diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index 71445603e2f15..b1b8bc13e2f13 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -86,7 +86,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     /// A convenient alternative to `try_fold_with` for use with infallible
     /// folders. Do not override this method, to ensure coherence with
     /// `try_fold_with`.
-    fn fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
+    fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
         self.try_fold_with(folder).into_ok()
     }
 
@@ -216,7 +216,7 @@ pub trait TypeSuperFoldable<'tcx>: TypeFoldable<'tcx> {
     /// A convenient alternative to `try_super_fold_with` for use with
     /// infallible folders. Do not override this method, to ensure coherence
     /// with `try_super_fold_with`.
-    fn super_fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
+    fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
         self.try_super_fold_with(folder).into_ok()
     }
 
@@ -229,16 +229,13 @@ pub trait TypeSuperFoldable<'tcx>: TypeFoldable<'tcx> {
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
 }
 
-/// This trait is implemented for every folding traversal. There is a fold
-/// method defined for every type of interest. Each such method has a default
-/// that does an "identity" fold. Implementations of these methods often fall
-/// back to a `super_fold_with` method if the primary argument doesn't
-/// satisfy a particular condition.
+/// This trait is implemented for every infallible folding traversal. There is
+/// a fold method defined for every type of interest. Each such method has a
+/// default that does an "identity" fold. Implementations of these methods
+/// often fall back to a `super_fold_with` method if the primary argument
+/// doesn't satisfy a particular condition.
 ///
-/// If this folder is fallible (and therefore its [`Error`][`TypeFolder::Error`]
-/// associated type is something other than the default `!`) then
-/// [`FallibleTypeFolder`] should be implemented manually. Otherwise,
-/// a blanket implementation of [`FallibleTypeFolder`] will defer to
+/// A blanket implementation of [`FallibleTypeFolder`] will defer to
 /// the infallible methods of this trait to ensure that the two APIs
 /// are coherent.
 pub trait TypeFolder<'tcx>: FallibleTypeFolder<'tcx, Error = !> {
@@ -341,43 +338,40 @@ where
         TypeFolder::tcx(self)
     }
 
-    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
+    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, !>
     where
         T: TypeFoldable<'tcx>,
     {
         Ok(self.fold_binder(t))
     }
 
-    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
+    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, !> {
         Ok(self.fold_ty(t))
     }
 
-    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
+    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, !> {
         Ok(self.fold_region(r))
     }
 
-    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
+    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, !> {
         Ok(self.fold_const(c))
     }
 
     fn try_fold_unevaluated(
         &mut self,
         c: ty::Unevaluated<'tcx>,
-    ) -> Result<ty::Unevaluated<'tcx>, Self::Error> {
+    ) -> Result<ty::Unevaluated<'tcx>, !> {
         Ok(self.fold_unevaluated(c))
     }
 
-    fn try_fold_predicate(
-        &mut self,
-        p: ty::Predicate<'tcx>,
-    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
+    fn try_fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> Result<ty::Predicate<'tcx>, !> {
         Ok(self.fold_predicate(p))
     }
 
     fn try_fold_mir_const(
         &mut self,
         c: mir::ConstantKind<'tcx>,
-    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
+    ) -> Result<mir::ConstantKind<'tcx>, !> {
         Ok(self.fold_mir_const(c))
     }
 }