data: &FunctionData) {
+        let body = Arc::clone(&self.body); // avoid borrow checker problem
+        for (type_ref, pat) in data.params.iter().zip(body.params.iter()) {
+            let ty = self.make_ty(type_ref);
+
+            self.infer_pat(*pat, &ty, BindingMode::default());
+        }
+        self.return_ty = self.make_ty(&data.ret_type);
+    }
+
+    fn infer_body(&mut self) {
+        self.infer_expr(self.body.body_expr, &Expectation::has_type(self.return_ty.clone()));
+    }
+
+    fn resolve_into_iter_item(&self) -> Option<TypeAliasId> {
+        let path = known::std_iter_into_iterator();
+        let trait_ = self.resolver.resolve_known_trait(self.db, &path)?;
+        self.db.trait_data(trait_).associated_type_by_name(&name::ITEM_TYPE)
+    }
+
+    fn resolve_ops_try_ok(&self) -> Option<TypeAliasId> {
+        let path = known::std_ops_try();
+        let trait_ = self.resolver.resolve_known_trait(self.db, &path)?;
+        self.db.trait_data(trait_).associated_type_by_name(&name::OK_TYPE)
+    }
+
+    fn resolve_future_future_output(&self) -> Option<TypeAliasId> {
+        let path = known::std_future_future();
+        let trait_ = self.resolver.resolve_known_trait(self.db, &path)?;
+        self.db.trait_data(trait_).associated_type_by_name(&name::OUTPUT_TYPE)
+    }
+
+    fn resolve_boxed_box(&self) -> Option<AdtId> {
+        let path = known::std_boxed_box();
+        let struct_ = self.resolver.resolve_known_struct(self.db, &path)?;
+        Some(struct_.into())
+    }
+}
+
+/// The ID of a type variable.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+pub struct TypeVarId(pub(super) u32);
+
+impl UnifyKey for TypeVarId {
+    type Value = TypeVarValue;
+
+    fn index(&self) -> u32 {
+        self.0
+    }
+
+    fn from_index(i: u32) -> Self {
+        TypeVarId(i)
+    }
+
+    fn tag() -> &'static str {
+        "TypeVarId"
+    }
+}
+
+/// The value of a type variable: either we already know the type, or we don't
+/// know it yet.
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum TypeVarValue {
+    Known(Ty),
+    Unknown,
+}
+
+impl TypeVarValue {
+    fn known(&self) -> Option<&Ty> {
+        match self {
+            TypeVarValue::Known(ty) => Some(ty),
+            TypeVarValue::Unknown => None,
+        }
+    }
+}
+
+impl UnifyValue for TypeVarValue {
+    type Error = NoError;
+
+    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
+        match (value1, value2) {
+            // We should never equate two type variables, both of which have
+            // known types. Instead, we recursively equate those types.
+            (TypeVarValue::Known(t1), TypeVarValue::Known(t2)) => panic!(
+                "equating two type variables, both of which have known types: {:?} and {:?}",
+                t1, t2
+            ),
+
+            // If one side is known, prefer that one.
+            (TypeVarValue::Known(..), TypeVarValue::Unknown) => Ok(value1.clone()),
+            (TypeVarValue::Unknown, TypeVarValue::Known(..)) => Ok(value2.clone()),
+
+            (TypeVarValue::Unknown, TypeVarValue::Unknown) => Ok(TypeVarValue::Unknown),
+        }
+    }
+}
+
+/// The kinds of placeholders we need during type inference. There's separate
+/// values for general types, and for integer and float variables. The latter
+/// two are used for inference of literal values (e.g. `100` could be one of
+/// several integer types).
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
+pub enum InferTy {
+    TypeVar(TypeVarId),
+    IntVar(TypeVarId),
+    FloatVar(TypeVarId),
+    MaybeNeverTypeVar(TypeVarId),
+}
+
+impl InferTy {
+    fn to_inner(self) -> TypeVarId {
+        match self {
+            InferTy::TypeVar(ty)
+            | InferTy::IntVar(ty)
+            | InferTy::FloatVar(ty)
+            | InferTy::MaybeNeverTypeVar(ty) => ty,
+        }
+    }
+
+    fn fallback_value(self) -> Ty {
+        match self {
+            InferTy::TypeVar(..) => Ty::Unknown,
+            InferTy::IntVar(..) => Ty::simple(TypeCtor::Int(Uncertain::Known(IntTy::i32()))),
+            InferTy::FloatVar(..) => Ty::simple(TypeCtor::Float(Uncertain::Known(FloatTy::f64()))),
+            InferTy::MaybeNeverTypeVar(..) => Ty::simple(TypeCtor::Never),
+        }
+    }
+}
+
+/// When inferring an expression, we propagate downward whatever type hint we
+/// are able in the form of an `Expectation`.
+#[derive(Clone, PartialEq, Eq, Debug)]
+struct Expectation {
+    ty: Ty,
+    // FIXME: In some cases, we need to be aware whether the expectation is that
+    // the type match exactly what we passed, or whether it just needs to be
+    // coercible to the expected type. See Expectation::rvalue_hint in rustc.
+}
+
+impl Expectation {
+    /// The expectation that the type of the expression needs to equal the given
+    /// type.
+    fn has_type(ty: Ty) -> Self {
+        Expectation { ty }
+    }
+
+    /// This expresses no expectation on the type.
+    fn none() -> Self {
+        Expectation { ty: Ty::Unknown }
+    }
+}
+
+mod diagnostics {
+    use hir_def::{expr::ExprId, FunctionId, HasSource, Lookup};
+    use hir_expand::diagnostics::DiagnosticSink;
+
+    use crate::{db::HirDatabase, diagnostics::NoSuchField};
+
+    #[derive(Debug, PartialEq, Eq, Clone)]
+    pub(super) enum InferenceDiagnostic {
+        NoSuchField { expr: ExprId, field: usize },
+    }
+
+    impl InferenceDiagnostic {
+        pub(super) fn add_to(
+            &self,
+            db: &impl HirDatabase,
+            owner: FunctionId,
+            sink: &mut DiagnosticSink,
+        ) {
+            match self {
+                InferenceDiagnostic::NoSuchField { expr, field } => {
+                    let file = owner.lookup(db).source(db).file_id;
+                    let (_, source_map) = db.body_with_source_map(owner.into());
+                    let field = source_map.field_syntax(*expr, *field);
+                    sink.push(NoSuchField { file, field })
+                }
+            }
+        }
+    }
+}