Pre-intern some commonly used type variables.

This requires some rearrangement of plumbing, such as adding
`mk_fresh_{,int_,float_}ty` and removing `mk_ty_infer`.

Author: nnethercote

compiler/rustc_hir_typeck/src/demand.rs

@@ -273,12 +273,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ct_op: |c| c,
             ty_op: |t| match *t.kind() {
                 ty::Infer(ty::TyVar(_)) => self.tcx.mk_ty_var(ty::TyVid::from_u32(0)),
-                ty::Infer(ty::IntVar(_)) => {
-                    self.tcx.mk_ty_infer(ty::IntVar(ty::IntVid { index: 0 }))
-                }
-                ty::Infer(ty::FloatVar(_)) => {
-                    self.tcx.mk_ty_infer(ty::FloatVar(ty::FloatVid { index: 0 }))
-                }
+                ty::Infer(ty::IntVar(_)) => self.tcx.mk_int_var(ty::IntVid { index: 0 }),
+                ty::Infer(ty::FloatVar(_)) => self.tcx.mk_float_var(ty::FloatVid { index: 0 }),
                 _ => t,
             },
         };

compiler/rustc_infer/src/infer/freshen.rs

@@ -58,14 +58,9 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         }
     }
 
-    fn freshen_ty<F>(
-        &mut self,
-        opt_ty: Option<Ty<'tcx>>,
-        key: ty::InferTy,
-        freshener: F,
-    ) -> Ty<'tcx>
+    fn freshen_ty<F>(&mut self, opt_ty: Option<Ty<'tcx>>, key: ty::InferTy, mk_fresh: F) -> Ty<'tcx>
     where
-        F: FnOnce(u32) -> ty::InferTy,
+        F: FnOnce(u32) -> Ty<'tcx>,
     {
         if let Some(ty) = opt_ty {
             return ty.fold_with(self);
@@ -76,7 +71,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
             Entry::Vacant(entry) => {
                 let index = self.ty_freshen_count;
                 self.ty_freshen_count += 1;
-                let t = self.infcx.tcx.mk_ty_infer(freshener(index));
+                let t = mk_fresh(index);
                 entry.insert(t);
                 t
             }
@@ -204,7 +199,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         match v {
             ty::TyVar(v) => {
                 let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
-                Some(self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy))
+                Some(self.freshen_ty(opt_ty, ty::TyVar(v), |n| self.infcx.tcx.mk_fresh_ty(n)))
             }
 
             ty::IntVar(v) => Some(
@@ -216,7 +211,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
                         .probe_value(v)
                         .map(|v| v.to_type(self.infcx.tcx)),
                     ty::IntVar(v),
-                    ty::FreshIntTy,
+                    |n| self.infcx.tcx.mk_fresh_int_ty(n),
                 ),
             ),
 
@@ -229,7 +224,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
                         .probe_value(v)
                         .map(|v| v.to_type(self.infcx.tcx)),
                     ty::FloatVar(v),
-                    ty::FreshFloatTy,
+                    |n| self.infcx.tcx.mk_fresh_float_ty(n),
                 ),
             ),
 

compiler/rustc_middle/src/ty/context.rs

@@ -241,6 +241,11 @@ impl<'tcx> CtxtInterners<'tcx> {
     }
 }
 
+const NUM_PREINTERNED_TY_VARS: u32 = 100;
+const NUM_PREINTERNED_FRESH_TYS: u32 = 20;
+const NUM_PREINTERNED_FRESH_INT_TYS: u32 = 3;
+const NUM_PREINTERNED_FRESH_FLOAT_TYS: u32 = 3;
+
 pub struct CommonTypes<'tcx> {
     pub unit: Ty<'tcx>,
     pub bool: Ty<'tcx>,
@@ -266,7 +271,20 @@ pub struct CommonTypes<'tcx> {
     /// Dummy type used for the `Self` of a `TraitRef` created for converting
     /// a trait object, and which gets removed in `ExistentialTraitRef`.
     /// This type must not appear anywhere in other converted types.
+    /// `Infer(ty::FreshTy(0))` does the job.
     pub trait_object_dummy_self: Ty<'tcx>,
+
+    /// Pre-interned `Infer(ty::TyVar(n))` for small values of `n`.
+    pub ty_vars: Vec<Ty<'tcx>>,
+
+    /// Pre-interned `Infer(ty::FreshTy(n))` for small values of `n`.
+    pub fresh_tys: Vec<Ty<'tcx>>,
+
+    /// Pre-interned `Infer(ty::FreshIntTy(n))` for small values of `n`.
+    pub fresh_int_tys: Vec<Ty<'tcx>>,
+
+    /// Pre-interned `Infer(ty::FreshFloatTy(n))` for small values of `n`.
+    pub fresh_float_tys: Vec<Ty<'tcx>>,
 }
 
 pub struct CommonLifetimes<'tcx> {
@@ -289,6 +307,15 @@ impl<'tcx> CommonTypes<'tcx> {
     ) -> CommonTypes<'tcx> {
         let mk = |ty| interners.intern_ty(ty, sess, untracked);
 
+        let ty_vars =
+            (0..NUM_PREINTERNED_TY_VARS).map(|n| mk(Infer(ty::TyVar(TyVid::from(n))))).collect();
+        let fresh_tys: Vec<_> =
+            (0..NUM_PREINTERNED_FRESH_TYS).map(|n| mk(Infer(ty::FreshTy(n)))).collect();
+        let fresh_int_tys: Vec<_> =
+            (0..NUM_PREINTERNED_FRESH_INT_TYS).map(|n| mk(Infer(ty::FreshIntTy(n)))).collect();
+        let fresh_float_tys: Vec<_> =
+            (0..NUM_PREINTERNED_FRESH_FLOAT_TYS).map(|n| mk(Infer(ty::FreshFloatTy(n)))).collect();
+
         CommonTypes {
             unit: mk(Tuple(List::empty())),
             bool: mk(Bool),
@@ -311,7 +338,12 @@ impl<'tcx> CommonTypes<'tcx> {
             str_: mk(Str),
             self_param: mk(ty::Param(ty::ParamTy { index: 0, name: kw::SelfUpper })),
 
-            trait_object_dummy_self: mk(Infer(ty::FreshTy(0))),
+            trait_object_dummy_self: fresh_tys[0],
+
+            ty_vars,
+            fresh_tys,
+            fresh_int_tys,
+            fresh_float_tys,
         }
     }
 }
@@ -1868,28 +1900,54 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     #[inline]
-    pub fn mk_ty_var(self, v: TyVid) -> Ty<'tcx> {
-        self.mk_ty_infer(TyVar(v))
+    pub fn mk_const(self, kind: impl Into<ty::ConstKind<'tcx>>, ty: Ty<'tcx>) -> Const<'tcx> {
+        self.mk_const_internal(ty::ConstData { kind: kind.into(), ty })
     }
 
     #[inline]
-    pub fn mk_const(self, kind: impl Into<ty::ConstKind<'tcx>>, ty: Ty<'tcx>) -> Const<'tcx> {
-        self.mk_const_internal(ty::ConstData { kind: kind.into(), ty })
+    pub fn mk_ty_var(self, v: TyVid) -> Ty<'tcx> {
+        // Use a pre-interned one when possible.
+        self.types.ty_vars.get(v.as_usize()).copied().unwrap_or_else(|| self.mk_ty(Infer(TyVar(v))))
     }
 
     #[inline]
     pub fn mk_int_var(self, v: IntVid) -> Ty<'tcx> {
-        self.mk_ty_infer(IntVar(v))
+        self.mk_ty(Infer(IntVar(v)))
     }
 
     #[inline]
     pub fn mk_float_var(self, v: FloatVid) -> Ty<'tcx> {
-        self.mk_ty_infer(FloatVar(v))
+        self.mk_ty(Infer(FloatVar(v)))
+    }
+
+    #[inline]
+    pub fn mk_fresh_ty(self, n: u32) -> Ty<'tcx> {
+        // Use a pre-interned one when possible.
+        self.types
+            .fresh_tys
+            .get(n as usize)
+            .copied()
+            .unwrap_or_else(|| self.mk_ty(Infer(ty::FreshTy(n))))
+    }
+
+    #[inline]
+    pub fn mk_fresh_int_ty(self, n: u32) -> Ty<'tcx> {
+        // Use a pre-interned one when possible.
+        self.types
+            .fresh_int_tys
+            .get(n as usize)
+            .copied()
+            .unwrap_or_else(|| self.mk_ty(Infer(ty::FreshIntTy(n))))
     }
 
     #[inline]
-    pub fn mk_ty_infer(self, it: InferTy) -> Ty<'tcx> {
-        self.mk_ty(Infer(it))
+    pub fn mk_fresh_float_ty(self, n: u32) -> Ty<'tcx> {
+        // Use a pre-interned one when possible.
+        self.types
+            .fresh_float_tys
+            .get(n as usize)
+            .copied()
+            .unwrap_or_else(|| self.mk_ty(Infer(ty::FreshFloatTy(n))))
     }
 
     #[inline]

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1209,7 +1209,7 @@ pub trait PrettyPrinter<'tcx>:
                 // in order to place the projections inside the `<...>`.
                 if !resugared {
                     // Use a type that can't appear in defaults of type parameters.
-                    let dummy_cx = cx.tcx().mk_ty_infer(ty::FreshTy(0));
+                    let dummy_cx = cx.tcx().mk_fresh_ty(0);
                     let principal = principal.with_self_ty(cx.tcx(), dummy_cx);
 
                     let args = cx
@@ -2696,7 +2696,7 @@ define_print_and_forward_display! {
 
     ty::ExistentialTraitRef<'tcx> {
         // Use a type that can't appear in defaults of type parameters.
-        let dummy_self = cx.tcx().mk_ty_infer(ty::FreshTy(0));
+        let dummy_self = cx.tcx().mk_fresh_ty(0);
         let trait_ref = self.with_self_ty(cx.tcx(), dummy_self);
         p!(print(trait_ref.print_only_trait_path()))
     }

compiler/rustc_symbol_mangling/src/v0.rs

@@ -540,7 +540,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
                 match predicate.as_ref().skip_binder() {
                     ty::ExistentialPredicate::Trait(trait_ref) => {
                         // Use a type that can't appear in defaults of type parameters.
-                        let dummy_self = cx.tcx.mk_ty_infer(ty::FreshTy(0));
+                        let dummy_self = cx.tcx.mk_fresh_ty(0);
                         let trait_ref = trait_ref.with_self_ty(cx.tcx, dummy_self);
                         cx = cx.print_def_path(trait_ref.def_id, trait_ref.substs)?;
                     }

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -879,7 +879,7 @@ pub fn object_region_bounds<'tcx>(
     // Since we don't actually *know* the self type for an object,
     // this "open(err)" serves as a kind of dummy standin -- basically
     // a placeholder type.
-    let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
+    let open_ty = tcx.mk_fresh_ty(0);
 
     let predicates = existential_predicates.iter().filter_map(|predicate| {
         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {