From 2ae298711f92a79ea5bc59479846bfeea49481ef Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Fri, 9 Jan 2026 10:42:48 +1100
Subject: [PATCH] Replace two `BottomUpFolder`s with `fold_regions`.

Because these folders only change regions.

Note: `BottomUpFolder` folds all regions, while `fold_regions` skips
some bound regions. But that's ok because these two folders only modify
`ReVar`s.
---
 .../src/interpret/intrinsics.rs                 | 11 +++--------
 compiler/rustc_hir_analysis/src/check/check.rs  | 17 ++++++-----------
 2 files changed, 9 insertions(+), 19 deletions(-)

diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index 94832c0c2577..fe1dd1b6eb35 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -13,7 +13,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::interpret::{CTFE_ALLOC_SALT, read_target_uint, write_target_uint};
 use rustc_middle::mir::{self, BinOp, ConstValue, NonDivergingIntrinsic};
 use rustc_middle::ty::layout::TyAndLayout;
-use rustc_middle::ty::{FloatTy, PolyExistentialPredicate, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{FloatTy, PolyExistentialPredicate, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug, ty};
 use rustc_span::{Symbol, sym};
 use rustc_trait_selection::traits::{Obligation, ObligationCause, ObligationCtxt};
@@ -243,13 +243,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 ocx.register_obligations(preds.iter().map(|pred: PolyExistentialPredicate<'_>| {
                     let pred = pred.with_self_ty(tcx, tp_ty);
                     // Lifetimes can only be 'static because of the bound on T
-                    let pred = pred.fold_with(&mut ty::BottomUpFolder {
-                        tcx,
-                        ty_op: |ty| ty,
-                        lt_op: |lt| {
-                            if lt == tcx.lifetimes.re_erased { tcx.lifetimes.re_static } else { lt }
-                        },
-                        ct_op: |ct| ct,
+                    let pred = ty::fold_regions(tcx, pred, |r, _| {
+                        if r == tcx.lifetimes.re_erased { tcx.lifetimes.re_static } else { r }
                     });
                     Obligation::new(tcx, ObligationCause::dummy(), param_env, pred)
                 }));
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index ad66003a6bf9..4664bfcce853 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -508,23 +508,18 @@ fn sanity_check_found_hidden_type<'tcx>(
             return Ok(());
         }
     }
-    let strip_vars = |ty: Ty<'tcx>| {
-        ty.fold_with(&mut BottomUpFolder {
-            tcx,
-            ty_op: |t| t,
-            ct_op: |c| c,
-            lt_op: |l| match l.kind() {
-                RegionKind::ReVar(_) => tcx.lifetimes.re_erased,
-                _ => l,
-            },
+    let erase_re_vars = |ty: Ty<'tcx>| {
+        fold_regions(tcx, ty, |r, _| match r.kind() {
+            RegionKind::ReVar(_) => tcx.lifetimes.re_erased,
+            _ => r,
         })
     };
     // Closures frequently end up containing erased lifetimes in their final representation.
     // These correspond to lifetime variables that never got resolved, so we patch this up here.
-    ty.ty = strip_vars(ty.ty);
+    ty.ty = erase_re_vars(ty.ty);
     // Get the hidden type.
     let hidden_ty = tcx.type_of(key.def_id).instantiate(tcx, key.args);
-    let hidden_ty = strip_vars(hidden_ty);
+    let hidden_ty = erase_re_vars(hidden_ty);
 
     // If the hidden types differ, emit a type mismatch diagnostic.
     if hidden_ty == ty.ty {