Auto merge of rust-lang#139291 - compiler-errors:folder-experiment-5, r=<try>

Folder experiment: Account for flags in const folders

**NOTE:** This is one of a series of perf experiments that I've come up with while sick in bed. I'm assigning them to lqd b/c you're a good reviewer and you'll hopefully be awake when these experiments finish, lol.

r? lqd

These are probably actually neutral given that we're not often folding large consts in the wild (since we're not in a GCE world), but they do run somewhat often, so let's see what perf says. I'm not actually totally certain when the abstract const expander runs, tbh.

Author: bors

compiler/rustc_middle/src/ty/abstract_const.rs

@@ -63,7 +63,12 @@ impl<'tcx> TyCtxt<'tcx> {
                     },
                     _ => c,
                 };
-                ct.super_fold_with(self)
+
+                if ct.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                    ct.super_fold_with(self)
+                } else {
+                    ct
+                }
             }
         }
         ac.fold_with(&mut Expander { tcx: self })

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -376,6 +376,22 @@ pub fn normalize_param_env_or_error<'tcx>(
 
                     c
                 }
+
+                fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+                    if t.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                        t.super_fold_with(self)
+                    } else {
+                        t
+                    }
+                }
+
+                fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+                    if p.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                        p.super_fold_with(self)
+                    } else {
+                        p
+                    }
+                }
             }
 
             // This whole normalization step is a hack to work around the fact that