Rollup merge of #116792 - JonasAlaif:renumber-fix, r=b-naber

Avoid unnecessary renumbering during borrowck

Currently, after renumbering there are always unused `RegionVid`s if the return type contains any regions, this is due to `visit_ty` being called twice on the same `Ty`: once with `TyContext::ReturnTy` and once with `TyContext::LocalDecl { local: _0 }`. This PR skips renumbering the first time around.

compiler/rustc_borrowck/src/renumber.rs

@@ -81,6 +81,10 @@ impl<'a, 'tcx> MutVisitor<'tcx> for RegionRenumberer<'a, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn visit_ty(&mut self, ty: &mut Ty<'tcx>, ty_context: TyContext) {
+        if matches!(ty_context, TyContext::ReturnTy(_)) {
+            // We will renumber the return ty when called again with `TyContext::LocalDecl`
+            return;
+        }
         *ty = self.renumber_regions(*ty, || RegionCtxt::TyContext(ty_context));
 
         debug!(?ty);