Make TypeFoldable::is_global() false when fresh tys/consts are present

This ensures that `ParamEnv::and` preserves the original `caller_bounds`
when we have a value containing fresh tys/consts. This ensures that when
we cache a `SelectionCandidate`, the cache key (a `ParamEnvAnd`)
contains all of the information that influenced the computation of our
result (e.g. we may end up choosing a `ParamCandidate`)

Author: Aaron1011

compiler/rustc_middle/src/ty/flags.rs

@@ -141,7 +141,9 @@ impl FlagComputation {
             &ty::Infer(infer) => {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
-                    ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {}
+                    ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
+                        self.add_flags(TypeFlags::HAS_TY_FRESH)
+                    }
 
                     ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => {
                         self.add_flags(TypeFlags::HAS_TY_INFER)
@@ -278,7 +280,7 @@ impl FlagComputation {
             ty::ConstKind::Infer(infer) => {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
-                    InferConst::Fresh(_) => {}
+                    InferConst::Fresh(_) => self.add_flags(TypeFlags::HAS_CT_FRESH),
                     InferConst::Var(_) => self.add_flags(TypeFlags::HAS_CT_INFER),
                 }
             }

compiler/rustc_type_ir/src/lib.rs

@@ -59,6 +59,15 @@ bitflags! {
                                           | TypeFlags::HAS_CT_INFER.bits
                                           | TypeFlags::HAS_TY_PLACEHOLDER.bits
                                           | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          // We consider 'freshened' types and constants
+                                          // to depend on a particular fn.
+                                          // The freshening process throws away information,
+                                          // which can make things unsuitable for use in a global
+                                          // cache. Note that there is no 'fresh lifetime' flag -
+                                          // freshening replaces all lifetimes with `ReErased`,
+                                          // which is different from how types/const are freshened.
+                                          | TypeFlags::HAS_TY_FRESH.bits
+                                          | TypeFlags::HAS_CT_FRESH.bits
                                           | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits;
 
         /// Does this have `Projection`?
@@ -90,6 +99,12 @@ 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 have `InferTy::FreshTy/FreshIntTy/FreshFloatTy`?
+        const HAS_TY_FRESH                = 1 << 18;
+
+        /// Does this value have `InferConst::Fresh`?
+        const HAS_CT_FRESH                = 1 << 19;
     }
 }