Auto merge of rust-lang#94799 - lcnr:list-ty-perf, r=petrochenkov

update `hash_stable` for `List<Ty<'tcx>>`

cc rust-lang#93505 (comment)

this is the hottest part changed since the pre-merge perf run

Author: bors

compiler/rustc_middle/src/ty/impls_ty.rs

@@ -86,18 +86,16 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::subst::GenericArgKin
             //
             // In order to make it very unlikely for the sequence of bytes being hashed for
             // a `GenericArgKind::Type` to be the same as the sequence of bytes being
-            // hashed for one of the other variants, we hash a `0xFF` byte before hashing
-            // their discriminant (since the discriminant of `TyKind` is unlikely to ever start
-            // with 0xFF).
+            // hashed for one of the other variants, we hash some very high number instead
+            // of their actual discriminant since `TyKind` should never start with anything
+            // that high.
             ty::subst::GenericArgKind::Type(ty) => ty.hash_stable(hcx, hasher),
             ty::subst::GenericArgKind::Const(ct) => {
-                0xFFu8.hash_stable(hcx, hasher);
-                mem::discriminant(self).hash_stable(hcx, hasher);
+                0xF3u8.hash_stable(hcx, hasher);
                 ct.hash_stable(hcx, hasher);
             }
             ty::subst::GenericArgKind::Lifetime(lt) => {
-                0xFFu8.hash_stable(hcx, hasher);
-                mem::discriminant(self).hash_stable(hcx, hasher);
+                0xF5u8.hash_stable(hcx, hasher);
                 lt.hash_stable(hcx, hasher);
             }
         }