diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index 4c7db4e803b8e..a6a1d1f73bb62 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -439,18 +439,18 @@ struct BoundVarReplacer<'a, 'tcx> {
     /// the ones we have visited.
     current_index: ty::DebruijnIndex,
 
-    fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
-    fld_t: &'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a),
-    fld_c: &'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a),
+    fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
+    fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
+    fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a)>,
 }
 
 impl<'a, 'tcx> BoundVarReplacer<'a, 'tcx> {
-    fn new<F, G, H>(tcx: TyCtxt<'tcx>, fld_r: &'a mut F, fld_t: &'a mut G, fld_c: &'a mut H) -> Self
-    where
-        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
-        G: FnMut(ty::BoundTy) -> Ty<'tcx>,
-        H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
-    {
+    fn new(
+        tcx: TyCtxt<'tcx>,
+        fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
+        fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
+        fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a)>,
+    ) -> Self {
         BoundVarReplacer { tcx, current_index: ty::INNERMOST, fld_r, fld_t, fld_c }
     }
 }
@@ -469,63 +469,58 @@ impl<'a, 'tcx> TypeFolder<'tcx> for BoundVarReplacer<'a, 'tcx> {
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         match *t.kind() {
-            ty::Bound(debruijn, bound_ty) => {
-                if debruijn == self.current_index {
-                    let fld_t = &mut self.fld_t;
+            ty::Bound(debruijn, bound_ty) if debruijn == self.current_index => {
+                if let Some(fld_t) = self.fld_t.as_mut() {
                     let ty = fld_t(bound_ty);
-                    ty::fold::shift_vars(self.tcx, &ty, self.current_index.as_u32())
-                } else {
-                    t
+                    return ty::fold::shift_vars(self.tcx, &ty, self.current_index.as_u32());
                 }
             }
-            _ => {
-                if !t.has_vars_bound_at_or_above(self.current_index) {
-                    // Nothing more to substitute.
-                    t
-                } else {
-                    t.super_fold_with(self)
-                }
+            _ if t.has_vars_bound_at_or_above(self.current_index) => {
+                return t.super_fold_with(self);
             }
+            _ => {}
         }
+        t
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         match *r {
             ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
-                let fld_r = &mut self.fld_r;
-                let region = fld_r(br);
-                if let ty::ReLateBound(debruijn1, br) = *region {
-                    // If the callback returns a late-bound region,
-                    // that region should always use the INNERMOST
-                    // debruijn index. Then we adjust it to the
-                    // correct depth.
-                    assert_eq!(debruijn1, ty::INNERMOST);
-                    self.tcx.mk_region(ty::ReLateBound(debruijn, br))
-                } else {
-                    region
+                if let Some(fld_r) = self.fld_r.as_mut() {
+                    let region = fld_r(br);
+                    return if let ty::ReLateBound(debruijn1, br) = *region {
+                        // If the callback returns a late-bound region,
+                        // that region should always use the INNERMOST
+                        // debruijn index. Then we adjust it to the
+                        // correct depth.
+                        assert_eq!(debruijn1, ty::INNERMOST);
+                        self.tcx.mk_region(ty::ReLateBound(debruijn, br))
+                    } else {
+                        region
+                    };
                 }
             }
-            _ => r,
+            _ => {}
         }
+        r
     }
 
     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
-        if let ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty } = *ct {
-            if debruijn == self.current_index {
-                let fld_c = &mut self.fld_c;
-                let ct = fld_c(bound_const, ty);
-                ty::fold::shift_vars(self.tcx, &ct, self.current_index.as_u32())
-            } else {
-                ct
+        match *ct {
+            ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty }
+                if debruijn == self.current_index =>
+            {
+                if let Some(fld_c) = self.fld_c.as_mut() {
+                    let ct = fld_c(bound_const, ty);
+                    return ty::fold::shift_vars(self.tcx, &ct, self.current_index.as_u32());
+                }
             }
-        } else {
-            if !ct.has_vars_bound_at_or_above(self.current_index) {
-                // Nothing more to substitute.
-                ct
-            } else {
-                ct.super_fold_with(self)
+            _ if ct.has_vars_bound_at_or_above(self.current_index) => {
+                return ct.super_fold_with(self);
             }
+            _ => {}
         }
+        ct
     }
 }
 
@@ -550,14 +545,16 @@ impl<'tcx> TyCtxt<'tcx> {
         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
         T: TypeFoldable<'tcx>,
     {
-        // identity for bound types and consts
-        let fld_t = |bound_ty| self.mk_ty(ty::Bound(ty::INNERMOST, bound_ty));
-        let fld_c = |bound_ct, ty| {
-            self.mk_const(ty::Const { val: ty::ConstKind::Bound(ty::INNERMOST, bound_ct), ty })
-        };
         let mut region_map = BTreeMap::new();
-        let real_fld_r = |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
-        let value = self.replace_escaping_bound_vars(value.skip_binder(), real_fld_r, fld_t, fld_c);
+        let mut real_fld_r =
+            |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
+        let value = value.skip_binder();
+        let value = if !value.has_escaping_bound_vars() {
+            value
+        } else {
+            let mut replacer = BoundVarReplacer::new(self, Some(&mut real_fld_r), None, None);
+            value.fold_with(&mut replacer)
+        };
         (value, region_map)
     }
 
@@ -580,7 +577,8 @@ impl<'tcx> TyCtxt<'tcx> {
         if !value.has_escaping_bound_vars() {
             value
         } else {
-            let mut replacer = BoundVarReplacer::new(self, &mut fld_r, &mut fld_t, &mut fld_c);
+            let mut replacer =
+                BoundVarReplacer::new(self, Some(&mut fld_r), Some(&mut fld_t), Some(&mut fld_c));
             value.fold_with(&mut replacer)
         }
     }