Make Ty wrap TyKind in an Arc

... like it will be in Chalk. We still keep `interned_mut` and
`into_inner` methods that will probably not exist with Chalk.

This worsens performance slightly (5ginstr inference on RA), but doesn't
include other simplifications we can do yet.

crates/hir_ty/src/infer.rs

@@ -108,6 +108,17 @@ pub struct TypeMismatch {
     pub actual: Ty,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug)]
+struct InternedStandardTypes {
+    unknown: Ty,
+}
+
+impl Default for InternedStandardTypes {
+    fn default() -> Self {
+        InternedStandardTypes { unknown: TyKind::Unknown.intern(&Interner) }
+    }
+}
+
 /// The result of type inference: A mapping from expressions and patterns to types.
 #[derive(Clone, PartialEq, Eq, Debug, Default)]
 pub struct InferenceResult {
@@ -126,6 +137,8 @@ pub struct InferenceResult {
     pub type_of_expr: ArenaMap<ExprId, Ty>,
     pub type_of_pat: ArenaMap<PatId, Ty>,
     pub(super) type_mismatches: ArenaMap<ExprId, TypeMismatch>,
+    /// Interned Unknown to return references to.
+    standard_types: InternedStandardTypes,
 }
 
 impl InferenceResult {
@@ -170,7 +183,7 @@ impl Index<ExprId> for InferenceResult {
     type Output = Ty;
 
     fn index(&self, expr: ExprId) -> &Ty {
-        self.type_of_expr.get(expr).unwrap_or(&Ty(TyKind::Unknown))
+        self.type_of_expr.get(expr).unwrap_or(&self.standard_types.unknown)
     }
 }
 
@@ -178,7 +191,7 @@ impl Index<PatId> for InferenceResult {
     type Output = Ty;
 
     fn index(&self, pat: PatId) -> &Ty {
-        self.type_of_pat.get(pat).unwrap_or(&Ty(TyKind::Unknown))
+        self.type_of_pat.get(pat).unwrap_or(&self.standard_types.unknown)
     }
 }
 
@@ -723,14 +736,19 @@ impl Expectation {
 
     /// This expresses no expectation on the type.
     fn none() -> Self {
-        Expectation { ty: TyKind::Unknown.intern(&Interner), rvalue_hint: false }
+        Expectation {
+            // FIXME
+            ty: TyKind::Unknown.intern(&Interner),
+            rvalue_hint: false,
+        }
     }
 
-    fn coercion_target(&self) -> &Ty {
+    fn coercion_target(&self) -> Ty {
         if self.rvalue_hint {
-            &Ty(TyKind::Unknown)
+            // FIXME
+            TyKind::Unknown.intern(&Interner)
         } else {
-            &self.ty
+            self.ty.clone()
         }
     }
 }

crates/hir_ty/src/infer/coerce.rs

@@ -71,7 +71,7 @@ impl<'a> InferenceContext<'a> {
         }
 
         // Pointer weakening and function to pointer
-        match (&mut from_ty.0, to_ty.interned(&Interner)) {
+        match (from_ty.interned_mut(), to_ty.interned(&Interner)) {
             // `*mut T` -> `*const T`
             // `&mut T` -> `&T`
             (TyKind::Raw(m1, ..), TyKind::Raw(m2 @ Mutability::Not, ..))

crates/hir_ty/src/infer/expr.rs

@@ -800,7 +800,7 @@ impl<'a> InferenceContext<'a> {
                 // we don't even make an attempt at coercion
                 self.table.new_maybe_never_var()
             } else {
-                self.coerce(&Ty::unit(), expected.coercion_target());
+                self.coerce(&Ty::unit(), &expected.coercion_target());
                 Ty::unit()
             }
         };

crates/hir_ty/src/infer/pat.rs

@@ -158,11 +158,11 @@ impl<'a> InferenceContext<'a> {
                         if mutability != exp_mut {
                             // FIXME: emit type error?
                         }
-                        inner_ty
+                        inner_ty.clone()
                     }
-                    _ => &Ty(TyKind::Unknown),
+                    _ => self.result.standard_types.unknown.clone(),
                 };
-                let subty = self.infer_pat(*pat, expectation, default_bm);
+                let subty = self.infer_pat(*pat, &expectation, default_bm);
                 TyKind::Ref(mutability, Substs::single(subty)).intern(&Interner)
             }
             Pat::TupleStruct { path: p, args: subpats, ellipsis } => self.infer_tuple_struct_pat(
@@ -232,11 +232,11 @@ impl<'a> InferenceContext<'a> {
             Pat::Box { inner } => match self.resolve_boxed_box() {
                 Some(box_adt) => {
                     let inner_expected = match expected.as_adt() {
-                        Some((adt, substs)) if adt == box_adt => substs.as_single(),
-                        _ => &Ty(TyKind::Unknown),
+                        Some((adt, substs)) if adt == box_adt => substs.as_single().clone(),
+                        _ => self.result.standard_types.unknown.clone(),
                     };
 
-                    let inner_ty = self.infer_pat(*inner, inner_expected, default_bm);
+                    let inner_ty = self.infer_pat(*inner, &inner_expected, default_bm);
                     Ty::adt_ty(box_adt, Substs::single(inner_ty))
                 }
                 None => self.err_ty(),

crates/hir_ty/src/infer/unify.rs

@@ -108,7 +108,7 @@ impl<T> Canonicalized<T> {
     pub(super) fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
         ty.walk_mut_binders(
             &mut |ty, binders| {
-                if let &mut TyKind::BoundVar(bound) = &mut ty.0 {
+                if let &mut TyKind::BoundVar(bound) = ty.interned_mut() {
                     if bound.debruijn >= binders {
                         let (v, k) = self.free_vars[bound.index];
                         *ty = TyKind::InferenceVar(v, k).intern(&Interner);
@@ -404,7 +404,7 @@ impl InferenceTable {
             if i > 0 {
                 cov_mark::hit!(type_var_resolves_to_int_var);
             }
-            match &ty.0 {
+            match ty.interned(&Interner) {
                 TyKind::InferenceVar(tv, _) => {
                     let inner = tv.to_inner();
                     match self.var_unification_table.inlined_probe_value(inner).known() {

crates/hir_ty/src/lib.rs

@@ -248,11 +248,11 @@ pub enum TyKind {
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
-pub struct Ty(TyKind);
+pub struct Ty(Arc<TyKind>);
 
 impl TyKind {
     pub fn intern(self, _interner: &Interner) -> Ty {
-        Ty(self)
+        Ty(Arc::new(self))
     }
 }
 
@@ -260,6 +260,14 @@ impl Ty {
     pub fn interned(&self, _interner: &Interner) -> &TyKind {
         &self.0
     }
+
+    pub fn interned_mut(&mut self) -> &mut TyKind {
+        Arc::make_mut(&mut self.0)
+    }
+
+    pub fn into_inner(self) -> TyKind {
+        Arc::try_unwrap(self.0).unwrap_or_else(|a| (*a).clone())
+    }
 }
 
 /// A list of substitutions for generic parameters.
@@ -814,7 +822,7 @@ impl Ty {
     /// `self` is `Option<_>` and the substs contain `u32`, we'll have
     /// `Option<u32>` afterwards.)
     pub fn apply_substs(mut self, new_substs: Substs) -> Ty {
-        match &mut self.0 {
+        match self.interned_mut() {
             TyKind::Adt(_, substs)
             | TyKind::Slice(substs)
             | TyKind::Array(substs)
@@ -854,7 +862,7 @@ impl Ty {
     }
 
     pub fn substs_mut(&mut self) -> Option<&mut Substs> {
-        match &mut self.0 {
+        match self.interned_mut() {
             TyKind::Adt(_, substs)
             | TyKind::Slice(substs)
             | TyKind::Array(substs)
@@ -988,7 +996,7 @@ pub trait TypeWalk {
     {
         self.walk_mut_binders(
             &mut |ty_mut, binders| {
-                let ty = mem::replace(ty_mut, Ty(TyKind::Unknown));
+                let ty = mem::replace(ty_mut, TyKind::Unknown.intern(&Interner));
                 *ty_mut = f(ty, binders);
             },
             binders,
@@ -1001,7 +1009,7 @@ pub trait TypeWalk {
         Self: Sized,
     {
         self.walk_mut(&mut |ty_mut| {
-            let ty = mem::replace(ty_mut, Ty(TyKind::Unknown));
+            let ty = mem::replace(ty_mut, TyKind::Unknown.intern(&Interner));
             *ty_mut = f(ty);
         });
         self
@@ -1022,7 +1030,7 @@ pub trait TypeWalk {
     {
         self.walk_mut_binders(
             &mut |ty, binders| {
-                if let &mut TyKind::BoundVar(bound) = &mut ty.0 {
+                if let &mut TyKind::BoundVar(bound) = ty.interned_mut() {
                     if bound.debruijn >= binders {
                         *ty = substs.0[bound.index].clone().shift_bound_vars(binders);
                     }
@@ -1039,7 +1047,7 @@ pub trait TypeWalk {
         Self: Sized,
     {
         self.fold_binders(
-            &mut |ty, binders| match &ty.0 {
+            &mut |ty, binders| match ty.interned(&Interner) {
                 TyKind::BoundVar(bound) if bound.debruijn >= binders => {
                     TyKind::BoundVar(bound.shifted_in_from(n)).intern(&Interner)
                 }
@@ -1084,7 +1092,7 @@ impl TypeWalk for Ty {
         f: &mut impl FnMut(&mut Ty, DebruijnIndex),
         binders: DebruijnIndex,
     ) {
-        match &mut self.0 {
+        match self.interned_mut() {
             TyKind::Alias(AliasTy::Projection(p_ty)) => {
                 p_ty.substitution.walk_mut_binders(f, binders);
             }

crates/hir_ty/src/lower.rs

@@ -993,7 +993,7 @@ pub(crate) fn generic_defaults_query(
 
             // Each default can only refer to previous parameters.
             ty.walk_mut_binders(
-                &mut |ty, binders| match &mut ty.0 {
+                &mut |ty, binders| match ty.interned_mut() {
                     TyKind::BoundVar(BoundVar { debruijn, index }) if *debruijn == binders => {
                         if *index >= idx {
                             // type variable default referring to parameter coming

crates/hir_ty/src/traits/chalk/mapping.rs

@@ -24,7 +24,7 @@ use super::*;
 impl ToChalk for Ty {
     type Chalk = chalk_ir::Ty<Interner>;
     fn to_chalk(self, db: &dyn HirDatabase) -> chalk_ir::Ty<Interner> {
-        match self.0 {
+        match self.into_inner() {
             TyKind::Ref(m, parameters) => ref_to_chalk(db, m, parameters),
             TyKind::Array(parameters) => array_to_chalk(db, parameters),
             TyKind::Function(FnPointer { sig, substs, .. }) => {