6 files changed, 312 insertions, 263 deletions
diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
index fe259371f..81afbd2b4 100644
--- a/crates/ra_hir_ty/src/infer.rs
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -18,7 +18,6 @@ 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::{
@@ -33,12 +32,11 @@ use hir_def::{
 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,
+    ApplicationTy, InEnvironment, ProjectionTy, TraitEnvironment, TraitRef, Ty, TypeCtor,
    TypeWalk, Uncertain,
 };
 use crate::{db::HirDatabase, infer::diagnostics::InferenceDiagnostic};
@@ -191,7 +189,7 @@ struct InferenceContext<'a, D: HirDatabase> {
    owner: DefWithBodyId,
    body: Arc<Body>,
    resolver: Resolver,
-    var_unification_table: InPlaceUnificationTable<TypeVarId>,
+    table: unify::InferenceTable,
    trait_env: Arc<TraitEnvironment>,
    obligations: Vec<Obligation>,
    result: InferenceResult,
@@ -209,7 +207,7 @@ 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(),
+            table: unify::InferenceTable::new(),
            obligations: Vec::default(),
            return_ty: Ty::Unknown, // set in collect_fn_signature
            trait_env: TraitEnvironment::lower(db, &resolver),
@@ -224,13 +222,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
    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));
+            let resolved = self.table.resolve_ty_completely(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));
+            let resolved = self.table.resolve_ty_completely(mem::replace(ty, Ty::Unknown));
            *ty = resolved;
        }
        result
@@ -275,96 +272,15 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        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::Unknown => self.table.new_type_var(),
            Ty::Apply(ApplicationTy { ctor: TypeCtor::Int(Uncertain::Unknown), .. }) => {
-                self.new_integer_var()
+                self.table.new_integer_var()
            }
            Ty::Apply(ApplicationTy { ctor: TypeCtor::Float(Uncertain::Unknown), .. }) => {
-                self.new_float_var()
+                self.table.new_float_var()
            }
            _ => ty,
        }
@@ -402,64 +318,22 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        }
    }
+    fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        self.table.unify(ty1, ty2)
+    }
    /// 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 {
+    fn resolve_ty_as_possible(&mut self, ty: Ty) -> Ty {
        self.resolve_obligations_as_possible();
-        ty.fold(&mut |ty| match ty {
+        self.table.resolve_ty_as_possible(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);
+        self.table.resolve_ty_shallow(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
@@ -469,7 +343,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
    /// 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);
+        let ty = self.resolve_ty_as_possible(ty);
        ty.fold(&mut |ty| match ty {
            Ty::Projection(proj_ty) => self.normalize_projection_ty(proj_ty),
            _ => ty,
@@ -477,40 +351,13 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
    }
    fn normalize_projection_ty(&mut self, proj_ty: ProjectionTy) -> Ty {
-        let var = self.new_type_var();
+        let var = self.table.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,
@@ -615,78 +462,20 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
    }
 }
-/// 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),
+    TypeVar(unify::TypeVarId),
-    IntVar(TypeVarId),
+    IntVar(unify::TypeVarId),
-    FloatVar(TypeVarId),
+    FloatVar(unify::TypeVarId),
-    MaybeNeverTypeVar(TypeVarId),
+    MaybeNeverTypeVar(unify::TypeVarId),
 }
 impl InferTy {
-    fn to_inner(self) -> TypeVarId {
+    fn to_inner(self) -> unify::TypeVarId {
        match self {
            InferTy::TypeVar(ty)
            | InferTy::IntVar(ty)
diff --git a/crates/ra_hir_ty/src/infer/coerce.rs b/crates/ra_hir_ty/src/infer/coerce.rs
index 064993d34..9bfc701cd 100644
--- a/crates/ra_hir_ty/src/infer/coerce.rs
+++ b/crates/ra_hir_ty/src/infer/coerce.rs
@@ -10,7 +10,7 @@ use test_utils::tested_by;
 use crate::{autoderef, db::HirDatabase, Substs, Ty, TypeCtor, TypeWalk};
-use super::{InEnvironment, InferTy, InferenceContext, TypeVarValue};
+use super::{InEnvironment, InferTy, InferenceContext, unify::TypeVarValue};
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
    /// Unify two types, but may coerce the first one to the second one
@@ -85,8 +85,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        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.new_maybe_never_type_var();
+                let var = self.table.new_maybe_never_type_var();
-                self.var_unification_table.union_value(*tv, TypeVarValue::Known(var));
+                self.table.var_unification_table.union_value(*tv, TypeVarValue::Known(var));
                return true;
            }
            (ty_app!(TypeCtor::Never), _) => return true,
@@ -94,7 +94,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
            // Trivial cases, this should go after `never` check to
            // avoid infer result type to be never
            _ => {
-                if self.unify_inner_trivial(&from_ty, &to_ty) {
+                if self.table.unify_inner_trivial(&from_ty, &to_ty) {
                    return true;
                }
            }
@@ -330,7 +330,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                // 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.unify_substs(st1, st2, 0);
+                    return self.table.unify_substs(st1, st2, 0);
                }
                _ => {}
            }
diff --git a/crates/ra_hir_ty/src/infer/expr.rs b/crates/ra_hir_ty/src/infer/expr.rs
index 4014f4732..1e78f6efd 100644
--- a/crates/ra_hir_ty/src/infer/expr.rs
+++ b/crates/ra_hir_ty/src/infer/expr.rs
@@ -32,7 +32,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() },
            );
        }
-        let ty = self.resolve_ty_as_possible(&mut vec![], ty);
+        let ty = self.resolve_ty_as_possible(ty);
        ty
    }
@@ -53,7 +53,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
            expected.ty.clone()
        };
-        self.resolve_ty_as_possible(&mut vec![], ty)
+        self.resolve_ty_as_possible(ty)
    }
    fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
@@ -94,7 +94,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                let pat_ty = match self.resolve_into_iter_item() {
                    Some(into_iter_item_alias) => {
-                        let pat_ty = self.new_type_var();
+                        let pat_ty = self.table.new_type_var();
                        let projection = ProjectionPredicate {
                            ty: pat_ty.clone(),
                            projection_ty: ProjectionTy {
@@ -103,7 +103,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                            },
                        };
                        self.obligations.push(Obligation::Projection(projection));
-                        self.resolve_ty_as_possible(&mut vec![], pat_ty)
+                        self.resolve_ty_as_possible(pat_ty)
                    }
                    None => Ty::Unknown,
                };
@@ -128,7 +128,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                }
                // add return type
-                let ret_ty = self.new_type_var();
+                let ret_ty = 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 },
@@ -167,7 +167,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
            Expr::Match { expr, arms } => {
                let input_ty = self.infer_expr(*expr, &Expectation::none());
-                let mut result_ty = self.new_maybe_never_type_var();
+                let mut result_ty = self.table.new_maybe_never_type_var();
                for arm in arms {
                    for &pat in &arm.pats {
@@ -283,7 +283,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                let inner_ty = self.infer_expr(*expr, &Expectation::none());
                let ty = match self.resolve_future_future_output() {
                    Some(future_future_output_alias) => {
-                        let ty = self.new_type_var();
+                        let ty = self.table.new_type_var();
                        let projection = ProjectionPredicate {
                            ty: ty.clone(),
                            projection_ty: ProjectionTy {
@@ -292,7 +292,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                            },
                        };
                        self.obligations.push(Obligation::Projection(projection));
-                        self.resolve_ty_as_possible(&mut vec![], ty)
+                        self.resolve_ty_as_possible(ty)
                    }
                    None => Ty::Unknown,
                };
@@ -302,7 +302,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                let inner_ty = self.infer_expr(*expr, &Expectation::none());
                let ty = match self.resolve_ops_try_ok() {
                    Some(ops_try_ok_alias) => {
-                        let ty = self.new_type_var();
+                        let ty = self.table.new_type_var();
                        let projection = ProjectionPredicate {
                            ty: ty.clone(),
                            projection_ty: ProjectionTy {
@@ -311,7 +311,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                            },
                        };
                        self.obligations.push(Obligation::Projection(projection));
-                        self.resolve_ty_as_possible(&mut vec![], ty)
+                        self.resolve_ty_as_possible(ty)
                    }
                    None => Ty::Unknown,
                };
@@ -465,10 +465,10 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                    ty_app!(TypeCtor::Tuple { .. }, st) => st
                        .iter()
                        .cloned()
-                        .chain(repeat_with(|| self.new_type_var()))
+                        .chain(repeat_with(|| self.table.new_type_var()))
                        .take(exprs.len())
                        .collect::<Vec<_>>(),
-                    _ => (0..exprs.len()).map(|_| self.new_type_var()).collect(),
+                    _ => (0..exprs.len()).map(|_| self.table.new_type_var()).collect(),
                };
                for (expr, ty) in exprs.iter().zip(tys.iter_mut()) {
@@ -482,7 +482,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                    ty_app!(TypeCtor::Array, st) | ty_app!(TypeCtor::Slice, st) => {
                        st.as_single().clone()
                    }
-                    _ => self.new_type_var(),
+                    _ => self.table.new_type_var(),
                };
                match array {
@@ -524,7 +524,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        };
        // 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(&mut vec![], ty);
+        let ty = self.resolve_ty_as_possible(ty);
        self.write_expr_ty(tgt_expr, ty.clone());
        ty
    }
@@ -553,7 +553,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                        }
                    }
-                    let ty = self.resolve_ty_as_possible(&mut vec![], ty);
+                    let ty = self.resolve_ty_as_possible(ty);
                    self.infer_pat(*pat, &ty, BindingMode::default());
                }
                Statement::Expr(expr) => {
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index 1ebb36239..a14662884 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -170,7 +170,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                    }
                    BindingMode::Move => inner_ty.clone(),
                };
-                let bound_ty = self.resolve_ty_as_possible(&mut vec![], bound_ty);
+                let bound_ty = self.resolve_ty_as_possible(bound_ty);
                self.write_pat_ty(pat, bound_ty);
                return inner_ty;
            }
@@ -179,7 +179,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        // use a new type variable if we got Ty::Unknown here
        let ty = self.insert_type_vars_shallow(ty);
        self.unify(&ty, expected);
-        let ty = self.resolve_ty_as_possible(&mut vec![], ty);
+        let ty = self.resolve_ty_as_possible(ty);
        self.write_pat_ty(pat, ty.clone());
        ty
    }
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index bbf146418..d0d7646a4 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -57,7 +57,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
        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(&mut vec![], ty);
+                let ty = self.resolve_ty_as_possible(ty);
                return Some(ty);
            }
            ValueNs::FunctionId(it) => it.into(),
@@ -211,7 +211,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                        // we're picking this method
                        let trait_substs = Substs::build_for_def(self.db, trait_)
                            .push(ty.clone())
-                            .fill(std::iter::repeat_with(|| self.new_type_var()))
+                            .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                            .build();
                        let substs = Substs::build_for_def(self.db, item)
                            .use_parent_substs(&trait_substs)
diff --git a/crates/ra_hir_ty/src/infer/unify.rs b/crates/ra_hir_ty/src/infer/unify.rs
index f3a875678..ff50138f5 100644
--- a/crates/ra_hir_ty/src/infer/unify.rs
+++ b/crates/ra_hir_ty/src/infer/unify.rs
@@ -1,9 +1,15 @@
 //! Unification and canonicalization logic.
+use std::borrow::Cow;
+use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
+use test_utils::tested_by;
 use super::{InferenceContext, Obligation};
 use crate::{
    db::HirDatabase, utils::make_mut_slice, Canonical, InEnvironment, InferTy, ProjectionPredicate,
-    ProjectionTy, Substs, TraitRef, Ty, TypeWalk,
+    ProjectionTy, Substs, TraitRef, Ty, TypeCtor, TypeWalk,
 };
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
@@ -24,7 +30,7 @@ where
    /// 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<super::TypeVarId>,
+    var_stack: Vec<TypeVarId>,
 }
 pub(super) struct Canonicalized<T> {
@@ -53,14 +59,14 @@ where
                    return tv.fallback_value();
                }
                if let Some(known_ty) =
-                    self.ctx.var_unification_table.inlined_probe_value(inner).known()
+                    self.ctx.table.var_unification_table.inlined_probe_value(inner).known()
                {
                    self.var_stack.push(inner);
                    let result = self.do_canonicalize_ty(known_ty.clone());
                    self.var_stack.pop();
                    result
                } else {
-                    let root = self.ctx.var_unification_table.find(inner);
+                    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),
@@ -153,10 +159,264 @@ impl<T> Canonicalized<T> {
        solution: Canonical<Vec<Ty>>,
    ) {
        // the solution may contain new variables, which we need to convert to new inference vars
-        let new_vars = Substs((0..solution.num_vars).map(|_| ctx.new_type_var()).collect());
+        let new_vars = Substs((0..solution.num_vars).map(|_| ctx.table.new_type_var()).collect());
        for (i, ty) in solution.value.into_iter().enumerate() {
            let var = self.free_vars[i];
-            ctx.unify(&Ty::Infer(var), &ty.subst_bound_vars(&new_vars));
+            ctx.table.unify(&Ty::Infer(var), &ty.subst_bound_vars(&new_vars));
+        }
+    }
+}
+pub fn unify(ty1: Canonical<&Ty>, ty2: &Ty) -> Substs {
+    let mut table = InferenceTable::new();
+    let vars = Substs::builder(ty1.num_vars)
+        .fill(std::iter::repeat_with(|| table.new_type_var())).build();
+    let ty_with_vars = ty1.value.clone().subst_bound_vars(&vars);
+    table.unify(&ty_with_vars, ty2);
+    Substs::builder(ty1.num_vars).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),
+        }
+    }
+    pub(super) 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,
+        }
+    }
+    /// 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 {
+                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
+    }
+    /// 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) {
+                    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_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) {
+                    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_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),
        }
    }
 }