Auto merge of #99232 - lcnr:no-bound-vars-check, r=jackh726

`replace_bound_vars` fast path: check predicates, don't check consts

split out from #98900

`ty::Const` doesn't have precomputed type flags, so
computing `has_vars_bound_at_or_above` for constants
requires us to visit the const and its contained types
and constants. A noop fold should be pretty much equally as
fast so removing it prevents us from walking the constant twice
in case it contains bound vars.

r? `@jackh726`

Author: bors

Diff for: compiler/rustc_middle/src/ty/fold.rs

@@ -445,10 +445,13 @@ impl<'a, 'tcx> TypeFolder<'tcx> for BoundVarReplacer<'a, 'tcx> {
                 let ct = (self.fld_c)(bound_const, ct.ty());
                 ty::fold::shift_vars(self.tcx, ct, self.current_index.as_u32())
             }
-            _ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
-            _ => ct,
+            _ => ct.super_fold_with(self),
         }
     }
+
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if p.has_vars_bound_at_or_above(self.current_index) { p.super_fold_with(self) } else { p }
+    }
 }
 
 impl<'tcx> TyCtxt<'tcx> {

Diff for: compiler/rustc_trait_selection/src/traits/project.rs

@@ -753,10 +753,13 @@ impl<'tcx> TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
                     .tcx
                     .mk_const(ty::ConstS { kind: ty::ConstKind::Placeholder(p), ty: ct.ty() })
             }
-            _ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
-            _ => ct,
+            _ => ct.super_fold_with(self),
         }
     }
+
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if p.has_vars_bound_at_or_above(self.current_index) { p.super_fold_with(self) } else { p }
+    }
 }
 
 // The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.