diff --git a/compiler/rustc_const_eval/src/lib.rs b/compiler/rustc_const_eval/src/lib.rs
index f308e764e861d..0d36466f6e3e7 100644
--- a/compiler/rustc_const_eval/src/lib.rs
+++ b/compiler/rustc_const_eval/src/lib.rs
@@ -23,6 +23,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(trusted_len)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index b09b2227f3e88..0ab077cf2bf40 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -95,7 +95,8 @@ pub fn equal_up_to_regions(
                 // Leave consts and types unchanged.
                 ct_op: |ct| ct,
                 ty_op: |ty| ty,
-            }),
+            })
+            .into_ok(),
         )
     };
     tcx.infer_ctxt().enter(|infcx| infcx.can_eq(param_env, normalize(src), normalize(dest)).is_ok())
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index dd1f0d63aff05..27e73738b7f35 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -503,7 +503,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             indices: FxHashMap::default(),
             binder_index: ty::INNERMOST,
         };
-        let out_value = value.fold_with(&mut canonicalizer);
+        let out_value = value.fold_with(&mut canonicalizer).into_ok();
 
         // Once we have canonicalized `out_value`, it should not
         // contain anything that ties it to this inference context
@@ -621,7 +621,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         let infcx = self.infcx;
         let bound_to = infcx.shallow_resolve(ty_var);
         if bound_to != ty_var {
-            self.fold_ty(bound_to)
+            self.fold_ty(bound_to).into_ok()
         } else {
             let var = self.canonical_var(info, ty_var.into());
             self.tcx().mk_ty(ty::Bound(self.binder_index, var.into()))
@@ -640,12 +640,12 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         let infcx = self.infcx;
         let bound_to = infcx.shallow_resolve(const_var);
         if bound_to != const_var {
-            self.fold_const(bound_to)
+            self.fold_const(bound_to).into_ok()
         } else {
             let var = self.canonical_var(info, const_var.into());
             self.tcx().mk_const(ty::Const {
                 val: ty::ConstKind::Bound(self.binder_index, var),
-                ty: self.fold_ty(const_var.ty),
+                ty: self.fold_ty(const_var.ty).into_ok(),
             })
         }
     }
diff --git a/compiler/rustc_infer/src/infer/freshen.rs b/compiler/rustc_infer/src/infer/freshen.rs
index fce6403575b79..7599e98167fb5 100644
--- a/compiler/rustc_infer/src/infer/freshen.rs
+++ b/compiler/rustc_infer/src/infer/freshen.rs
@@ -72,7 +72,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         F: FnOnce(u32) -> ty::InferTy,
     {
         if let Some(ty) = opt_ty {
-            return ty.fold_with(self);
+            return ty.fold_with(self).into_ok();
         }
 
         match self.ty_freshen_map.entry(key) {
@@ -98,7 +98,7 @@ impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
         F: FnOnce(u32) -> ty::InferConst<'tcx>,
     {
         if let Some(ct) = opt_ct {
-            return ct.fold_with(self);
+            return ct.fold_with(self).into_ok();
         }
 
         match self.const_freshen_map.entry(key) {
diff --git a/compiler/rustc_infer/src/infer/fudge.rs b/compiler/rustc_infer/src/infer/fudge.rs
index c889abf3d3c88..4e6f1315d1024 100644
--- a/compiler/rustc_infer/src/infer/fudge.rs
+++ b/compiler/rustc_infer/src/infer/fudge.rs
@@ -161,7 +161,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         {
             Ok(value)
         } else {
-            Ok(value.fold_with(&mut fudger))
+            Ok(value.fold_with(&mut fudger).into_ok())
         }
     }
 }
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index afac40b0ebd0d..4a9a63e1c7602 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -681,7 +681,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     pub fn freshen<T: TypeFoldable<'tcx>>(&self, t: T) -> T {
-        t.fold_with(&mut self.freshener())
+        t.fold_with(&mut self.freshener()).into_ok()
     }
 
     /// Returns the origin of the type variable identified by `vid`, or `None`
@@ -1381,7 +1381,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        value.fold_with(&mut ShallowResolver { infcx: self })
+        value.fold_with(&mut ShallowResolver { infcx: self }).into_ok()
     }
 
     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
@@ -1402,7 +1402,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             return value; // Avoid duplicated subst-folding.
         }
         let mut r = resolve::OpportunisticVarResolver::new(self);
-        value.fold_with(&mut r)
+        value.fold_with(&mut r).into_ok()
     }
 
     /// Returns the first unresolved variable contained in `T`. In the
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index e2e07f2072e49..932f26d555010 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -418,92 +418,94 @@ struct Instantiator<'a, 'tcx> {
 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
     fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
         let tcx = self.infcx.tcx;
-        value.fold_with(&mut BottomUpFolder {
-            tcx,
-            ty_op: |ty| {
-                if ty.references_error() {
-                    return tcx.ty_error();
-                } else if let ty::Opaque(def_id, substs) = ty.kind() {
-                    // Check that this is `impl Trait` type is
-                    // declared by `parent_def_id` -- i.e., one whose
-                    // value we are inferring.  At present, this is
-                    // always true during the first phase of
-                    // type-check, but not always true later on during
-                    // NLL. Once we support named opaque types more fully,
-                    // this same scenario will be able to arise during all phases.
-                    //
-                    // Here is an example using type alias `impl Trait`
-                    // that indicates the distinction we are checking for:
-                    //
-                    // ```rust
-                    // mod a {
-                    //   pub type Foo = impl Iterator;
-                    //   pub fn make_foo() -> Foo { .. }
-                    // }
-                    //
-                    // mod b {
-                    //   fn foo() -> a::Foo { a::make_foo() }
-                    // }
-                    // ```
-                    //
-                    // Here, the return type of `foo` references an
-                    // `Opaque` indeed, but not one whose value is
-                    // presently being inferred. You can get into a
-                    // similar situation with closure return types
-                    // today:
-                    //
-                    // ```rust
-                    // fn foo() -> impl Iterator { .. }
-                    // fn bar() {
-                    //     let x = || foo(); // returns the Opaque assoc with `foo`
-                    // }
-                    // ```
-                    if let Some(def_id) = def_id.as_local() {
-                        let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
-                        let parent_def_id = self.infcx.defining_use_anchor;
-                        let def_scope_default = || {
-                            let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
-                            parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
-                        };
-                        let (in_definition_scope, origin) =
-                            match tcx.hir().expect_item(opaque_hir_id).kind {
-                                // Anonymous `impl Trait`
-                                hir::ItemKind::OpaqueTy(hir::OpaqueTy {
-                                    impl_trait_fn: Some(parent),
-                                    origin,
-                                    ..
-                                }) => (parent == parent_def_id.to_def_id(), origin),
-                                // Named `type Foo = impl Bar;`
-                                hir::ItemKind::OpaqueTy(hir::OpaqueTy {
-                                    impl_trait_fn: None,
-                                    origin,
-                                    ..
-                                }) => (
-                                    may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
-                                    origin,
-                                ),
-                                _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
+        value
+            .fold_with(&mut BottomUpFolder {
+                tcx,
+                ty_op: |ty| {
+                    if ty.references_error() {
+                        return tcx.ty_error();
+                    } else if let ty::Opaque(def_id, substs) = ty.kind() {
+                        // Check that this is `impl Trait` type is
+                        // declared by `parent_def_id` -- i.e., one whose
+                        // value we are inferring.  At present, this is
+                        // always true during the first phase of
+                        // type-check, but not always true later on during
+                        // NLL. Once we support named opaque types more fully,
+                        // this same scenario will be able to arise during all phases.
+                        //
+                        // Here is an example using type alias `impl Trait`
+                        // that indicates the distinction we are checking for:
+                        //
+                        // ```rust
+                        // mod a {
+                        //   pub type Foo = impl Iterator;
+                        //   pub fn make_foo() -> Foo { .. }
+                        // }
+                        //
+                        // mod b {
+                        //   fn foo() -> a::Foo { a::make_foo() }
+                        // }
+                        // ```
+                        //
+                        // Here, the return type of `foo` references an
+                        // `Opaque` indeed, but not one whose value is
+                        // presently being inferred. You can get into a
+                        // similar situation with closure return types
+                        // today:
+                        //
+                        // ```rust
+                        // fn foo() -> impl Iterator { .. }
+                        // fn bar() {
+                        //     let x = || foo(); // returns the Opaque assoc with `foo`
+                        // }
+                        // ```
+                        if let Some(def_id) = def_id.as_local() {
+                            let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+                            let parent_def_id = self.infcx.defining_use_anchor;
+                            let def_scope_default = || {
+                                let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
+                                parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
                             };
-                        if in_definition_scope {
-                            let opaque_type_key =
-                                OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
-                            return self.fold_opaque_ty(ty, opaque_type_key, origin);
-                        }
-
-                        debug!(
-                            "instantiate_opaque_types_in_map: \
+                            let (in_definition_scope, origin) =
+                                match tcx.hir().expect_item(opaque_hir_id).kind {
+                                    // Anonymous `impl Trait`
+                                    hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                                        impl_trait_fn: Some(parent),
+                                        origin,
+                                        ..
+                                    }) => (parent == parent_def_id.to_def_id(), origin),
+                                    // Named `type Foo = impl Bar;`
+                                    hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                                        impl_trait_fn: None,
+                                        origin,
+                                        ..
+                                    }) => (
+                                        may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
+                                        origin,
+                                    ),
+                                    _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
+                                };
+                            if in_definition_scope {
+                                let opaque_type_key =
+                                    OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
+                                return self.fold_opaque_ty(ty, opaque_type_key, origin);
+                            }
+
+                            debug!(
+                                "instantiate_opaque_types_in_map: \
                              encountered opaque outside its definition scope \
                              def_id={:?}",
-                            def_id,
-                        );
+                                def_id,
+                            );
+                        }
                     }
-                }
 
-                ty
-            },
-            lt_op: |lt| lt,
-            ct_op: |ct| ct,
-        })
+                    ty
+                },
+                lt_op: |lt| lt,
+                ct_op: |ct| ct,
+            })
+            .into_ok()
     }
 
     #[instrument(skip(self), level = "debug")]
@@ -556,21 +558,23 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
             debug!(?predicate);
 
             // We can't normalize associated types from `rustc_infer`, but we can eagerly register inference variables for them.
-            let predicate = predicate.fold_with(&mut BottomUpFolder {
-                tcx,
-                ty_op: |ty| match ty.kind() {
-                    ty::Projection(projection_ty) => infcx.infer_projection(
-                        self.param_env,
-                        *projection_ty,
-                        traits::ObligationCause::misc(self.value_span, self.body_id),
-                        0,
-                        &mut self.obligations,
-                    ),
-                    _ => ty,
-                },
-                lt_op: |lt| lt,
-                ct_op: |ct| ct,
-            });
+            let predicate = predicate
+                .fold_with(&mut BottomUpFolder {
+                    tcx,
+                    ty_op: |ty| match ty.kind() {
+                        ty::Projection(projection_ty) => infcx.infer_projection(
+                            self.param_env,
+                            *projection_ty,
+                            traits::ObligationCause::misc(self.value_span, self.body_id),
+                            0,
+                            &mut self.obligations,
+                        ),
+                        _ => ty,
+                    },
+                    lt_op: |lt| lt,
+                    ct_op: |ct| ct,
+                })
+                .into_ok();
             debug!(?predicate);
 
             if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
diff --git a/compiler/rustc_infer/src/infer/resolve.rs b/compiler/rustc_infer/src/infer/resolve.rs
index d09c585a02598..554c5b162f755 100644
--- a/compiler/rustc_infer/src/infer/resolve.rs
+++ b/compiler/rustc_infer/src/infer/resolve.rs
@@ -182,7 +182,7 @@ where
     T: TypeFoldable<'tcx>,
 {
     let mut full_resolver = FullTypeResolver { infcx, err: None };
-    let result = value.fold_with(&mut full_resolver);
+    let result = value.fold_with(&mut full_resolver).into_ok();
     match full_resolver.err {
         None => Ok(result),
         Some(e) => Err(e),
diff --git a/compiler/rustc_infer/src/lib.rs b/compiler/rustc_infer/src/lib.rs
index e4b407e7c112d..5153427954ccc 100644
--- a/compiler/rustc_infer/src/lib.rs
+++ b/compiler/rustc_infer/src/lib.rs
@@ -24,6 +24,7 @@
 #![feature(control_flow_enum)]
 #![feature(min_specialization)]
 #![feature(label_break_value)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 9ce9f65a49066..b67ad8b770ea1 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -56,6 +56,7 @@
 #![feature(try_blocks)]
 #![feature(try_reserve_kind)]
 #![feature(nonzero_ops)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "512"]
 
 #[macro_use]
diff --git a/compiler/rustc_middle/src/ty/erase_regions.rs b/compiler/rustc_middle/src/ty/erase_regions.rs
index 559534a7aa4e3..25b460cf16d3c 100644
--- a/compiler/rustc_middle/src/ty/erase_regions.rs
+++ b/compiler/rustc_middle/src/ty/erase_regions.rs
@@ -9,7 +9,7 @@ pub(super) fn provide(providers: &mut ty::query::Providers) {
 fn erase_regions_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
     // N.B., use `super_fold_with` here. If we used `fold_with`, it
     // could invoke the `erase_regions_ty` query recursively.
-    ty.super_fold_with(&mut RegionEraserVisitor { tcx })
+    ty.super_fold_with(&mut RegionEraserVisitor { tcx }).into_ok()
 }
 
 impl<'tcx> TyCtxt<'tcx> {
@@ -27,7 +27,7 @@ impl<'tcx> TyCtxt<'tcx> {
             return value;
         }
         debug!("erase_regions({:?})", value);
-        let value1 = value.fold_with(&mut RegionEraserVisitor { tcx: self });
+        let value1 = value.fold_with(&mut RegionEraserVisitor { tcx: self }).into_ok();
         debug!("erase_regions = {:?}", value1);
         value1
     }
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index 639606f6b891e..31055c03a5d36 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -336,7 +336,7 @@ impl<'tcx> TyCtxt<'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
+        value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f)).into_ok()
     }
 
     /// Invoke `callback` on every region appearing free in `value`.
@@ -638,7 +638,7 @@ impl<'tcx> TyCtxt<'tcx> {
             value
         } else {
             let mut replacer = BoundVarReplacer::new(self, Some(&mut real_fld_r), None, None);
-            value.fold_with(&mut replacer)
+            value.fold_with(&mut replacer).into_ok()
         };
         (value, region_map)
     }
@@ -664,7 +664,7 @@ impl<'tcx> TyCtxt<'tcx> {
         } else {
             let mut replacer =
                 BoundVarReplacer::new(self, Some(&mut fld_r), Some(&mut fld_t), Some(&mut fld_c));
-            value.fold_with(&mut replacer)
+            value.fold_with(&mut replacer).into_ok()
         }
     }
 
@@ -1030,7 +1030,7 @@ where
 {
     debug!("shift_vars(value={:?}, amount={})", value, amount);
 
-    value.fold_with(&mut Shifter::new(tcx, amount))
+    value.fold_with(&mut Shifter::new(tcx, amount)).into_ok()
 }
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
@@ -1293,7 +1293,7 @@ impl<'tcx> TyCtxt<'tcx> {
     ///
     /// FIXME(@lcnr): explain this function a bit more
     pub fn expose_default_const_substs<T: TypeFoldable<'tcx>>(self, v: T) -> T {
-        v.fold_with(&mut ExposeDefaultConstSubstsFolder { tcx: self })
+        v.fold_with(&mut ExposeDefaultConstSubstsFolder { tcx: self }).into_ok()
     }
 }
 
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 23863bf389e5a..15931b8d2c86a 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -669,7 +669,7 @@ fn polymorphize<'tcx>(
                     // ..and polymorphize any closures/generators captured as upvars.
                     let upvars_ty = upvars_ty.unwrap();
                     let polymorphized_upvars_ty = upvars_ty.fold_with(
-                        &mut PolymorphizationFolder { tcx });
+                        &mut PolymorphizationFolder { tcx }).into_ok();
                     debug!("polymorphize: polymorphized_upvars_ty={:?}", polymorphized_upvars_ty);
                     ty::GenericArg::from(polymorphized_upvars_ty)
                 },
diff --git a/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
index c5d845f97559d..e6f67adae93da 100644
--- a/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
+++ b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
@@ -35,7 +35,9 @@ impl<'tcx> TyCtxt<'tcx> {
         if !value.has_projections() {
             value
         } else {
-            value.fold_with(&mut NormalizeAfterErasingRegionsFolder { tcx: self, param_env })
+            value
+                .fold_with(&mut NormalizeAfterErasingRegionsFolder { tcx: self, param_env })
+                .into_ok()
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 13c6b4d6b01ba..f454b95862bab 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2193,7 +2193,7 @@ impl<F: fmt::Write> FmtPrinter<'_, 'tcx, F> {
                 name: &mut name,
                 region_map: BTreeMap::new(),
             };
-            let new_value = value.clone().skip_binder().fold_with(&mut folder);
+            let new_value = value.clone().skip_binder().fold_with(&mut folder).into_ok();
             let region_map = folder.region_map;
             start_or_continue(&mut self, "", "> ");
             (new_value, region_map)
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 419cf164db3b6..0767224ad9288 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -439,7 +439,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for T {
         span: Option<Span>,
     ) -> T {
         let mut folder = SubstFolder { tcx, substs, span, binders_passed: 0 };
-        self.fold_with(&mut folder)
+        self.fold_with(&mut folder).into_ok()
     }
 }
 
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index b90123b4cee82..5137f9650633a 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -574,14 +574,14 @@ impl<'tcx> OpaqueTypeExpander<'tcx> {
         if self.found_any_recursion {
             return None;
         }
-        let substs = substs.fold_with(self);
+        let substs = substs.fold_with(self).into_ok();
         if !self.check_recursion || self.seen_opaque_tys.insert(def_id) {
             let expanded_ty = match self.expanded_cache.get(&(def_id, substs)) {
                 Some(expanded_ty) => expanded_ty,
                 None => {
                     let generic_ty = self.tcx.type_of(def_id);
                     let concrete_ty = generic_ty.subst(self.tcx, substs);
-                    let expanded_ty = self.fold_ty(concrete_ty);
+                    let expanded_ty = self.fold_ty(concrete_ty).into_ok();
                     self.expanded_cache.insert((def_id, substs), expanded_ty);
                     expanded_ty
                 }
@@ -1092,7 +1092,7 @@ pub fn normalize_opaque_types(
         check_recursion: false,
         tcx,
     };
-    val.fold_with(&mut visitor)
+    val.fold_with(&mut visitor).into_ok()
 }
 
 pub fn provide(providers: &mut ty::query::Providers) {
diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index 1820e33b19bf4..ecc352c1a49b5 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -22,6 +22,7 @@
 #![feature(never_type)]
 #![feature(crate_visibility_modifier)]
 #![feature(control_flow_enum)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index eb7293ae86e69..9052dff0aaa83 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -65,14 +65,16 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         // Convert the type from the function into a type valid outside
         // the function, by replacing invalid regions with 'static,
         // after producing an error for each of them.
-        let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
-            self.tcx,
-            self.is_tainted_by_errors(),
-            def_id,
-            map,
-            instantiated_ty,
-            span,
-        ));
+        let definition_ty = instantiated_ty
+            .fold_with(&mut ReverseMapper::new(
+                self.tcx,
+                self.is_tainted_by_errors(),
+                def_id,
+                map,
+                instantiated_ty,
+                span,
+            ))
+            .into_ok();
         debug!(?definition_ty);
 
         definition_ty
@@ -123,14 +125,14 @@ impl ReverseMapper<'tcx> {
     ) -> GenericArg<'tcx> {
         assert!(!self.map_missing_regions_to_empty);
         self.map_missing_regions_to_empty = true;
-        let kind = kind.fold_with(self);
+        let kind = kind.fold_with(self).into_ok();
         self.map_missing_regions_to_empty = false;
         kind
     }
 
     fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
         assert!(!self.map_missing_regions_to_empty);
-        kind.fold_with(self)
+        kind.fold_with(self).into_ok()
     }
 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 1876b0ce80eed..866bcde1bfd4f 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1916,8 +1916,9 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
         self.probe(|_| {
             let mut selcx = SelectionContext::new(self);
 
-            let cleaned_pred =
-                pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
+            let cleaned_pred = pred
+                .fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() })
+                .into_ok();
 
             let cleaned_pred = super::project::normalize(
                 &mut selcx,
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 2baa21a43d61d..0911b2c529ab2 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -339,7 +339,7 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
         if !needs_normalization(&value, self.param_env.reveal()) {
             value
         } else {
-            value.fold_with(self)
+            value.fold_with(self).into_ok()
         }
     }
 }
@@ -555,7 +555,7 @@ impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
             universe_indices,
         };
 
-        let value = value.super_fold_with(&mut replacer);
+        let value = value.super_fold_with(&mut replacer).into_ok();
 
         (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
     }
@@ -681,7 +681,7 @@ impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
             universe_indices,
             current_index: ty::INNERMOST,
         };
-        value.super_fold_with(&mut replacer)
+        value.super_fold_with(&mut replacer).into_ok()
     }
 }
 
@@ -1546,7 +1546,8 @@ fn confirm_candidate<'cx, 'tcx>(
     // when possible for this to work. See `auto-trait-projection-recursion.rs`
     // for a case where this matters.
     if progress.ty.has_infer_regions() {
-        progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
+        progress.ty =
+            OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty).into_ok();
     }
     progress
 }
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index f3a90935a9fdf..dd9cd51936ec3 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -88,7 +88,7 @@ impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
                 normalizer.universes.extend((0..max_visitor.escaping).map(|_| None));
             }
         }
-        let result = value.fold_with(&mut normalizer);
+        let result = value.fold_with(&mut normalizer).into_ok();
         info!(
             "normalize::<{}>: result={:?} with {} obligations",
             std::any::type_name::<T>(),
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 2aa214694cb14..767cb1618bb67 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -2222,6 +2222,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .predicate
             .to_poly_trait_ref()
             .fold_with(&mut self.freshener)
+            .into_ok()
             .with_constness(obligation.predicate.skip_binder().constness);
 
         let dfn = previous_stack.cache.next_dfn();
diff --git a/compiler/rustc_traits/src/chalk/db.rs b/compiler/rustc_traits/src/chalk/db.rs
index 1d457d6761fd0..fdff07302c240 100644
--- a/compiler/rustc_traits/src/chalk/db.rs
+++ b/compiler/rustc_traits/src/chalk/db.rs
@@ -45,7 +45,7 @@ impl<'tcx> RustIrDatabase<'tcx> {
         predicates
             .iter()
             .map(|(wc, _)| wc.subst(self.interner.tcx, bound_vars))
-            .map(|wc| wc.fold_with(&mut regions_substitutor))
+            .map(|wc| wc.fold_with(&mut regions_substitutor).into_ok())
             .filter_map(|wc| LowerInto::<Option<chalk_ir::QuantifiedWhereClause<RustInterner<'tcx>>>>::lower_into(wc, &self.interner)).collect()
     }
 
@@ -287,7 +287,7 @@ impl<'tcx> chalk_solve::RustIrDatabase<RustInterner<'tcx>> for RustIrDatabase<'t
         let trait_ref = trait_ref.subst(self.interner.tcx, bound_vars);
         let mut regions_substitutor =
             lowering::RegionsSubstitutor::new(self.interner.tcx, self.reempty_placeholder);
-        let trait_ref = trait_ref.fold_with(&mut regions_substitutor);
+        let trait_ref = trait_ref.fold_with(&mut regions_substitutor).into_ok();
 
         let where_clauses = self.where_clauses_for(def_id, bound_vars);
 
@@ -335,7 +335,7 @@ impl<'tcx> chalk_solve::RustIrDatabase<RustInterner<'tcx>> for RustIrDatabase<'t
             let self_ty = self_ty.subst(self.interner.tcx, bound_vars);
             let mut regions_substitutor =
                 lowering::RegionsSubstitutor::new(self.interner.tcx, self.reempty_placeholder);
-            let self_ty = self_ty.fold_with(&mut regions_substitutor);
+            let self_ty = self_ty.fold_with(&mut regions_substitutor).into_ok();
             let lowered_ty = self_ty.lower_into(&self.interner);
 
             parameters[0].assert_ty_ref(&self.interner).could_match(
@@ -501,22 +501,24 @@ impl<'tcx> chalk_solve::RustIrDatabase<RustInterner<'tcx>> for RustIrDatabase<'t
                 .iter()
                 .map(|(bound, _)| bound.subst(self.interner.tcx, &bound_vars))
                 .map(|bound| {
-                    bound.fold_with(&mut ty::fold::BottomUpFolder {
-                        tcx: self.interner.tcx,
-                        ty_op: |ty| {
-                            if let ty::Opaque(def_id, substs) = *ty.kind() {
-                                if def_id == opaque_ty_id.0 && substs == identity_substs {
-                                    return self.interner.tcx.mk_ty(ty::Bound(
-                                        ty::INNERMOST,
-                                        ty::BoundTy::from(ty::BoundVar::from_u32(0)),
-                                    ));
+                    bound
+                        .fold_with(&mut ty::fold::BottomUpFolder {
+                            tcx: self.interner.tcx,
+                            ty_op: |ty| {
+                                if let ty::Opaque(def_id, substs) = *ty.kind() {
+                                    if def_id == opaque_ty_id.0 && substs == identity_substs {
+                                        return self.interner.tcx.mk_ty(ty::Bound(
+                                            ty::INNERMOST,
+                                            ty::BoundTy::from(ty::BoundVar::from_u32(0)),
+                                        ));
+                                    }
                                 }
-                            }
-                            ty
-                        },
-                        lt_op: |lt| lt,
-                        ct_op: |ct| ct,
-                    })
+                                ty
+                            },
+                            lt_op: |lt| lt,
+                            ct_op: |ct| ct,
+                        })
+                        .into_ok()
                 })
                 .filter_map(|bound| {
                     LowerInto::<
diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index 033a94658425b..66073facf4b0c 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -817,7 +817,7 @@ crate fn collect_bound_vars<'tcx, T: TypeFoldable<'tcx>>(
         .collect();
 
     let mut bound_var_substitutor = NamedBoundVarSubstitutor::new(tcx, &named_parameters);
-    let new_ty = ty.skip_binder().fold_with(&mut bound_var_substitutor);
+    let new_ty = ty.skip_binder().fold_with(&mut bound_var_substitutor).into_ok();
 
     for var in named_parameters.values() {
         parameters.insert(*var, chalk_ir::VariableKind::Lifetime);
diff --git a/compiler/rustc_traits/src/chalk/mod.rs b/compiler/rustc_traits/src/chalk/mod.rs
index b7275bac19048..a6f7c4d7988f6 100644
--- a/compiler/rustc_traits/src/chalk/mod.rs
+++ b/compiler/rustc_traits/src/chalk/mod.rs
@@ -49,12 +49,12 @@ crate fn evaluate_goal<'tcx>(
 
     let mut params_substitutor =
         ParamsSubstitutor::new(tcx, placeholders_collector.next_ty_placeholder);
-    let obligation = obligation.fold_with(&mut params_substitutor);
+    let obligation = obligation.fold_with(&mut params_substitutor).into_ok();
     // FIXME(chalk): we really should be substituting these back in the solution
     let _params: FxHashMap<usize, ParamTy> = params_substitutor.params;
 
     let mut regions_substitutor = RegionsSubstitutor::new(tcx, reempty_placeholder);
-    let obligation = obligation.fold_with(&mut regions_substitutor);
+    let obligation = obligation.fold_with(&mut regions_substitutor).into_ok();
 
     let max_universe = obligation.max_universe.index();
 
diff --git a/compiler/rustc_traits/src/lib.rs b/compiler/rustc_traits/src/lib.rs
index 8612499623be6..ea70a8d9e3a0d 100644
--- a/compiler/rustc_traits/src/lib.rs
+++ b/compiler/rustc_traits/src/lib.rs
@@ -4,6 +4,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/compiler/rustc_typeck/src/check/op.rs b/compiler/rustc_typeck/src/check/op.rs
index 8f852c0b51c98..1c4e0522bef8f 100644
--- a/compiler/rustc_typeck/src/check/op.rs
+++ b/compiler/rustc_typeck/src/check/op.rs
@@ -442,8 +442,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             let mut eraser = TypeParamEraser(self, expr.span);
                             let needs_bound = self
                                 .lookup_op_method(
-                                    eraser.fold_ty(lhs_ty),
-                                    &[eraser.fold_ty(rhs_ty)],
+                                    eraser.fold_ty(lhs_ty).into_ok(),
+                                    &[eraser.fold_ty(rhs_ty).into_ok()],
                                     Op::Binary(op, is_assign),
                                 )
                                 .is_ok();
diff --git a/compiler/rustc_typeck/src/check/writeback.rs b/compiler/rustc_typeck/src/check/writeback.rs
index ecf0332db7b8b..a5965411020a4 100644
--- a/compiler/rustc_typeck/src/check/writeback.rs
+++ b/compiler/rustc_typeck/src/check/writeback.rs
@@ -658,7 +658,7 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
         T: TypeFoldable<'tcx>,
     {
         let mut resolver = Resolver::new(self.fcx, span, self.body);
-        let x = x.fold_with(&mut resolver);
+        let x = x.fold_with(&mut resolver).into_ok();
         if cfg!(debug_assertions) && x.needs_infer() {
             span_bug!(span.to_span(self.fcx.tcx), "writeback: `{:?}` has inference variables", x);
         }
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index 1dce63416dc96..1ea379c57389a 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -761,7 +761,7 @@ fn infer_placeholder_type<'a>(
 
                 // Suggesting unnameable types won't help.
                 let mut mk_nameable = MakeNameable::new(tcx);
-                let ty = mk_nameable.fold_ty(ty);
+                let ty = mk_nameable.fold_ty(ty).into_ok();
                 let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
                 if let Some(sugg_ty) = sugg_ty {
                     err.span_suggestion(
@@ -785,7 +785,7 @@ fn infer_placeholder_type<'a>(
 
             if !ty.references_error() {
                 let mut mk_nameable = MakeNameable::new(tcx);
-                let ty = mk_nameable.fold_ty(ty);
+                let ty = mk_nameable.fold_ty(ty).into_ok();
                 let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
                 if let Some(sugg_ty) = sugg_ty {
                     diag.span_suggestion(
diff --git a/compiler/rustc_typeck/src/hir_wf_check.rs b/compiler/rustc_typeck/src/hir_wf_check.rs
index 5c229c86ddc1a..f9de6376b0ff5 100644
--- a/compiler/rustc_typeck/src/hir_wf_check.rs
+++ b/compiler/rustc_typeck/src/hir_wf_check.rs
@@ -71,8 +71,11 @@ fn diagnostic_hir_wf_check<'tcx>(
         fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
             self.tcx.infer_ctxt().enter(|infcx| {
                 let mut fulfill = traits::FulfillmentContext::new();
-                let tcx_ty =
-                    self.icx.to_ty(ty).fold_with(&mut EraseAllBoundRegions { tcx: self.tcx });
+                let tcx_ty = self
+                    .icx
+                    .to_ty(ty)
+                    .fold_with(&mut EraseAllBoundRegions { tcx: self.tcx })
+                    .into_ok();
                 let cause = traits::ObligationCause::new(
                     ty.span,
                     self.hir_id,
diff --git a/compiler/rustc_typeck/src/lib.rs b/compiler/rustc_typeck/src/lib.rs
index 0881cf07586b3..c0f0b3fe7046e 100644
--- a/compiler/rustc_typeck/src/lib.rs
+++ b/compiler/rustc_typeck/src/lib.rs
@@ -71,6 +71,7 @@ This API is completely unstable and subject to change.
 #![feature(slice_partition_dedup)]
 #![feature(control_flow_enum)]
 #![feature(hash_drain_filter)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/src/librustdoc/clean/auto_trait.rs b/src/librustdoc/clean/auto_trait.rs
index 3a6f611660ece..04fe1dccce44b 100644
--- a/src/librustdoc/clean/auto_trait.rs
+++ b/src/librustdoc/clean/auto_trait.rs
@@ -449,7 +449,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                         _ => false,
                     }
             })
-            .map(|p| p.fold_with(&mut replacer));
+            .map(|p| p.fold_with(&mut replacer).into_ok());
 
         let mut generic_params =
             (tcx.generics_of(item_def_id), tcx.explicit_predicates_of(item_def_id))
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index b6311abb5c3e8..6ba56fe01a935 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -18,6 +18,7 @@
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
 #![recursion_limit = "256"]
+#![feature(unwrap_infallible)]
 #![warn(rustc::internal)]
 
 #[macro_use]