diff --git a/compiler/rustc_borrowck/src/type_check/relate_tys.rs b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
index 02a41469c97f4..5a13512ecd503 100644
--- a/compiler/rustc_borrowck/src/type_check/relate_tys.rs
+++ b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
@@ -167,7 +167,7 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
             let infcx = self.type_checker.infcx;
             let mut lazy_universe = None;
             let delegate = FnMutDelegate {
-                regions: &mut |br: ty::BoundRegion| {
+                regions: |br: ty::BoundRegion| {
                     // The first time this closure is called, create a
                     // new universe for the placeholders we will make
                     // from here out.
@@ -184,10 +184,10 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
 
                     placeholder_reg
                 },
-                types: &mut |_bound_ty: ty::BoundTy| {
+                types: |_bound_ty: ty::BoundTy| {
                     unreachable!("we only replace regions in nll_relate, not types")
                 },
-                consts: &mut |_bound_var: ty::BoundVar| {
+                consts: |_bound_var: ty::BoundVar| {
                     unreachable!("we only replace regions in nll_relate, not consts")
                 },
             };
@@ -211,7 +211,7 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
         let infcx = self.type_checker.infcx;
         let mut reg_map = FxHashMap::default();
         let delegate = FnMutDelegate {
-            regions: &mut |br: ty::BoundRegion| {
+            regions: |br: ty::BoundRegion| {
                 if let Some(ex_reg_var) = reg_map.get(&br) {
                     *ex_reg_var
                 } else {
@@ -222,10 +222,10 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
                     ex_reg_var
                 }
             },
-            types: &mut |_bound_ty: ty::BoundTy| {
+            types: |_bound_ty: ty::BoundTy| {
                 unreachable!("we only replace regions in nll_relate, not types")
             },
-            consts: &mut |_bound_var: ty::BoundVar| {
+            consts: |_bound_var: ty::BoundVar| {
                 unreachable!("we only replace regions in nll_relate, not consts")
             },
         };
diff --git a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
index 2143f72a3b0a5..1deb28e59ba99 100644
--- a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
+++ b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
@@ -33,19 +33,19 @@ impl<'tcx> InferCtxt<'tcx> {
         let next_universe = self.create_next_universe();
 
         let delegate = FnMutDelegate {
-            regions: &mut |br: ty::BoundRegion| {
+            regions: |br: ty::BoundRegion| {
                 ty::Region::new_placeholder(
                     self.tcx,
                     ty::PlaceholderRegion { universe: next_universe, bound: br },
                 )
             },
-            types: &mut |bound_ty: ty::BoundTy| {
+            types: |bound_ty: ty::BoundTy| {
                 Ty::new_placeholder(
                     self.tcx,
                     ty::PlaceholderType { universe: next_universe, bound: bound_ty },
                 )
             },
-            consts: &mut |bound_var: ty::BoundVar| {
+            consts: |bound_var: ty::BoundVar| {
                 ty::Const::new_placeholder(
                     self.tcx,
                     ty::PlaceholderConst { universe: next_universe, bound: bound_var },
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index b2057fa36d7fc..df39ebb21ba60 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -66,13 +66,18 @@ pub trait BoundVarReplacerDelegate<'tcx> {
 /// A simple delegate taking 3 mutable functions. The used functions must
 /// always return the same result for each bound variable, no matter how
 /// frequently they are called.
-pub struct FnMutDelegate<'a, 'tcx> {
-    pub regions: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
-    pub types: &'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a),
-    pub consts: &'a mut (dyn FnMut(ty::BoundVar) -> ty::Const<'tcx> + 'a),
+pub struct FnMutDelegate<R, T, C> {
+    pub regions: R,
+    pub types: T,
+    pub consts: C,
 }
 
-impl<'a, 'tcx> BoundVarReplacerDelegate<'tcx> for FnMutDelegate<'a, 'tcx> {
+impl<'tcx, R, T, C> BoundVarReplacerDelegate<'tcx> for FnMutDelegate<R, T, C>
+where
+    R: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
+    T: FnMut(ty::BoundTy) -> Ty<'tcx>,
+    C: FnMut(ty::BoundVar) -> ty::Const<'tcx>,
+{
     fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx> {
         (self.regions)(br)
     }
@@ -215,7 +220,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn instantiate_bound_regions_uncached<T, F>(
         self,
         value: Binder<'tcx, T>,
-        mut replace_regions: F,
+        replace_regions: F,
     ) -> T
     where
         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
@@ -226,9 +231,9 @@ impl<'tcx> TyCtxt<'tcx> {
             value
         } else {
             let delegate = FnMutDelegate {
-                regions: &mut replace_regions,
-                types: &mut |b| bug!("unexpected bound ty in binder: {b:?}"),
-                consts: &mut |b| bug!("unexpected bound ct in binder: {b:?}"),
+                regions: replace_regions,
+                types: |b| bug!("unexpected bound ty in binder: {b:?}"),
+                consts: |b| bug!("unexpected bound ct in binder: {b:?}"),
             };
             let mut replacer = BoundVarReplacer::new(self, delegate);
             value.fold_with(&mut replacer)
@@ -286,21 +291,21 @@ impl<'tcx> TyCtxt<'tcx> {
         self.replace_escaping_bound_vars_uncached(
             value,
             FnMutDelegate {
-                regions: &mut |r: ty::BoundRegion| {
+                regions: |r: ty::BoundRegion| {
                     ty::Region::new_bound(
                         self,
                         ty::INNERMOST,
                         ty::BoundRegion { var: shift_bv(r.var), kind: r.kind },
                     )
                 },
-                types: &mut |t: ty::BoundTy| {
+                types: |t: ty::BoundTy| {
                     Ty::new_bound(
                         self,
                         ty::INNERMOST,
                         ty::BoundTy { var: shift_bv(t.var), kind: t.kind },
                     )
                 },
-                consts: &mut |c| ty::Const::new_bound(self, ty::INNERMOST, shift_bv(c)),
+                consts: |c| ty::Const::new_bound(self, ty::INNERMOST, shift_bv(c)),
             },
         )
     }