5 files changed, 2072 insertions, 0 deletions

diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
new file mode 100644
index 000000000..32c7c57cd
--- /dev/null
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -0,0 +1,197 @@
+//! Coercion logic. Coercions are certain type conversions that can implicitly
+//! happen in certain places, e.g. weakening `&mut` to `&` or deref coercions
+//! like going from `&Vec<T>` to `&[T]`.
+//!
+//! See: https://doc.rust-lang.org/nomicon/coercions.html
+
+use hir_def::{lang_item::LangItemTarget, type_ref::Mutability};
+use test_utils::mark;
+
+use crate::{autoderef, traits::Solution, Obligation, Substs, TraitRef, Ty, TypeCtor};
+
+use super::{unify::TypeVarValue, InEnvironment, InferTy, InferenceContext};
+
+impl<'a> InferenceContext<'a> {
+    /// Unify two types, but may coerce the first one to the second one
+    /// using "implicit coercion rules" if needed.
+    pub(super) fn coerce(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+        let from_ty = self.resolve_ty_shallow(from_ty).into_owned();
+        let to_ty = self.resolve_ty_shallow(to_ty);
+        self.coerce_inner(from_ty, &to_ty)
+    }
+
+    /// Merge two types from different branches, with possible coercion.
+    ///
+    /// Mostly this means trying to coerce one to the other, but
+    ///  - if we have two function types for different functions, we need to
+    ///    coerce both to function pointers;
+    ///  - if we were concerned with lifetime subtyping, we'd need to look for a
+    ///    least upper bound.
+    pub(super) fn coerce_merge_branch(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
+        if self.coerce(ty1, ty2) {
+            ty2.clone()
+        } else if self.coerce(ty2, ty1) {
+            ty1.clone()
+        } else {
+            if let (ty_app!(TypeCtor::FnDef(_)), ty_app!(TypeCtor::FnDef(_))) = (ty1, ty2) {
+                mark::hit!(coerce_fn_reification);
+                // Special case: two function types. Try to coerce both to
+                // pointers to have a chance at getting a match. See
+                // https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
+                let sig1 = ty1.callable_sig(self.db).expect("FnDef without callable sig");
+                let sig2 = ty2.callable_sig(self.db).expect("FnDef without callable sig");
+                let ptr_ty1 = Ty::fn_ptr(sig1);
+                let ptr_ty2 = Ty::fn_ptr(sig2);
+                self.coerce_merge_branch(&ptr_ty1, &ptr_ty2)
+            } else {
+                mark::hit!(coerce_merge_fail_fallback);
+                ty1.clone()
+            }
+        }
+    }
+
+    fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
+        match (&from_ty, to_ty) {
+            // Never type will make type variable to fallback to Never Type instead of Unknown.
+            (ty_app!(TypeCtor::Never), Ty::Infer(InferTy::TypeVar(tv))) => {
+                let var = self.table.new_maybe_never_type_var();
+                self.table.var_unification_table.union_value(*tv, TypeVarValue::Known(var));
+                return true;
+            }
+            (ty_app!(TypeCtor::Never), _) => return true,
+
+            // Trivial cases, this should go after `never` check to
+            // avoid infer result type to be never
+            _ => {
+                if self.table.unify_inner_trivial(&from_ty, &to_ty, 0) {
+                    return true;
+                }
+            }
+        }
+
+        // Pointer weakening and function to pointer
+        match (&mut from_ty, to_ty) {
+            // `*mut T`, `&mut T, `&T`` -> `*const T`
+            // `&mut T` -> `&T`
+            // `&mut T` -> `*mut T`
+            (ty_app!(c1@TypeCtor::RawPtr(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::Ref(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(Mutability::Mut)), ty_app!(c2@TypeCtor::RawPtr(_))) => {
+                *c1 = *c2;
+            }
+
+            // Illegal mutablity conversion
+            (
+                ty_app!(TypeCtor::RawPtr(Mutability::Shared)),
+                ty_app!(TypeCtor::RawPtr(Mutability::Mut)),
+            )
+            | (
+                ty_app!(TypeCtor::Ref(Mutability::Shared)),
+                ty_app!(TypeCtor::Ref(Mutability::Mut)),
+            ) => return false,
+
+            // `{function_type}` -> `fn()`
+            (ty_app!(TypeCtor::FnDef(_)), ty_app!(TypeCtor::FnPtr { .. })) => {
+                match from_ty.callable_sig(self.db) {
+                    None => return false,
+                    Some(sig) => {
+                        from_ty = Ty::fn_ptr(sig);
+                    }
+                }
+            }
+
+            (ty_app!(TypeCtor::Closure { .. }, params), ty_app!(TypeCtor::FnPtr { .. })) => {
+                from_ty = params[0].clone();
+            }
+
+            _ => {}
+        }
+
+        if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
+            return ret;
+        }
+
+        // Auto Deref if cannot coerce
+        match (&from_ty, to_ty) {
+            // FIXME: DerefMut
+            (ty_app!(TypeCtor::Ref(_), st1), ty_app!(TypeCtor::Ref(_), st2)) => {
+                self.unify_autoderef_behind_ref(&st1[0], &st2[0])
+            }
+
+            // Otherwise, normal unify
+            _ => self.unify(&from_ty, to_ty),
+        }
+    }
+
+    /// Coerce a type using `from_ty: CoerceUnsized<ty_ty>`
+    ///
+    /// See: https://doc.rust-lang.org/nightly/std/marker/trait.CoerceUnsized.html
+    fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> Option<bool> {
+        let krate = self.resolver.krate().unwrap();
+        let coerce_unsized_trait = match self.db.lang_item(krate, "coerce_unsized".into()) {
+            Some(LangItemTarget::TraitId(trait_)) => trait_,
+            _ => return None,
+        };
+
+        let generic_params = crate::utils::generics(self.db.upcast(), coerce_unsized_trait.into());
+        if generic_params.len() != 2 {
+            // The CoerceUnsized trait should have two generic params: Self and T.
+            return None;
+        }
+
+        let substs = Substs::build_for_generics(&generic_params)
+            .push(from_ty.clone())
+            .push(to_ty.clone())
+            .build();
+        let trait_ref = TraitRef { trait_: coerce_unsized_trait, substs };
+        let goal = InEnvironment::new(self.trait_env.clone(), Obligation::Trait(trait_ref));
+
+        let canonicalizer = self.canonicalizer();
+        let canonicalized = canonicalizer.canonicalize_obligation(goal);
+
+        let solution = self.db.trait_solve(krate, canonicalized.value.clone())?;
+
+        match solution {
+            Solution::Unique(v) => {
+                canonicalized.apply_solution(self, v.0);
+            }
+            _ => return None,
+        };
+
+        Some(true)
+    }
+
+    /// Unify `from_ty` to `to_ty` with optional auto Deref
+    ///
+    /// Note that the parameters are already stripped the outer reference.
+    fn unify_autoderef_behind_ref(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+        let canonicalized = self.canonicalizer().canonicalize_ty(from_ty.clone());
+        let to_ty = self.resolve_ty_shallow(&to_ty);
+        // FIXME: Auto DerefMut
+        for derefed_ty in autoderef::autoderef(
+            self.db,
+            self.resolver.krate(),
+            InEnvironment {
+                value: canonicalized.value.clone(),
+                environment: self.trait_env.clone(),
+            },
+        ) {
+            let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
+            match (&*self.resolve_ty_shallow(&derefed_ty), &*to_ty) {
+                // Stop when constructor matches.
+                (ty_app!(from_ctor, st1), ty_app!(to_ctor, st2)) if from_ctor == to_ctor => {
+                    // It will not recurse to `coerce`.
+                    return self.table.unify_substs(st1, st2, 0);
+                }
+                _ => {
+                    if self.table.unify_inner_trivial(&derefed_ty, &to_ty, 0) {
+                        return true;
+                    }
+                }
+            }
+        }
+
+        false
+    }
+}
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
new file mode 100644
index 000000000..a2f849d02
--- /dev/null
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -0,0 +1,873 @@
+//! Type inference for expressions.
+
+use std::iter::{repeat, repeat_with};
+use std::{mem, sync::Arc};
+
+use hir_def::{
+    builtin_type::Signedness,
+    expr::{Array, BinaryOp, Expr, ExprId, Literal, Statement, UnaryOp},
+    path::{GenericArg, GenericArgs},
+    resolver::resolver_for_expr,
+    AdtId, AssocContainerId, FieldId, Lookup,
+};
+use hir_expand::name::{name, Name};
+use syntax::ast::RangeOp;
+
+use crate::{
+    autoderef, method_resolution, op,
+    traits::{FnTrait, InEnvironment},
+    utils::{generics, variant_data, Generics},
+    ApplicationTy, Binders, CallableDefId, InferTy, IntTy, Mutability, Obligation, Rawness, Substs,
+    TraitRef, Ty, TypeCtor,
+};
+
+use super::{
+    find_breakable, BindingMode, BreakableContext, Diverges, Expectation, InferenceContext,
+    InferenceDiagnostic, TypeMismatch,
+};
+
+impl<'a> InferenceContext<'a> {
+    pub(super) fn infer_expr(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
+        let ty = self.infer_expr_inner(tgt_expr, expected);
+        if ty.is_never() {
+            // Any expression that produces a value of type `!` must have diverged
+            self.diverges = Diverges::Always;
+        }
+        let could_unify = self.unify(&ty, &expected.ty);
+        if !could_unify {
+            self.result.type_mismatches.insert(
+                tgt_expr,
+                TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() },
+            );
+        }
+        self.resolve_ty_as_possible(ty)
+    }
+
+    /// Infer type of expression with possibly implicit coerce to the expected type.
+    /// Return the type after possible coercion.
+    pub(super) fn infer_expr_coerce(&mut self, expr: ExprId, expected: &Expectation) -> Ty {
+        let ty = self.infer_expr_inner(expr, &expected);
+        let ty = if !self.coerce(&ty, &expected.coercion_target()) {
+            self.result
+                .type_mismatches
+                .insert(expr, TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() });
+            // Return actual type when type mismatch.
+            // This is needed for diagnostic when return type mismatch.
+            ty
+        } else if expected.coercion_target() == &Ty::Unknown {
+            ty
+        } else {
+            expected.ty.clone()
+        };
+
+        self.resolve_ty_as_possible(ty)
+    }
+
+    fn callable_sig_from_fn_trait(&mut self, ty: &Ty, num_args: usize) -> Option<(Vec<Ty>, Ty)> {
+        let krate = self.resolver.krate()?;
+        let fn_once_trait = FnTrait::FnOnce.get_id(self.db, krate)?;
+        let output_assoc_type =
+            self.db.trait_data(fn_once_trait).associated_type_by_name(&name![Output])?;
+        let generic_params = generics(self.db.upcast(), fn_once_trait.into());
+        if generic_params.len() != 2 {
+            return None;
+        }
+
+        let mut param_builder = Substs::builder(num_args);
+        let mut arg_tys = vec![];
+        for _ in 0..num_args {
+            let arg = self.table.new_type_var();
+            param_builder = param_builder.push(arg.clone());
+            arg_tys.push(arg);
+        }
+        let parameters = param_builder.build();
+        let arg_ty = Ty::Apply(ApplicationTy {
+            ctor: TypeCtor::Tuple { cardinality: num_args as u16 },
+            parameters,
+        });
+        let substs =
+            Substs::build_for_generics(&generic_params).push(ty.clone()).push(arg_ty).build();
+
+        let trait_env = Arc::clone(&self.trait_env);
+        let implements_fn_trait =
+            Obligation::Trait(TraitRef { trait_: fn_once_trait, substs: substs.clone() });
+        let goal = self.canonicalizer().canonicalize_obligation(InEnvironment {
+            value: implements_fn_trait.clone(),
+            environment: trait_env,
+        });
+        if self.db.trait_solve(krate, goal.value).is_some() {
+            self.obligations.push(implements_fn_trait);
+            let output_proj_ty =
+                crate::ProjectionTy { associated_ty: output_assoc_type, parameters: substs };
+            let return_ty = self.normalize_projection_ty(output_proj_ty);
+            Some((arg_tys, return_ty))
+        } else {
+            None
+        }
+    }
+
+    pub fn callable_sig(&mut self, ty: &Ty, num_args: usize) -> Option<(Vec<Ty>, Ty)> {
+        match ty.callable_sig(self.db) {
+            Some(sig) => Some((sig.params().to_vec(), sig.ret().clone())),
+            None => self.callable_sig_from_fn_trait(ty, num_args),
+        }
+    }
+
+    fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
+        let body = Arc::clone(&self.body); // avoid borrow checker problem
+        let ty = match &body[tgt_expr] {
+            Expr::Missing => Ty::Unknown,
+            Expr::If { condition, then_branch, else_branch } => {
+                // if let is desugared to match, so this is always simple if
+                self.infer_expr(*condition, &Expectation::has_type(Ty::simple(TypeCtor::Bool)));
+
+                let condition_diverges = mem::replace(&mut self.diverges, Diverges::Maybe);
+                let mut both_arms_diverge = Diverges::Always;
+
+                let then_ty = self.infer_expr_inner(*then_branch, &expected);
+                both_arms_diverge &= mem::replace(&mut self.diverges, Diverges::Maybe);
+                let else_ty = match else_branch {
+                    Some(else_branch) => self.infer_expr_inner(*else_branch, &expected),
+                    None => Ty::unit(),
+                };
+                both_arms_diverge &= self.diverges;
+
+                self.diverges = condition_diverges | both_arms_diverge;
+
+                self.coerce_merge_branch(&then_ty, &else_ty)
+            }
+            Expr::Block { statements, tail, .. } => {
+                // FIXME: Breakable block inference
+                self.infer_block(statements, *tail, expected)
+            }
+            Expr::Unsafe { body } => self.infer_expr(*body, expected),
+            Expr::TryBlock { body } => {
+                let _inner = self.infer_expr(*body, expected);
+                // FIXME should be std::result::Result<{inner}, _>
+                Ty::Unknown
+            }
+            Expr::Loop { body, label } => {
+                self.breakables.push(BreakableContext {
+                    may_break: false,
+                    break_ty: self.table.new_type_var(),
+                    label: label.clone(),
+                });
+                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+
+                let ctxt = self.breakables.pop().expect("breakable stack broken");
+                if ctxt.may_break {
+                    self.diverges = Diverges::Maybe;
+                }
+
+                if ctxt.may_break {
+                    ctxt.break_ty
+                } else {
+                    Ty::simple(TypeCtor::Never)
+                }
+            }
+            Expr::While { condition, body, label } => {
+                self.breakables.push(BreakableContext {
+                    may_break: false,
+                    break_ty: Ty::Unknown,
+                    label: label.clone(),
+                });
+                // while let is desugared to a match loop, so this is always simple while
+                self.infer_expr(*condition, &Expectation::has_type(Ty::simple(TypeCtor::Bool)));
+                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+                let _ctxt = self.breakables.pop().expect("breakable stack broken");
+                // the body may not run, so it diverging doesn't mean we diverge
+                self.diverges = Diverges::Maybe;
+                Ty::unit()
+            }
+            Expr::For { iterable, body, pat, label } => {
+                let iterable_ty = self.infer_expr(*iterable, &Expectation::none());
+
+                self.breakables.push(BreakableContext {
+                    may_break: false,
+                    break_ty: Ty::Unknown,
+                    label: label.clone(),
+                });
+                let pat_ty =
+                    self.resolve_associated_type(iterable_ty, self.resolve_into_iter_item());
+
+                self.infer_pat(*pat, &pat_ty, BindingMode::default());
+
+                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+                let _ctxt = self.breakables.pop().expect("breakable stack broken");
+                // the body may not run, so it diverging doesn't mean we diverge
+                self.diverges = Diverges::Maybe;
+                Ty::unit()
+            }
+            Expr::Lambda { body, args, ret_type, arg_types } => {
+                assert_eq!(args.len(), arg_types.len());
+
+                let mut sig_tys = Vec::new();
+
+                // collect explicitly written argument types
+                for arg_type in arg_types.iter() {
+                    let arg_ty = if let Some(type_ref) = arg_type {
+                        self.make_ty(type_ref)
+                    } else {
+                        self.table.new_type_var()
+                    };
+                    sig_tys.push(arg_ty);
+                }
+
+                // add return type
+                let ret_ty = match ret_type {
+                    Some(type_ref) => self.make_ty(type_ref),
+                    None => self.table.new_type_var(),
+                };
+                sig_tys.push(ret_ty.clone());
+                let sig_ty = Ty::apply(
+                    TypeCtor::FnPtr { num_args: sig_tys.len() as u16 - 1, is_varargs: false },
+                    Substs(sig_tys.clone().into()),
+                );
+                let closure_ty =
+                    Ty::apply_one(TypeCtor::Closure { def: self.owner, expr: tgt_expr }, sig_ty);
+
+                // Eagerly try to relate the closure type with the expected
+                // type, otherwise we often won't have enough information to
+                // infer the body.
+                self.coerce(&closure_ty, &expected.ty);
+
+                // Now go through the argument patterns
+                for (arg_pat, arg_ty) in args.iter().zip(sig_tys) {
+                    let resolved = self.resolve_ty_as_possible(arg_ty);
+                    self.infer_pat(*arg_pat, &resolved, BindingMode::default());
+                }
+
+                let prev_diverges = mem::replace(&mut self.diverges, Diverges::Maybe);
+                let prev_ret_ty = mem::replace(&mut self.return_ty, ret_ty.clone());
+
+                self.infer_expr_coerce(*body, &Expectation::has_type(ret_ty));
+
+                self.diverges = prev_diverges;
+                self.return_ty = prev_ret_ty;
+
+                closure_ty
+            }
+            Expr::Call { callee, args } => {
+                let callee_ty = self.infer_expr(*callee, &Expectation::none());
+                let canonicalized = self.canonicalizer().canonicalize_ty(callee_ty.clone());
+                let mut derefs = autoderef(
+                    self.db,
+                    self.resolver.krate(),
+                    InEnvironment {
+                        value: canonicalized.value.clone(),
+                        environment: self.trait_env.clone(),
+                    },
+                );
+                let (param_tys, ret_ty): (Vec<Ty>, Ty) = derefs
+                    .find_map(|callee_deref_ty| {
+                        self.callable_sig(
+                            &canonicalized.decanonicalize_ty(callee_deref_ty.value),
+                            args.len(),
+                        )
+                    })
+                    .unwrap_or((Vec::new(), Ty::Unknown));
+                self.register_obligations_for_call(&callee_ty);
+                self.check_call_arguments(args, &param_tys);
+                self.normalize_associated_types_in(ret_ty)
+            }
+            Expr::MethodCall { receiver, args, method_name, generic_args } => self
+                .infer_method_call(tgt_expr, *receiver, &args, &method_name, generic_args.as_ref()),
+            Expr::Match { expr, arms } => {
+                let input_ty = self.infer_expr(*expr, &Expectation::none());
+
+                let mut result_ty = if arms.is_empty() {
+                    Ty::simple(TypeCtor::Never)
+                } else {
+                    self.table.new_type_var()
+                };
+
+                let matchee_diverges = self.diverges;
+                let mut all_arms_diverge = Diverges::Always;
+
+                for arm in arms {
+                    self.diverges = Diverges::Maybe;
+                    let _pat_ty = self.infer_pat(arm.pat, &input_ty, BindingMode::default());
+                    if let Some(guard_expr) = arm.guard {
+                        self.infer_expr(
+                            guard_expr,
+                            &Expectation::has_type(Ty::simple(TypeCtor::Bool)),
+                        );
+                    }
+
+                    let arm_ty = self.infer_expr_inner(arm.expr, &expected);
+                    all_arms_diverge &= self.diverges;
+                    result_ty = self.coerce_merge_branch(&result_ty, &arm_ty);
+                }
+
+                self.diverges = matchee_diverges | all_arms_diverge;
+
+                result_ty
+            }
+            Expr::Path(p) => {
+                // FIXME this could be more efficient...
+                let resolver = resolver_for_expr(self.db.upcast(), self.owner, tgt_expr);
+                self.infer_path(&resolver, p, tgt_expr.into()).unwrap_or(Ty::Unknown)
+            }
+            Expr::Continue { .. } => Ty::simple(TypeCtor::Never),
+            Expr::Break { expr, label } => {
+                let val_ty = if let Some(expr) = expr {
+                    self.infer_expr(*expr, &Expectation::none())
+                } else {
+                    Ty::unit()
+                };
+
+                let last_ty =
+                    if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
+                        ctxt.break_ty.clone()
+                    } else {
+                        Ty::Unknown
+                    };
+
+                let merged_type = self.coerce_merge_branch(&last_ty, &val_ty);
+
+                if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
+                    ctxt.break_ty = merged_type;
+                    ctxt.may_break = true;
+                } else {
+                    self.push_diagnostic(InferenceDiagnostic::BreakOutsideOfLoop {
+                        expr: tgt_expr,
+                    });
+                }
+
+                Ty::simple(TypeCtor::Never)
+            }
+            Expr::Return { expr } => {
+                if let Some(expr) = expr {
+                    self.infer_expr_coerce(*expr, &Expectation::has_type(self.return_ty.clone()));
+                } else {
+                    let unit = Ty::unit();
+                    self.coerce(&unit, &self.return_ty.clone());
+                }
+                Ty::simple(TypeCtor::Never)
+            }
+            Expr::RecordLit { path, fields, spread } => {
+                let (ty, def_id) = self.resolve_variant(path.as_ref());
+                if let Some(variant) = def_id {
+                    self.write_variant_resolution(tgt_expr.into(), variant);
+                }
+
+                self.unify(&ty, &expected.ty);
+
+                let substs = ty.substs().unwrap_or_else(Substs::empty);
+                let field_types = def_id.map(|it| self.db.field_types(it)).unwrap_or_default();
+                let variant_data = def_id.map(|it| variant_data(self.db.upcast(), it));
+                for (field_idx, field) in fields.iter().enumerate() {
+                    let field_def =
+                        variant_data.as_ref().and_then(|it| match it.field(&field.name) {
+                            Some(local_id) => Some(FieldId { parent: def_id.unwrap(), local_id }),
+                            None => {
+                                self.push_diagnostic(InferenceDiagnostic::NoSuchField {
+                                    expr: tgt_expr,
+                                    field: field_idx,
+                                });
+                                None
+                            }
+                        });
+                    if let Some(field_def) = field_def {
+                        self.result.record_field_resolutions.insert(field.expr, field_def);
+                    }
+                    let field_ty = field_def
+                        .map_or(Ty::Unknown, |it| field_types[it.local_id].clone().subst(&substs));
+                    self.infer_expr_coerce(field.expr, &Expectation::has_type(field_ty));
+                }
+                if let Some(expr) = spread {
+                    self.infer_expr(*expr, &Expectation::has_type(ty.clone()));
+                }
+                ty
+            }
+            Expr::Field { expr, name } => {
+                let receiver_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                let canonicalized = self.canonicalizer().canonicalize_ty(receiver_ty);
+                let ty = autoderef::autoderef(
+                    self.db,
+                    self.resolver.krate(),
+                    InEnvironment {
+                        value: canonicalized.value.clone(),
+                        environment: self.trait_env.clone(),
+                    },
+                )
+                .find_map(|derefed_ty| match canonicalized.decanonicalize_ty(derefed_ty.value) {
+                    Ty::Apply(a_ty) => match a_ty.ctor {
+                        TypeCtor::Tuple { .. } => name
+                            .as_tuple_index()
+                            .and_then(|idx| a_ty.parameters.0.get(idx).cloned()),
+                        TypeCtor::Adt(AdtId::StructId(s)) => {
+                            self.db.struct_data(s).variant_data.field(name).map(|local_id| {
+                                let field = FieldId { parent: s.into(), local_id };
+                                self.write_field_resolution(tgt_expr, field);
+                                self.db.field_types(s.into())[field.local_id]
+                                    .clone()
+                                    .subst(&a_ty.parameters)
+                            })
+                        }
+                        TypeCtor::Adt(AdtId::UnionId(u)) => {
+                            self.db.union_data(u).variant_data.field(name).map(|local_id| {
+                                let field = FieldId { parent: u.into(), local_id };
+                                self.write_field_resolution(tgt_expr, field);
+                                self.db.field_types(u.into())[field.local_id]
+                                    .clone()
+                                    .subst(&a_ty.parameters)
+                            })
+                        }
+                        _ => None,
+                    },
+                    _ => None,
+                })
+                .unwrap_or(Ty::Unknown);
+                let ty = self.insert_type_vars(ty);
+                self.normalize_associated_types_in(ty)
+            }
+            Expr::Await { expr } => {
+                let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                self.resolve_associated_type(inner_ty, self.resolve_future_future_output())
+            }
+            Expr::Try { expr } => {
+                let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                self.resolve_associated_type(inner_ty, self.resolve_ops_try_ok())
+            }
+            Expr::Cast { expr, type_ref } => {
+                let _inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                let cast_ty = self.make_ty(type_ref);
+                // FIXME check the cast...
+                cast_ty
+            }
+            Expr::Ref { expr, rawness, mutability } => {
+                let expectation = if let Some((exp_inner, exp_rawness, exp_mutability)) =
+                    &expected.ty.as_reference_or_ptr()
+                {
+                    if *exp_mutability == Mutability::Mut && *mutability == Mutability::Shared {
+                        // FIXME: throw type error - expected mut reference but found shared ref,
+                        // which cannot be coerced
+                    }
+                    if *exp_rawness == Rawness::Ref && *rawness == Rawness::RawPtr {
+                        // FIXME: throw type error - expected reference but found ptr,
+                        // which cannot be coerced
+                    }
+                    Expectation::rvalue_hint(Ty::clone(exp_inner))
+                } else {
+                    Expectation::none()
+                };
+                let inner_ty = self.infer_expr_inner(*expr, &expectation);
+                let ty = match rawness {
+                    Rawness::RawPtr => TypeCtor::RawPtr(*mutability),
+                    Rawness::Ref => TypeCtor::Ref(*mutability),
+                };
+                Ty::apply_one(ty, inner_ty)
+            }
+            Expr::Box { expr } => {
+                let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                if let Some(box_) = self.resolve_boxed_box() {
+                    Ty::apply_one(TypeCtor::Adt(box_), inner_ty)
+                } else {
+                    Ty::Unknown
+                }
+            }
+            Expr::UnaryOp { expr, op } => {
+                let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                match op {
+                    UnaryOp::Deref => match self.resolver.krate() {
+                        Some(krate) => {
+                            let canonicalized = self.canonicalizer().canonicalize_ty(inner_ty);
+                            match autoderef::deref(
+                                self.db,
+                                krate,
+                                InEnvironment {
+                                    value: &canonicalized.value,
+                                    environment: self.trait_env.clone(),
+                                },
+                            ) {
+                                Some(derefed_ty) => {
+                                    canonicalized.decanonicalize_ty(derefed_ty.value)
+                                }
+                                None => Ty::Unknown,
+                            }
+                        }
+                        None => Ty::Unknown,
+                    },
+                    UnaryOp::Neg => {
+                        match &inner_ty {
+                            // Fast path for builtins
+                            Ty::Apply(ApplicationTy {
+                                ctor: TypeCtor::Int(IntTy { signedness: Signedness::Signed, .. }),
+                                ..
+                            })
+                            | Ty::Apply(ApplicationTy { ctor: TypeCtor::Float(_), .. })
+                            | Ty::Infer(InferTy::IntVar(..))
+                            | Ty::Infer(InferTy::FloatVar(..)) => inner_ty,
+                            // Otherwise we resolve via the std::ops::Neg trait
+                            _ => self
+                                .resolve_associated_type(inner_ty, self.resolve_ops_neg_output()),
+                        }
+                    }
+                    UnaryOp::Not => {
+                        match &inner_ty {
+                            // Fast path for builtins
+                            Ty::Apply(ApplicationTy { ctor: TypeCtor::Bool, .. })
+                            | Ty::Apply(ApplicationTy { ctor: TypeCtor::Int(_), .. })
+                            | Ty::Infer(InferTy::IntVar(..)) => inner_ty,
+                            // Otherwise we resolve via the std::ops::Not trait
+                            _ => self
+                                .resolve_associated_type(inner_ty, self.resolve_ops_not_output()),
+                        }
+                    }
+                }
+            }
+            Expr::BinaryOp { lhs, rhs, op } => match op {
+                Some(op) => {
+                    let lhs_expectation = match op {
+                        BinaryOp::LogicOp(..) => Expectation::has_type(Ty::simple(TypeCtor::Bool)),
+                        _ => Expectation::none(),
+                    };
+                    let lhs_ty = self.infer_expr(*lhs, &lhs_expectation);
+                    // FIXME: find implementation of trait corresponding to operation
+                    // symbol and resolve associated `Output` type
+                    let rhs_expectation = op::binary_op_rhs_expectation(*op, lhs_ty.clone());
+                    let rhs_ty = self.infer_expr(*rhs, &Expectation::has_type(rhs_expectation));
+
+                    // FIXME: similar as above, return ty is often associated trait type
+                    op::binary_op_return_ty(*op, lhs_ty, rhs_ty)
+                }
+                _ => Ty::Unknown,
+            },
+            Expr::Range { lhs, rhs, range_type } => {
+                let lhs_ty = lhs.map(|e| self.infer_expr_inner(e, &Expectation::none()));
+                let rhs_expect = lhs_ty
+                    .as_ref()
+                    .map_or_else(Expectation::none, |ty| Expectation::has_type(ty.clone()));
+                let rhs_ty = rhs.map(|e| self.infer_expr(e, &rhs_expect));
+                match (range_type, lhs_ty, rhs_ty) {
+                    (RangeOp::Exclusive, None, None) => match self.resolve_range_full() {
+                        Some(adt) => Ty::simple(TypeCtor::Adt(adt)),
+                        None => Ty::Unknown,
+                    },
+                    (RangeOp::Exclusive, None, Some(ty)) => match self.resolve_range_to() {
+                        Some(adt) => Ty::apply_one(TypeCtor::Adt(adt), ty),
+                        None => Ty::Unknown,
+                    },
+                    (RangeOp::Inclusive, None, Some(ty)) => {
+                        match self.resolve_range_to_inclusive() {
+                            Some(adt) => Ty::apply_one(TypeCtor::Adt(adt), ty),
+                            None => Ty::Unknown,
+                        }
+                    }
+                    (RangeOp::Exclusive, Some(_), Some(ty)) => match self.resolve_range() {
+                        Some(adt) => Ty::apply_one(TypeCtor::Adt(adt), ty),
+                        None => Ty::Unknown,
+                    },
+                    (RangeOp::Inclusive, Some(_), Some(ty)) => {
+                        match self.resolve_range_inclusive() {
+                            Some(adt) => Ty::apply_one(TypeCtor::Adt(adt), ty),
+                            None => Ty::Unknown,
+                        }
+                    }
+                    (RangeOp::Exclusive, Some(ty), None) => match self.resolve_range_from() {
+                        Some(adt) => Ty::apply_one(TypeCtor::Adt(adt), ty),
+                        None => Ty::Unknown,
+                    },
+                    (RangeOp::Inclusive, _, None) => Ty::Unknown,
+                }
+            }
+            Expr::Index { base, index } => {
+                let base_ty = self.infer_expr_inner(*base, &Expectation::none());
+                let index_ty = self.infer_expr(*index, &Expectation::none());
+
+                if let (Some(index_trait), Some(krate)) =
+                    (self.resolve_ops_index(), self.resolver.krate())
+                {
+                    let canonicalized = self.canonicalizer().canonicalize_ty(base_ty);
+                    let self_ty = method_resolution::resolve_indexing_op(
+                        self.db,
+                        &canonicalized.value,
+                        self.trait_env.clone(),
+                        krate,
+                        index_trait,
+                    );
+                    let self_ty =
+                        self_ty.map_or(Ty::Unknown, |t| canonicalized.decanonicalize_ty(t.value));
+                    self.resolve_associated_type_with_params(
+                        self_ty,
+                        self.resolve_ops_index_output(),
+                        &[index_ty],
+                    )
+                } else {
+                    Ty::Unknown
+                }
+            }
+            Expr::Tuple { exprs } => {
+                let mut tys = match &expected.ty {
+                    ty_app!(TypeCtor::Tuple { .. }, st) => st
+                        .iter()
+                        .cloned()
+                        .chain(repeat_with(|| self.table.new_type_var()))
+                        .take(exprs.len())
+                        .collect::<Vec<_>>(),
+                    _ => (0..exprs.len()).map(|_| self.table.new_type_var()).collect(),
+                };
+
+                for (expr, ty) in exprs.iter().zip(tys.iter_mut()) {
+                    self.infer_expr_coerce(*expr, &Expectation::has_type(ty.clone()));
+                }
+
+                Ty::apply(TypeCtor::Tuple { cardinality: tys.len() as u16 }, Substs(tys.into()))
+            }
+            Expr::Array(array) => {
+                let elem_ty = match &expected.ty {
+                    ty_app!(TypeCtor::Array, st) | ty_app!(TypeCtor::Slice, st) => {
+                        st.as_single().clone()
+                    }
+                    _ => self.table.new_type_var(),
+                };
+
+                match array {
+                    Array::ElementList(items) => {
+                        for expr in items.iter() {
+                            self.infer_expr_coerce(*expr, &Expectation::has_type(elem_ty.clone()));
+                        }
+                    }
+                    Array::Repeat { initializer, repeat } => {
+                        self.infer_expr_coerce(
+                            *initializer,
+                            &Expectation::has_type(elem_ty.clone()),
+                        );
+                        self.infer_expr(
+                            *repeat,
+                            &Expectation::has_type(Ty::simple(TypeCtor::Int(IntTy::usize()))),
+                        );
+                    }
+                }
+
+                Ty::apply_one(TypeCtor::Array, elem_ty)
+            }
+            Expr::Literal(lit) => match lit {
+                Literal::Bool(..) => Ty::simple(TypeCtor::Bool),
+                Literal::String(..) => {
+                    Ty::apply_one(TypeCtor::Ref(Mutability::Shared), Ty::simple(TypeCtor::Str))
+                }
+                Literal::ByteString(..) => {
+                    let byte_type = Ty::simple(TypeCtor::Int(IntTy::u8()));
+                    let array_type = Ty::apply_one(TypeCtor::Array, byte_type);
+                    Ty::apply_one(TypeCtor::Ref(Mutability::Shared), array_type)
+                }
+                Literal::Char(..) => Ty::simple(TypeCtor::Char),
+                Literal::Int(_v, ty) => match ty {
+                    Some(int_ty) => Ty::simple(TypeCtor::Int((*int_ty).into())),
+                    None => self.table.new_integer_var(),
+                },
+                Literal::Float(_v, ty) => match ty {
+                    Some(float_ty) => Ty::simple(TypeCtor::Float((*float_ty).into())),
+                    None => self.table.new_float_var(),
+                },
+            },
+        };
+        // use a new type variable if we got Ty::Unknown here
+        let ty = self.insert_type_vars_shallow(ty);
+        let ty = self.resolve_ty_as_possible(ty);
+        self.write_expr_ty(tgt_expr, ty.clone());
+        ty
+    }
+
+    fn infer_block(
+        &mut self,
+        statements: &[Statement],
+        tail: Option<ExprId>,
+        expected: &Expectation,
+    ) -> Ty {
+        for stmt in statements {
+            match stmt {
+                Statement::Let { pat, type_ref, initializer } => {
+                    let decl_ty =
+                        type_ref.as_ref().map(|tr| self.make_ty(tr)).unwrap_or(Ty::Unknown);
+
+                    // Always use the declared type when specified
+                    let mut ty = decl_ty.clone();
+
+                    if let Some(expr) = initializer {
+                        let actual_ty =
+                            self.infer_expr_coerce(*expr, &Expectation::has_type(decl_ty.clone()));
+                        if decl_ty == Ty::Unknown {
+                            ty = actual_ty;
+                        }
+                    }
+
+                    let ty = self.resolve_ty_as_possible(ty);
+                    self.infer_pat(*pat, &ty, BindingMode::default());
+                }
+                Statement::Expr(expr) => {
+                    self.infer_expr(*expr, &Expectation::none());
+                }
+            }
+        }
+
+        let ty = if let Some(expr) = tail {
+            self.infer_expr_coerce(expr, expected)
+        } else {
+            // Citing rustc: if there is no explicit tail expression,
+            // that is typically equivalent to a tail expression
+            // of `()` -- except if the block diverges. In that
+            // case, there is no value supplied from the tail
+            // expression (assuming there are no other breaks,
+            // this implies that the type of the block will be
+            // `!`).
+            if self.diverges.is_always() {
+                // we don't even make an attempt at coercion
+                self.table.new_maybe_never_type_var()
+            } else {
+                self.coerce(&Ty::unit(), expected.coercion_target());
+                Ty::unit()
+            }
+        };
+        ty
+    }
+
+    fn infer_method_call(
+        &mut self,
+        tgt_expr: ExprId,
+        receiver: ExprId,
+        args: &[ExprId],
+        method_name: &Name,
+        generic_args: Option<&GenericArgs>,
+    ) -> Ty {
+        let receiver_ty = self.infer_expr(receiver, &Expectation::none());
+        let canonicalized_receiver = self.canonicalizer().canonicalize_ty(receiver_ty.clone());
+
+        let traits_in_scope = self.resolver.traits_in_scope(self.db.upcast());
+
+        let resolved = self.resolver.krate().and_then(|krate| {
+            method_resolution::lookup_method(
+                &canonicalized_receiver.value,
+                self.db,
+                self.trait_env.clone(),
+                krate,
+                &traits_in_scope,
+                method_name,
+            )
+        });
+        let (derefed_receiver_ty, method_ty, def_generics) = match resolved {
+            Some((ty, func)) => {
+                let ty = canonicalized_receiver.decanonicalize_ty(ty);
+                self.write_method_resolution(tgt_expr, func);
+                (ty, self.db.value_ty(func.into()), Some(generics(self.db.upcast(), func.into())))
+            }
+            None => (receiver_ty, Binders::new(0, Ty::Unknown), None),
+        };
+        let substs = self.substs_for_method_call(def_generics, generic_args, &derefed_receiver_ty);
+        let method_ty = method_ty.subst(&substs);
+        let method_ty = self.insert_type_vars(method_ty);
+        self.register_obligations_for_call(&method_ty);
+        let (expected_receiver_ty, param_tys, ret_ty) = match method_ty.callable_sig(self.db) {
+            Some(sig) => {
+                if !sig.params().is_empty() {
+                    (sig.params()[0].clone(), sig.params()[1..].to_vec(), sig.ret().clone())
+                } else {
+                    (Ty::Unknown, Vec::new(), sig.ret().clone())
+                }
+            }
+            None => (Ty::Unknown, Vec::new(), Ty::Unknown),
+        };
+        // Apply autoref so the below unification works correctly
+        // FIXME: return correct autorefs from lookup_method
+        let actual_receiver_ty = match expected_receiver_ty.as_reference() {
+            Some((_, mutability)) => Ty::apply_one(TypeCtor::Ref(mutability), derefed_receiver_ty),
+            _ => derefed_receiver_ty,
+        };
+        self.unify(&expected_receiver_ty, &actual_receiver_ty);
+
+        self.check_call_arguments(args, &param_tys);
+        self.normalize_associated_types_in(ret_ty)
+    }
+
+    fn check_call_arguments(&mut self, args: &[ExprId], param_tys: &[Ty]) {
+        // Quoting https://github.com/rust-lang/rust/blob/6ef275e6c3cb1384ec78128eceeb4963ff788dca/src/librustc_typeck/check/mod.rs#L3325 --
+        // We do this in a pretty awful way: first we type-check any arguments
+        // that are not closures, then we type-check the closures. This is so
+        // that we have more information about the types of arguments when we
+        // type-check the functions. This isn't really the right way to do this.
+        for &check_closures in &[false, true] {
+            let param_iter = param_tys.iter().cloned().chain(repeat(Ty::Unknown));
+            for (&arg, param_ty) in args.iter().zip(param_iter) {
+                let is_closure = matches!(&self.body[arg], Expr::Lambda { .. });
+                if is_closure != check_closures {
+                    continue;
+                }
+
+                let param_ty = self.normalize_associated_types_in(param_ty);
+                self.infer_expr_coerce(arg, &Expectation::has_type(param_ty.clone()));
+            }
+        }
+    }
+
+    fn substs_for_method_call(
+        &mut self,
+        def_generics: Option<Generics>,
+        generic_args: Option<&GenericArgs>,
+        receiver_ty: &Ty,
+    ) -> Substs {
+        let (parent_params, self_params, type_params, impl_trait_params) =
+            def_generics.as_ref().map_or((0, 0, 0, 0), |g| g.provenance_split());
+        assert_eq!(self_params, 0); // method shouldn't have another Self param
+        let total_len = parent_params + type_params + impl_trait_params;
+        let mut substs = Vec::with_capacity(total_len);
+        // Parent arguments are unknown, except for the receiver type
+        if let Some(parent_generics) = def_generics.as_ref().map(|p| p.iter_parent()) {
+            for (_id, param) in parent_generics {
+                if param.provenance == hir_def::generics::TypeParamProvenance::TraitSelf {
+                    substs.push(receiver_ty.clone());
+                } else {
+                    substs.push(Ty::Unknown);
+                }
+            }
+        }
+        // handle provided type arguments
+        if let Some(generic_args) = generic_args {
+            // if args are provided, it should be all of them, but we can't rely on that
+            for arg in generic_args.args.iter().take(type_params) {
+                match arg {
+                    GenericArg::Type(type_ref) => {
+                        let ty = self.make_ty(type_ref);
+                        substs.push(ty);
+                    }
+                }
+            }
+        };
+        let supplied_params = substs.len();
+        for _ in supplied_params..total_len {
+            substs.push(Ty::Unknown);
+        }
+        assert_eq!(substs.len(), total_len);
+        Substs(substs.into())
+    }
+
+    fn register_obligations_for_call(&mut self, callable_ty: &Ty) {
+        if let Ty::Apply(a_ty) = callable_ty {
+            if let TypeCtor::FnDef(def) = a_ty.ctor {
+                let generic_predicates = self.db.generic_predicates(def.into());
+                for predicate in generic_predicates.iter() {
+                    let predicate = predicate.clone().subst(&a_ty.parameters);
+                    if let Some(obligation) = Obligation::from_predicate(predicate) {
+                        self.obligations.push(obligation);
+                    }
+                }
+                // add obligation for trait implementation, if this is a trait method
+                match def {
+                    CallableDefId::FunctionId(f) => {
+                        if let AssocContainerId::TraitId(trait_) =
+                            f.lookup(self.db.upcast()).container
+                        {
+                            // construct a TraitDef
+                            let substs = a_ty
+                                .parameters
+                                .prefix(generics(self.db.upcast(), trait_.into()).len());
+                            self.obligations.push(Obligation::Trait(TraitRef { trait_, substs }));
+                        }
+                    }
+                    CallableDefId::StructId(_) | CallableDefId::EnumVariantId(_) => {}
+                }
+            }
+        }
+    }
+}
diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
new file mode 100644
index 000000000..4dd4f9802
--- /dev/null
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -0,0 +1,241 @@
+//! Type inference for patterns.
+
+use std::iter::repeat;
+use std::sync::Arc;
+
+use hir_def::{
+    expr::{BindingAnnotation, Expr, Literal, Pat, PatId, RecordFieldPat},
+    path::Path,
+    type_ref::Mutability,
+    FieldId,
+};
+use hir_expand::name::Name;
+use test_utils::mark;
+
+use super::{BindingMode, Expectation, InferenceContext};
+use crate::{utils::variant_data, Substs, Ty, TypeCtor};
+
+impl<'a> InferenceContext<'a> {
+    fn infer_tuple_struct_pat(
+        &mut self,
+        path: Option<&Path>,
+        subpats: &[PatId],
+        expected: &Ty,
+        default_bm: BindingMode,
+        id: PatId,
+    ) -> Ty {
+        let (ty, def) = self.resolve_variant(path);
+        let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        if let Some(variant) = def {
+            self.write_variant_resolution(id.into(), variant);
+        }
+        self.unify(&ty, expected);
+
+        let substs = ty.substs().unwrap_or_else(Substs::empty);
+
+        let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
+
+        for (i, &subpat) in subpats.iter().enumerate() {
+            let expected_ty = var_data
+                .as_ref()
+                .and_then(|d| d.field(&Name::new_tuple_field(i)))
+                .map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
+            let expected_ty = self.normalize_associated_types_in(expected_ty);
+            self.infer_pat(subpat, &expected_ty, default_bm);
+        }
+
+        ty
+    }
+
+    fn infer_record_pat(
+        &mut self,
+        path: Option<&Path>,
+        subpats: &[RecordFieldPat],
+        expected: &Ty,
+        default_bm: BindingMode,
+        id: PatId,
+    ) -> Ty {
+        let (ty, def) = self.resolve_variant(path);
+        let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        if let Some(variant) = def {
+            self.write_variant_resolution(id.into(), variant);
+        }
+
+        self.unify(&ty, expected);
+
+        let substs = ty.substs().unwrap_or_else(Substs::empty);
+
+        let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
+        for subpat in subpats {
+            let matching_field = var_data.as_ref().and_then(|it| it.field(&subpat.name));
+            if let Some(local_id) = matching_field {
+                let field_def = FieldId { parent: def.unwrap(), local_id };
+                self.result.record_field_pat_resolutions.insert(subpat.pat, field_def);
+            }
+
+            let expected_ty =
+                matching_field.map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
+            let expected_ty = self.normalize_associated_types_in(expected_ty);
+            self.infer_pat(subpat.pat, &expected_ty, default_bm);
+        }
+
+        ty
+    }
+
+    pub(super) fn infer_pat(
+        &mut self,
+        pat: PatId,
+        mut expected: &Ty,
+        mut default_bm: BindingMode,
+    ) -> Ty {
+        let body = Arc::clone(&self.body); // avoid borrow checker problem
+
+        if is_non_ref_pat(&body, pat) {
+            while let Some((inner, mutability)) = expected.as_reference() {
+                expected = inner;
+                default_bm = match default_bm {
+                    BindingMode::Move => BindingMode::Ref(mutability),
+                    BindingMode::Ref(Mutability::Shared) => BindingMode::Ref(Mutability::Shared),
+                    BindingMode::Ref(Mutability::Mut) => BindingMode::Ref(mutability),
+                }
+            }
+        } else if let Pat::Ref { .. } = &body[pat] {
+            mark::hit!(match_ergonomics_ref);
+            // When you encounter a `&pat` pattern, reset to Move.
+            // This is so that `w` is by value: `let (_, &w) = &(1, &2);`
+            default_bm = BindingMode::Move;
+        }
+
+        // Lose mutability.
+        let default_bm = default_bm;
+        let expected = expected;
+
+        let ty = match &body[pat] {
+            Pat::Tuple { ref args, .. } => {
+                let expectations = match expected.as_tuple() {
+                    Some(parameters) => &*parameters.0,
+                    _ => &[],
+                };
+                let expectations_iter = expectations.iter().chain(repeat(&Ty::Unknown));
+
+                let inner_tys = args
+                    .iter()
+                    .zip(expectations_iter)
+                    .map(|(&pat, ty)| self.infer_pat(pat, ty, default_bm))
+                    .collect();
+
+                Ty::apply(TypeCtor::Tuple { cardinality: args.len() as u16 }, Substs(inner_tys))
+            }
+            Pat::Or(ref pats) => {
+                if let Some((first_pat, rest)) = pats.split_first() {
+                    let ty = self.infer_pat(*first_pat, expected, default_bm);
+                    for pat in rest {
+                        self.infer_pat(*pat, expected, default_bm);
+                    }
+                    ty
+                } else {
+                    Ty::Unknown
+                }
+            }
+            Pat::Ref { pat, mutability } => {
+                let expectation = match expected.as_reference() {
+                    Some((inner_ty, exp_mut)) => {
+                        if *mutability != exp_mut {
+                            // FIXME: emit type error?
+                        }
+                        inner_ty
+                    }
+                    _ => &Ty::Unknown,
+                };
+                let subty = self.infer_pat(*pat, expectation, default_bm);
+                Ty::apply_one(TypeCtor::Ref(*mutability), subty)
+            }
+            Pat::TupleStruct { path: p, args: subpats, .. } => {
+                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm, pat)
+            }
+            Pat::Record { path: p, args: fields, ellipsis: _ } => {
+                self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
+            }
+            Pat::Path(path) => {
+                // FIXME use correct resolver for the surrounding expression
+                let resolver = self.resolver.clone();
+                self.infer_path(&resolver, &path, pat.into()).unwrap_or(Ty::Unknown)
+            }
+            Pat::Bind { mode, name: _, subpat } => {
+                let mode = if mode == &BindingAnnotation::Unannotated {
+                    default_bm
+                } else {
+                    BindingMode::convert(*mode)
+                };
+                let inner_ty = if let Some(subpat) = subpat {
+                    self.infer_pat(*subpat, expected, default_bm)
+                } else {
+                    expected.clone()
+                };
+                let inner_ty = self.insert_type_vars_shallow(inner_ty);
+
+                let bound_ty = match mode {
+                    BindingMode::Ref(mutability) => {
+                        Ty::apply_one(TypeCtor::Ref(mutability), inner_ty.clone())
+                    }
+                    BindingMode::Move => inner_ty.clone(),
+                };
+                let bound_ty = self.resolve_ty_as_possible(bound_ty);
+                self.write_pat_ty(pat, bound_ty);
+                return inner_ty;
+            }
+            Pat::Slice { prefix, slice, suffix } => {
+                let (container_ty, elem_ty) = match &expected {
+                    ty_app!(TypeCtor::Array, st) => (TypeCtor::Array, st.as_single().clone()),
+                    ty_app!(TypeCtor::Slice, st) => (TypeCtor::Slice, st.as_single().clone()),
+                    _ => (TypeCtor::Slice, Ty::Unknown),
+                };
+
+                for pat_id in prefix.iter().chain(suffix) {
+                    self.infer_pat(*pat_id, &elem_ty, default_bm);
+                }
+
+                let pat_ty = Ty::apply_one(container_ty, elem_ty);
+                if let Some(slice_pat_id) = slice {
+                    self.infer_pat(*slice_pat_id, &pat_ty, default_bm);
+                }
+
+                pat_ty
+            }
+            Pat::Wild => expected.clone(),
+            Pat::Range { start, end } => {
+                let start_ty = self.infer_expr(*start, &Expectation::has_type(expected.clone()));
+                let end_ty = self.infer_expr(*end, &Expectation::has_type(start_ty));
+                end_ty
+            }
+            Pat::Lit(expr) => self.infer_expr(*expr, &Expectation::has_type(expected.clone())),
+            Pat::Missing => Ty::Unknown,
+        };
+        // use a new type variable if we got Ty::Unknown here
+        let ty = self.insert_type_vars_shallow(ty);
+        if !self.unify(&ty, expected) {
+            // FIXME record mismatch, we need to change the type of self.type_mismatches for that
+        }
+        let ty = self.resolve_ty_as_possible(ty);
+        self.write_pat_ty(pat, ty.clone());
+        ty
+    }
+}
+
+fn is_non_ref_pat(body: &hir_def::body::Body, pat: PatId) -> bool {
+    match &body[pat] {
+        Pat::Tuple { .. }
+        | Pat::TupleStruct { .. }
+        | Pat::Record { .. }
+        | Pat::Range { .. }
+        | Pat::Slice { .. } => true,
+        Pat::Or(pats) => pats.iter().all(|p| is_non_ref_pat(body, *p)),
+        // FIXME: Path/Lit might actually evaluate to ref, but inference is unimplemented.
+        Pat::Path(..) => true,
+        Pat::Lit(expr) => match body[*expr] {
+            Expr::Literal(Literal::String(..)) => false,
+            _ => true,
+        },
+        Pat::Wild | Pat::Bind { .. } | Pat::Ref { .. } | Pat::Missing => false,
+    }
+}
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
new file mode 100644
index 000000000..80d7ed10e
--- /dev/null
+++ b/crates/hir_ty/src/infer/path.rs
@@ -0,0 +1,287 @@
+//! Path expression resolution.
+
+use std::iter;
+
+use hir_def::{
+    path::{Path, PathSegment},
+    resolver::{ResolveValueResult, Resolver, TypeNs, ValueNs},
+    AdtId, AssocContainerId, AssocItemId, EnumVariantId, Lookup,
+};
+use hir_expand::name::Name;
+
+use crate::{method_resolution, Substs, Ty, ValueTyDefId};
+
+use super::{ExprOrPatId, InferenceContext, TraitRef};
+
+impl<'a> InferenceContext<'a> {
+    pub(super) fn infer_path(
+        &mut self,
+        resolver: &Resolver,
+        path: &Path,
+        id: ExprOrPatId,
+    ) -> Option<Ty> {
+        let ty = self.resolve_value_path(resolver, path, id)?;
+        let ty = self.insert_type_vars(ty);
+        let ty = self.normalize_associated_types_in(ty);
+        Some(ty)
+    }
+
+    fn resolve_value_path(
+        &mut self,
+        resolver: &Resolver,
+        path: &Path,
+        id: ExprOrPatId,
+    ) -> Option<Ty> {
+        let (value, self_subst) = if let Some(type_ref) = path.type_anchor() {
+            if path.segments().is_empty() {
+                // This can't actually happen syntax-wise
+                return None;
+            }
+            let ty = self.make_ty(type_ref);
+            let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
+            let ctx = crate::lower::TyLoweringContext::new(self.db, &resolver);
+            let (ty, _) = Ty::from_type_relative_path(&ctx, ty, None, remaining_segments_for_ty);
+            self.resolve_ty_assoc_item(
+                ty,
+                &path.segments().last().expect("path had at least one segment").name,
+                id,
+            )?
+        } else {
+            let value_or_partial =
+                resolver.resolve_path_in_value_ns(self.db.upcast(), path.mod_path())?;
+
+            match value_or_partial {
+                ResolveValueResult::ValueNs(it) => (it, None),
+                ResolveValueResult::Partial(def, remaining_index) => {
+                    self.resolve_assoc_item(def, path, remaining_index, id)?
+                }
+            }
+        };
+
+        let typable: ValueTyDefId = match value {
+            ValueNs::LocalBinding(pat) => {
+                let ty = self.result.type_of_pat.get(pat)?.clone();
+                let ty = self.resolve_ty_as_possible(ty);
+                return Some(ty);
+            }
+            ValueNs::FunctionId(it) => it.into(),
+            ValueNs::ConstId(it) => it.into(),
+            ValueNs::StaticId(it) => it.into(),
+            ValueNs::StructId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
+            ValueNs::EnumVariantId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
+            ValueNs::ImplSelf(impl_id) => {
+                let generics = crate::utils::generics(self.db.upcast(), impl_id.into());
+                let substs = Substs::type_params_for_generics(&generics);
+                let ty = self.db.impl_self_ty(impl_id).subst(&substs);
+                if let Some((AdtId::StructId(struct_id), substs)) = ty.as_adt() {
+                    let ty = self.db.value_ty(struct_id.into()).subst(&substs);
+                    return Some(ty);
+                } else {
+                    // FIXME: diagnostic, invalid Self reference
+                    return None;
+                }
+            }
+        };
+
+        let ty = self.db.value_ty(typable);
+        // self_subst is just for the parent
+        let parent_substs = self_subst.unwrap_or_else(Substs::empty);
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+        let substs = Ty::substs_from_path(&ctx, path, typable, true);
+        let full_substs = Substs::builder(substs.len())
+            .use_parent_substs(&parent_substs)
+            .fill(substs.0[parent_substs.len()..].iter().cloned())
+            .build();
+        let ty = ty.subst(&full_substs);
+        Some(ty)
+    }
+
+    fn resolve_assoc_item(
+        &mut self,
+        def: TypeNs,
+        path: &Path,
+        remaining_index: usize,
+        id: ExprOrPatId,
+    ) -> Option<(ValueNs, Option<Substs>)> {
+        assert!(remaining_index < path.segments().len());
+        // there may be more intermediate segments between the resolved one and
+        // the end. Only the last segment needs to be resolved to a value; from
+        // the segments before that, we need to get either a type or a trait ref.
+
+        let resolved_segment = path.segments().get(remaining_index - 1).unwrap();
+        let remaining_segments = path.segments().skip(remaining_index);
+        let is_before_last = remaining_segments.len() == 1;
+
+        match (def, is_before_last) {
+            (TypeNs::TraitId(trait_), true) => {
+                let segment =
+                    remaining_segments.last().expect("there should be at least one segment here");
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+                let trait_ref = TraitRef::from_resolved_path(&ctx, trait_, resolved_segment, None);
+                self.resolve_trait_assoc_item(trait_ref, segment, id)
+            }
+            (def, _) => {
+                // Either we already have a type (e.g. `Vec::new`), or we have a
+                // trait but it's not the last segment, so the next segment
+                // should resolve to an associated type of that trait (e.g. `<T
+                // as Iterator>::Item::default`)
+                let remaining_segments_for_ty =
+                    remaining_segments.take(remaining_segments.len() - 1);
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+                let (ty, _) = Ty::from_partly_resolved_hir_path(
+                    &ctx,
+                    def,
+                    resolved_segment,
+                    remaining_segments_for_ty,
+                    true,
+                );
+                if let Ty::Unknown = ty {
+                    return None;
+                }
+
+                let ty = self.insert_type_vars(ty);
+                let ty = self.normalize_associated_types_in(ty);
+
+                let segment =
+                    remaining_segments.last().expect("there should be at least one segment here");
+
+                self.resolve_ty_assoc_item(ty, &segment.name, id)
+            }
+        }
+    }
+
+    fn resolve_trait_assoc_item(
+        &mut self,
+        trait_ref: TraitRef,
+        segment: PathSegment<'_>,
+        id: ExprOrPatId,
+    ) -> Option<(ValueNs, Option<Substs>)> {
+        let trait_ = trait_ref.trait_;
+        let item =
+            self.db.trait_data(trait_).items.iter().map(|(_name, id)| (*id)).find_map(|item| {
+                match item {
+                    AssocItemId::FunctionId(func) => {
+                        if segment.name == &self.db.function_data(func).name {
+                            Some(AssocItemId::FunctionId(func))
+                        } else {
+                            None
+                        }
+                    }
+
+                    AssocItemId::ConstId(konst) => {
+                        if self
+                            .db
+                            .const_data(konst)
+                            .name
+                            .as_ref()
+                            .map_or(false, |n| n == segment.name)
+                        {
+                            Some(AssocItemId::ConstId(konst))
+                        } else {
+                            None
+                        }
+                    }
+                    AssocItemId::TypeAliasId(_) => None,
+                }
+            })?;
+        let def = match item {
+            AssocItemId::FunctionId(f) => ValueNs::FunctionId(f),
+            AssocItemId::ConstId(c) => ValueNs::ConstId(c),
+            AssocItemId::TypeAliasId(_) => unreachable!(),
+        };
+
+        self.write_assoc_resolution(id, item);
+        Some((def, Some(trait_ref.substs)))
+    }
+
+    fn resolve_ty_assoc_item(
+        &mut self,
+        ty: Ty,
+        name: &Name,
+        id: ExprOrPatId,
+    ) -> Option<(ValueNs, Option<Substs>)> {
+        if let Ty::Unknown = ty {
+            return None;
+        }
+
+        if let Some(result) = self.resolve_enum_variant_on_ty(&ty, name, id) {
+            return Some(result);
+        }
+
+        let canonical_ty = self.canonicalizer().canonicalize_ty(ty.clone());
+        let krate = self.resolver.krate()?;
+        let traits_in_scope = self.resolver.traits_in_scope(self.db.upcast());
+
+        method_resolution::iterate_method_candidates(
+            &canonical_ty.value,
+            self.db,
+            self.trait_env.clone(),
+            krate,
+            &traits_in_scope,
+            Some(name),
+            method_resolution::LookupMode::Path,
+            move |_ty, item| {
+                let (def, container) = match item {
+                    AssocItemId::FunctionId(f) => {
+                        (ValueNs::FunctionId(f), f.lookup(self.db.upcast()).container)
+                    }
+                    AssocItemId::ConstId(c) => {
+                        (ValueNs::ConstId(c), c.lookup(self.db.upcast()).container)
+                    }
+                    AssocItemId::TypeAliasId(_) => unreachable!(),
+                };
+                let substs = match container {
+                    AssocContainerId::ImplId(impl_id) => {
+                        let impl_substs = Substs::build_for_def(self.db, impl_id)
+                            .fill(iter::repeat_with(|| self.table.new_type_var()))
+                            .build();
+                        let impl_self_ty = self.db.impl_self_ty(impl_id).subst(&impl_substs);
+                        self.unify(&impl_self_ty, &ty);
+                        Some(impl_substs)
+                    }
+                    AssocContainerId::TraitId(trait_) => {
+                        // we're picking this method
+                        let trait_substs = Substs::build_for_def(self.db, trait_)
+                            .push(ty.clone())
+                            .fill(std::iter::repeat_with(|| self.table.new_type_var()))
+                            .build();
+                        self.obligations.push(super::Obligation::Trait(TraitRef {
+                            trait_,
+                            substs: trait_substs.clone(),
+                        }));
+                        Some(trait_substs)
+                    }
+                    AssocContainerId::ContainerId(_) => None,
+                };
+
+                self.write_assoc_resolution(id, item);
+                Some((def, substs))
+            },
+        )
+    }
+
+    fn resolve_enum_variant_on_ty(
+        &mut self,
+        ty: &Ty,
+        name: &Name,
+        id: ExprOrPatId,
+    ) -> Option<(ValueNs, Option<Substs>)> {
+        let (enum_id, subst) = match ty.as_adt() {
+            Some((AdtId::EnumId(e), subst)) => (e, subst),
+            _ => return None,
+        };
+        let enum_data = self.db.enum_data(enum_id);
+        let local_id = enum_data.variant(name)?;
+        let variant = EnumVariantId { parent: enum_id, local_id };
+        self.write_variant_resolution(id, variant.into());
+        Some((ValueNs::EnumVariantId(variant), Some(subst.clone())))
+    }
+}
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
new file mode 100644
index 000000000..2e895d911
--- /dev/null
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -0,0 +1,474 @@
+//! Unification and canonicalization logic.
+
+use std::borrow::Cow;
+
+use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
+
+use test_utils::mark;
+
+use super::{InferenceContext, Obligation};
+use crate::{
+    BoundVar, Canonical, DebruijnIndex, GenericPredicate, InEnvironment, InferTy, Substs, Ty,
+    TyKind, TypeCtor, TypeWalk,
+};
+
+impl<'a> InferenceContext<'a> {
+    pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b>
+    where
+        'a: 'b,
+    {
+        Canonicalizer { ctx: self, free_vars: Vec::new(), var_stack: Vec::new() }
+    }
+}
+
+pub(super) struct Canonicalizer<'a, 'b>
+where
+    'a: 'b,
+{
+    ctx: &'b mut InferenceContext<'a>,
+    free_vars: Vec<InferTy>,
+    /// A stack of type variables that is used to detect recursive types (which
+    /// are an error, but we need to protect against them to avoid stack
+    /// overflows).
+    var_stack: Vec<TypeVarId>,
+}
+
+#[derive(Debug)]
+pub(super) struct Canonicalized<T> {
+    pub value: Canonical<T>,
+    free_vars: Vec<InferTy>,
+}
+
+impl<'a, 'b> Canonicalizer<'a, 'b>
+where
+    'a: 'b,
+{
+    fn add(&mut self, free_var: InferTy) -> usize {
+        self.free_vars.iter().position(|&v| v == free_var).unwrap_or_else(|| {
+            let next_index = self.free_vars.len();
+            self.free_vars.push(free_var);
+            next_index
+        })
+    }
+
+    fn do_canonicalize<T: TypeWalk>(&mut self, t: T, binders: DebruijnIndex) -> T {
+        t.fold_binders(
+            &mut |ty, binders| match ty {
+                Ty::Infer(tv) => {
+                    let inner = tv.to_inner();
+                    if self.var_stack.contains(&inner) {
+                        // recursive type
+                        return tv.fallback_value();
+                    }
+                    if let Some(known_ty) =
+                        self.ctx.table.var_unification_table.inlined_probe_value(inner).known()
+                    {
+                        self.var_stack.push(inner);
+                        let result = self.do_canonicalize(known_ty.clone(), binders);
+                        self.var_stack.pop();
+                        result
+                    } else {
+                        let root = self.ctx.table.var_unification_table.find(inner);
+                        let free_var = match tv {
+                            InferTy::TypeVar(_) => InferTy::TypeVar(root),
+                            InferTy::IntVar(_) => InferTy::IntVar(root),
+                            InferTy::FloatVar(_) => InferTy::FloatVar(root),
+                            InferTy::MaybeNeverTypeVar(_) => InferTy::MaybeNeverTypeVar(root),
+                        };
+                        let position = self.add(free_var);
+                        Ty::Bound(BoundVar::new(binders, position))
+                    }
+                }
+                _ => ty,
+            },
+            binders,
+        )
+    }
+
+    fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
+        let kinds = self
+            .free_vars
+            .iter()
+            .map(|v| match v {
+                // mapping MaybeNeverTypeVar to the same kind as general ones
+                // should be fine, because as opposed to int or float type vars,
+                // they don't restrict what kind of type can go into them, they
+                // just affect fallback.
+                InferTy::TypeVar(_) | InferTy::MaybeNeverTypeVar(_) => TyKind::General,
+                InferTy::IntVar(_) => TyKind::Integer,
+                InferTy::FloatVar(_) => TyKind::Float,
+            })
+            .collect();
+        Canonicalized { value: Canonical { value: result, kinds }, free_vars: self.free_vars }
+    }
+
+    pub(crate) fn canonicalize_ty(mut self, ty: Ty) -> Canonicalized<Ty> {
+        let result = self.do_canonicalize(ty, DebruijnIndex::INNERMOST);
+        self.into_canonicalized(result)
+    }
+
+    pub(crate) fn canonicalize_obligation(
+        mut self,
+        obligation: InEnvironment<Obligation>,
+    ) -> Canonicalized<InEnvironment<Obligation>> {
+        let result = match obligation.value {
+            Obligation::Trait(tr) => {
+                Obligation::Trait(self.do_canonicalize(tr, DebruijnIndex::INNERMOST))
+            }
+            Obligation::Projection(pr) => {
+                Obligation::Projection(self.do_canonicalize(pr, DebruijnIndex::INNERMOST))
+            }
+        };
+        self.into_canonicalized(InEnvironment {
+            value: result,
+            environment: obligation.environment,
+        })
+    }
+}
+
+impl<T> Canonicalized<T> {
+    pub fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
+        ty.walk_mut_binders(
+            &mut |ty, binders| {
+                if let &mut Ty::Bound(bound) = ty {
+                    if bound.debruijn >= binders {
+                        *ty = Ty::Infer(self.free_vars[bound.index]);
+                    }
+                }
+            },
+            DebruijnIndex::INNERMOST,
+        );
+        ty
+    }
+
+    pub fn apply_solution(&self, ctx: &mut InferenceContext<'_>, solution: Canonical<Substs>) {
+        // the solution may contain new variables, which we need to convert to new inference vars
+        let new_vars = Substs(
+            solution
+                .kinds
+                .iter()
+                .map(|k| match k {
+                    TyKind::General => ctx.table.new_type_var(),
+                    TyKind::Integer => ctx.table.new_integer_var(),
+                    TyKind::Float => ctx.table.new_float_var(),
+                })
+                .collect(),
+        );
+        for (i, ty) in solution.value.into_iter().enumerate() {
+            let var = self.free_vars[i];
+            // eagerly replace projections in the type; we may be getting types
+            // e.g. from where clauses where this hasn't happened yet
+            let ty = ctx.normalize_associated_types_in(ty.clone().subst_bound_vars(&new_vars));
+            ctx.table.unify(&Ty::Infer(var), &ty);
+        }
+    }
+}
+
+pub fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substs> {
+    let mut table = InferenceTable::new();
+    let vars = Substs(
+        tys.kinds
+            .iter()
+            // we always use type vars here because we want everything to
+            // fallback to Unknown in the end (kind of hacky, as below)
+            .map(|_| table.new_type_var())
+            .collect(),
+    );
+    let ty1_with_vars = tys.value.0.clone().subst_bound_vars(&vars);
+    let ty2_with_vars = tys.value.1.clone().subst_bound_vars(&vars);
+    if !table.unify(&ty1_with_vars, &ty2_with_vars) {
+        return None;
+    }
+    // default any type vars that weren't unified back to their original bound vars
+    // (kind of hacky)
+    for (i, var) in vars.iter().enumerate() {
+        if &*table.resolve_ty_shallow(var) == var {
+            table.unify(var, &Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i)));
+        }
+    }
+    Some(
+        Substs::builder(tys.kinds.len())
+            .fill(vars.iter().map(|v| table.resolve_ty_completely(v.clone())))
+            .build(),
+    )
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct InferenceTable {
+    pub(super) var_unification_table: InPlaceUnificationTable<TypeVarId>,
+}
+
+impl InferenceTable {
+    pub fn new() -> Self {
+        InferenceTable { var_unification_table: InPlaceUnificationTable::new() }
+    }
+
+    pub fn new_type_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::TypeVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    pub fn new_integer_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::IntVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    pub fn new_float_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::FloatVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
+    }
+
+    pub fn new_maybe_never_type_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::MaybeNeverTypeVar(
+            self.var_unification_table.new_key(TypeVarValue::Unknown),
+        ))
+    }
+
+    pub fn resolve_ty_completely(&mut self, ty: Ty) -> Ty {
+        self.resolve_ty_completely_inner(&mut Vec::new(), ty)
+    }
+
+    pub fn resolve_ty_as_possible(&mut self, ty: Ty) -> Ty {
+        self.resolve_ty_as_possible_inner(&mut Vec::new(), ty)
+    }
+
+    pub fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        self.unify_inner(ty1, ty2, 0)
+    }
+
+    pub 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_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, depth),
+        }
+    }
+
+    pub(super) fn unify_inner_trivial(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
+        match (ty1, ty2) {
+            (Ty::Unknown, _) | (_, Ty::Unknown) => true,
+
+            (Ty::Placeholder(p1), Ty::Placeholder(p2)) if *p1 == *p2 => true,
+
+            (Ty::Dyn(dyn1), Ty::Dyn(dyn2)) if dyn1.len() == dyn2.len() => {
+                for (pred1, pred2) in dyn1.iter().zip(dyn2.iter()) {
+                    if !self.unify_preds(pred1, pred2, depth + 1) {
+                        return false;
+                    }
+                }
+                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 unify_preds(
+        &mut self,
+        pred1: &GenericPredicate,
+        pred2: &GenericPredicate,
+        depth: usize,
+    ) -> bool {
+        match (pred1, pred2) {
+            (GenericPredicate::Implemented(tr1), GenericPredicate::Implemented(tr2))
+                if tr1.trait_ == tr2.trait_ =>
+            {
+                self.unify_substs(&tr1.substs, &tr2.substs, depth + 1)
+            }
+            (GenericPredicate::Projection(proj1), GenericPredicate::Projection(proj2))
+                if proj1.projection_ty.associated_ty == proj2.projection_ty.associated_ty =>
+            {
+                self.unify_substs(
+                    &proj1.projection_ty.parameters,
+                    &proj2.projection_ty.parameters,
+                    depth + 1,
+                ) && self.unify_inner(&proj1.ty, &proj2.ty, depth + 1)
+            }
+            _ => false,
+        }
+    }
+
+    /// If `ty` is a type variable with known type, returns that type;
+    /// otherwise, return ty.
+    pub 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 {
+                mark::hit!(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
+    }
+
+    /// 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_inner(&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) {
+                    mark::hit!(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_inner(tv_stack, known_ty.clone());
+                    tv_stack.pop();
+                    result
+                } else {
+                    ty
+                }
+            }
+            _ => ty,
+        })
+    }
+
+    /// Resolves the type completely; type variables without known type are
+    /// replaced by Ty::Unknown.
+    fn resolve_ty_completely_inner(&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) {
+                    mark::hit!(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_inner(tv_stack, known_ty.clone());
+                    tv_stack.pop();
+                    result
+                } else {
+                    tv.fallback_value()
+                }
+            }
+            _ => ty,
+        })
+    }
+}
+
+/// 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),
+        }
+    }
+}