ty: add MAY_POLYMORPHIZE flag

This commit adds a `MAY_POLYMORPHIZE` which checks for closures and
generators so that polymorphization of substs does not need to traverse
every substs.

Signed-off-by: David Wood <david@davidtw.co>

Author: davidtwco

src/librustc_middle/ty/flags.rs

@@ -85,6 +85,8 @@ impl FlagComputation {
             }
 
             &ty::Generator(_, ref substs, _) => {
+                self.add_flags(TypeFlags::MAY_POLYMORPHIZE);
+
                 let substs = substs.as_generator();
                 let should_remove_further_specializable =
                     !self.flags.contains(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
@@ -107,6 +109,8 @@ impl FlagComputation {
             }
 
             &ty::Closure(_, substs) => {
+                self.add_flags(TypeFlags::MAY_POLYMORPHIZE);
+
                 let substs = substs.as_closure();
                 let should_remove_further_specializable =
                     !self.flags.contains(TypeFlags::STILL_FURTHER_SPECIALIZABLE);

src/librustc_middle/ty/fold.rs

@@ -150,6 +150,12 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
     }
 
+    /// Does this value contain closures or generators such that it may require
+    /// polymorphization?
+    fn may_polymorphize(&self) -> bool {
+        self.has_type_flags(TypeFlags::MAY_POLYMORPHIZE)
+    }
+
     /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
     fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> bool) -> bool {
         pub struct Visitor<F>(F);

src/librustc_middle/ty/instance.rs

@@ -528,7 +528,11 @@ fn polymorphize<'tcx>(
                     // ..then use the identity for this parameter.
                     tcx.mk_param_from_def(param),
 
-            // Otherwise, use the parameter as before (polymorphizing any closures or generators).
+            // If the parameter does not contain any closures or generators, then use the
+            // substitution directly.
+            _ if !substs.may_polymorphize() => substs[param.index as usize],
+
+            // Otherwise, use the substitution after polymorphizing.
             _ => {
                 let arg = substs[param.index as usize];
                 let polymorphized_arg = arg.fold_with(&mut PolymorphizationFolder { tcx });

src/librustc_middle/ty/mod.rs

@@ -575,6 +575,10 @@ bitflags! {
         /// Does this value have parameters/placeholders/inference variables which could be
         /// replaced later, in a way that would change the results of `impl` specialization?
         const STILL_FURTHER_SPECIALIZABLE = 1 << 17;
+
+        /// Does this value contain closures or generators such that it may require
+        /// polymorphization?
+        const MAY_POLYMORPHIZE = 1 << 18;
     }
 }