Rollup merge of #109240 - compiler-errors:dont-binder-twice, r=oli-obk

Walk un-shifted nested `impl Trait` in trait when setting up default trait method assumptions

Fixes a double subtraction in some binder math in return-position `impl Trait` in trait handling code.

Fixes #109239

compiler/rustc_middle/src/ty/fold.rs

@@ -51,9 +51,7 @@ where
 // Region folder
 
 impl<'tcx> TyCtxt<'tcx> {
-    /// Folds the escaping and free regions in `value` using `f`, and
-    /// sets `skipped_regions` to true if any late-bound region was found
-    /// and skipped.
+    /// Folds the escaping and free regions in `value` using `f`.
     pub fn fold_regions<T>(
         self,
         value: T,
@@ -64,17 +62,6 @@ impl<'tcx> TyCtxt<'tcx> {
     {
         value.fold_with(&mut RegionFolder::new(self, &mut f))
     }
-
-    pub fn super_fold_regions<T>(
-        self,
-        value: T,
-        mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
-    ) -> T
-    where
-        T: TypeSuperFoldable<TyCtxt<'tcx>>,
-    {
-        value.super_fold_with(&mut RegionFolder::new(self, &mut f))
-    }
 }
 
 /// Folds over the substructure of a type, visiting its component