Auto merge of #78779 - LeSeulArtichaut:ty-visitor-return, r=oli-obk

Introduce `TypeVisitor::BreakTy`

Implements MCP rust-lang/compiler-team#383.
r? `@ghost`
cc `@lcnr` `@oli-obk`

~~Blocked on FCP in rust-lang/compiler-team#383.~~

9 files changed, 116 insertions, 91 deletions

diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 4a1d5459d1e..cdc5940d9ba 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -51,6 +51,7 @@
 #![feature(half_open_range_patterns)]
 #![feature(exclusive_range_pattern)]
 #![feature(control_flow_enum)]
+#![feature(associated_type_defaults)]
 #![recursion_limit = "512"]
 
 #[macro_use]
diff --git a/compiler/rustc_middle/src/macros.rs b/compiler/rustc_middle/src/macros.rs
index 9cb8fd7eb9f..c0f2a76c19d 100644
--- a/compiler/rustc_middle/src/macros.rs
+++ b/compiler/rustc_middle/src/macros.rs
@@ -62,7 +62,7 @@ macro_rules! TrivialTypeFoldableImpls {
                 fn super_visit_with<F: $crate::ty::fold::TypeVisitor<$tcx>>(
                     &self,
                     _: &mut F)
-                    -> ::std::ops::ControlFlow<()>
+                    -> ::std::ops::ControlFlow<F::BreakTy>
                 {
                     ::std::ops::ControlFlow::CONTINUE
                 }
@@ -105,7 +105,7 @@ macro_rules! EnumTypeFoldableImpl {
             fn super_visit_with<V: $crate::ty::fold::TypeVisitor<$tcx>>(
                 &self,
                 visitor: &mut V,
-            ) -> ::std::ops::ControlFlow<()> {
+            ) -> ::std::ops::ControlFlow<V::BreakTy> {
                 EnumTypeFoldableImpl!(@VisitVariants(self, visitor) input($($variants)*) output())
             }
         }
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index f30b23591d2..9289d4708de 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -2464,7 +2464,10 @@ impl<'tcx> TypeFoldable<'tcx> for UserTypeProjection {
         }
     }
 
-    fn super_visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<()> {
+    fn super_visit_with<Vs: TypeVisitor<'tcx>>(
+        &self,
+        visitor: &mut Vs,
+    ) -> ControlFlow<Vs::BreakTy> {
         self.base.visit_with(visitor)
         // Note: there's nothing in `self.proj` to visit.
     }
diff --git a/compiler/rustc_middle/src/mir/type_foldable.rs b/compiler/rustc_middle/src/mir/type_foldable.rs
index dca46cb0a99..da8e189ba9d 100644
--- a/compiler/rustc_middle/src/mir/type_foldable.rs
+++ b/compiler/rustc_middle/src/mir/type_foldable.rs
@@ -87,7 +87,7 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
         Terminator { source_info: self.source_info, kind }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::mir::TerminatorKind::*;
 
         match self.kind {
@@ -144,7 +144,7 @@ impl<'tcx> TypeFoldable<'tcx> for GeneratorKind {
         self
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -154,7 +154,7 @@ impl<'tcx> TypeFoldable<'tcx> for Place<'tcx> {
         Place { local: self.local.fold_with(folder), projection: self.projection.fold_with(folder) }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.local.visit_with(visitor)?;
         self.projection.visit_with(visitor)
     }
@@ -165,7 +165,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<PlaceElem<'tcx>> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_place_elems(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -211,7 +211,7 @@ impl<'tcx> TypeFoldable<'tcx> for Rvalue<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::mir::Rvalue::*;
         match *self {
             Use(ref op) => op.visit_with(visitor),
@@ -266,7 +266,7 @@ impl<'tcx> TypeFoldable<'tcx> for Operand<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match *self {
             Operand::Copy(ref place) | Operand::Move(ref place) => place.visit_with(visitor),
             Operand::Constant(ref c) => c.visit_with(visitor),
@@ -290,7 +290,10 @@ impl<'tcx> TypeFoldable<'tcx> for PlaceElem<'tcx> {
         }
     }
 
-    fn super_visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<()> {
+    fn super_visit_with<Vs: TypeVisitor<'tcx>>(
+        &self,
+        visitor: &mut Vs,
+    ) -> ControlFlow<Vs::BreakTy> {
         use crate::mir::ProjectionElem::*;
 
         match self {
@@ -305,7 +308,7 @@ impl<'tcx> TypeFoldable<'tcx> for Field {
     fn super_fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self {
         self
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -314,7 +317,7 @@ impl<'tcx> TypeFoldable<'tcx> for GeneratorSavedLocal {
     fn super_fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self {
         self
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -323,7 +326,7 @@ impl<'tcx, R: Idx, C: Idx> TypeFoldable<'tcx> for BitMatrix<R, C> {
     fn super_fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self {
         self
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -336,7 +339,7 @@ impl<'tcx> TypeFoldable<'tcx> for Constant<'tcx> {
             literal: self.literal.fold_with(folder),
         }
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.literal.visit_with(visitor)
     }
 }
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index ee5c7a5473a..1883c89a151 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -49,8 +49,8 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         self.super_fold_with(folder)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()>;
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.super_visit_with(visitor)
     }
 
@@ -73,7 +73,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     }
 
     fn has_type_flags(&self, flags: TypeFlags) -> bool {
-        self.visit_with(&mut HasTypeFlagsVisitor { flags }).is_break()
+        self.visit_with(&mut HasTypeFlagsVisitor { flags }).break_value() == Some(FoundFlags)
     }
     fn has_projections(&self) -> bool {
         self.has_type_flags(TypeFlags::HAS_PROJECTION)
@@ -142,26 +142,13 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     fn still_further_specializable(&self) -> bool {
         self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
     }
-
-    /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
-    fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> ControlFlow<()>) -> ControlFlow<()> {
-        pub struct Visitor<F>(F);
-
-        impl<'tcx, F: FnMut(Ty<'tcx>) -> ControlFlow<()>> TypeVisitor<'tcx> for Visitor<F> {
-            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<()> {
-                self.0(ty)
-            }
-        }
-
-        self.visit_with(&mut Visitor(visit))
-    }
 }
 
 impl TypeFoldable<'tcx> for hir::Constness {
     fn super_fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self {
         self
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -195,23 +182,25 @@ pub trait TypeFolder<'tcx>: Sized {
 }
 
 pub trait TypeVisitor<'tcx>: Sized {
-    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<()> {
+    type BreakTy = !;
+
+    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<Self::BreakTy> {
         t.super_visit_with(self)
     }
 
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<()> {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         t.super_visit_with(self)
     }
 
-    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
         r.super_visit_with(self)
     }
 
-    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<()> {
+    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         c.super_visit_with(self)
     }
 
-    fn visit_predicate(&mut self, p: ty::Predicate<'tcx>) -> ControlFlow<()> {
+    fn visit_predicate(&mut self, p: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
         p.super_visit_with(self)
     }
 }
@@ -329,14 +318,19 @@ impl<'tcx> TyCtxt<'tcx> {
         where
             F: FnMut(ty::Region<'tcx>) -> bool,
         {
-            fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<()> {
+            type BreakTy = ();
+
+            fn visit_binder<T: TypeFoldable<'tcx>>(
+                &mut self,
+                t: &Binder<T>,
+            ) -> ControlFlow<Self::BreakTy> {
                 self.outer_index.shift_in(1);
                 let result = t.as_ref().skip_binder().visit_with(self);
                 self.outer_index.shift_out(1);
                 result
             }
 
-            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
                 match *r {
                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
                         ControlFlow::CONTINUE
@@ -351,7 +345,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 }
             }
 
-            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<()> {
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                 // We're only interested in types involving regions
                 if ty.flags().intersects(TypeFlags::HAS_FREE_REGIONS) {
                     ty.super_visit_with(self)
@@ -809,6 +803,9 @@ where
     value.fold_with(&mut Shifter::new(tcx, amount))
 }
 
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+struct FoundEscapingVars;
+
 /// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
 /// bound region or a bound type.
 ///
@@ -840,93 +837,114 @@ struct HasEscapingVarsVisitor {
 }
 
 impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
-    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<()> {
+    type BreakTy = FoundEscapingVars;
+
+    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<Self::BreakTy> {
         self.outer_index.shift_in(1);
         let result = t.super_visit_with(self);
         self.outer_index.shift_out(1);
         result
     }
 
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<()> {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         // If the outer-exclusive-binder is *strictly greater* than
         // `outer_index`, that means that `t` contains some content
         // bound at `outer_index` or above (because
         // `outer_exclusive_binder` is always 1 higher than the
         // content in `t`). Therefore, `t` has some escaping vars.
         if t.outer_exclusive_binder > self.outer_index {
-            ControlFlow::BREAK
+            ControlFlow::Break(FoundEscapingVars)
         } else {
             ControlFlow::CONTINUE
         }
     }
 
-    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
         // If the region is bound by `outer_index` or anything outside
         // of outer index, then it escapes the binders we have
         // visited.
         if r.bound_at_or_above_binder(self.outer_index) {
-            ControlFlow::BREAK
+            ControlFlow::Break(FoundEscapingVars)
         } else {
             ControlFlow::CONTINUE
         }
     }
 
-    fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> ControlFlow<()> {
+    fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         // we don't have a `visit_infer_const` callback, so we have to
         // hook in here to catch this case (annoying...), but
         // otherwise we do want to remember to visit the rest of the
         // const, as it has types/regions embedded in a lot of other
         // places.
         match ct.val {
-            ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => ControlFlow::BREAK,
+            ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
+                ControlFlow::Break(FoundEscapingVars)
+            }
             _ => ct.super_visit_with(self),
         }
     }
 
-    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<()> {
+    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
         if predicate.inner.outer_exclusive_binder > self.outer_index {
-            ControlFlow::BREAK
+            ControlFlow::Break(FoundEscapingVars)
         } else {
             ControlFlow::CONTINUE
         }
     }
 }
 
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+struct FoundFlags;
+
 // FIXME: Optimize for checking for infer flags
 struct HasTypeFlagsVisitor {
     flags: ty::TypeFlags,
 }
 
 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
-    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<()> {
+    type BreakTy = FoundFlags;
+
+    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
         debug!(
             "HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}",
             t,
             t.flags(),
             self.flags
         );
-        if t.flags().intersects(self.flags) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
+        if t.flags().intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
     }
 
-    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
         let flags = r.type_flags();
         debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
-        if flags.intersects(self.flags) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
     }
 
-    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<()> {
+    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         let flags = FlagComputation::for_const(c);
         debug!("HasTypeFlagsVisitor: c={:?} c.flags={:?} self.flags={:?}", c, flags, self.flags);
-        if flags.intersects(self.flags) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
     }
 
-    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<()> {
+    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
         debug!(
             "HasTypeFlagsVisitor: predicate={:?} predicate.flags={:?} self.flags={:?}",
             predicate, predicate.inner.flags, self.flags
         );
         if predicate.inner.flags.intersects(self.flags) {
-            ControlFlow::BREAK
+            ControlFlow::Break(FoundFlags)
         } else {
             ControlFlow::CONTINUE
         }
@@ -960,14 +978,14 @@ impl LateBoundRegionsCollector {
 }
 
 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
-    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<()> {
+    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<Self::BreakTy> {
         self.current_index.shift_in(1);
         let result = t.super_visit_with(self);
         self.current_index.shift_out(1);
         result
     }
 
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<()> {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         // if we are only looking for "constrained" region, we have to
         // ignore the inputs to a projection, as they may not appear
         // in the normalized form
@@ -980,7 +998,7 @@ impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
         t.super_visit_with(self)
     }
 
-    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<()> {
+    fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         // if we are only looking for "constrained" region, we have to
         // ignore the inputs of an unevaluated const, as they may not appear
         // in the normalized form
@@ -993,7 +1011,7 @@ impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
         c.super_visit_with(self)
     }
 
-    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
         if let ty::ReLateBound(debruijn, br) = *r {
             if debruijn == self.current_index {
                 self.regions.insert(br);
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index c39f66ec896..6a67935cd98 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1849,7 +1849,7 @@ impl<'tcx> TypeFoldable<'tcx> for ParamEnv<'tcx> {
         ParamEnv::new(self.caller_bounds().fold_with(folder), self.reveal().fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.caller_bounds().visit_with(visitor)?;
         self.reveal().visit_with(visitor)
     }
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 42952ac2cd2..38f8e779f6a 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1796,7 +1796,7 @@ impl<F: fmt::Write> FmtPrinter<'_, 'tcx, F> {
     {
         struct LateBoundRegionNameCollector<'a>(&'a mut FxHashSet<Symbol>);
         impl<'tcx> ty::fold::TypeVisitor<'tcx> for LateBoundRegionNameCollector<'_> {
-            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<()> {
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
                 if let ty::ReLateBound(_, ty::BrNamed(_, name)) = *r {
                     self.0.insert(name);
                 }
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index fdadc00dc39..94e69a93a6b 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -729,7 +729,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::AdtDef {
         self
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -739,7 +739,7 @@ impl<'tcx, T: TypeFoldable<'tcx>, U: TypeFoldable<'tcx>> TypeFoldable<'tcx> for
         (self.0.fold_with(folder), self.1.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.0.visit_with(visitor)?;
         self.1.visit_with(visitor)
     }
@@ -752,7 +752,7 @@ impl<'tcx, A: TypeFoldable<'tcx>, B: TypeFoldable<'tcx>, C: TypeFoldable<'tcx>>
         (self.0.fold_with(folder), self.1.fold_with(folder), self.2.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.0.visit_with(visitor)?;
         self.1.visit_with(visitor)?;
         self.2.visit_with(visitor)
@@ -779,7 +779,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Rc<T> {
         Rc::new((*self).clone().fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
@@ -790,7 +790,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Arc<T> {
         Arc::new((*self).clone().fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
@@ -800,7 +800,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<T> {
         self.map_id(|value| value.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
@@ -810,7 +810,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Vec<T> {
         self.map_id(|t| t.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -820,7 +820,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<[T]> {
         self.map_id(|t| t.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -834,11 +834,11 @@ impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::Binder<T> {
         folder.fold_binder(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.as_ref().skip_binder().visit_with(visitor)
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_binder(self)
     }
 }
@@ -848,7 +848,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>>
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_existential_predicates(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|p| p.visit_with(visitor))
     }
 }
@@ -858,7 +858,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_type_list(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -868,7 +868,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_projs(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -894,7 +894,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::ty::InstanceDef::*;
         self.substs.visit_with(visitor)?;
         match self.def {
@@ -920,7 +920,7 @@ impl<'tcx> TypeFoldable<'tcx> for interpret::GlobalId<'tcx> {
         Self { instance: self.instance.fold_with(folder), promoted: self.promoted }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.instance.visit_with(visitor)
     }
 }
@@ -969,7 +969,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
         folder.fold_ty(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match self.kind() {
             ty::RawPtr(ref tm) => tm.visit_with(visitor),
             ty::Array(typ, sz) => {
@@ -1011,7 +1011,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
         }
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_ty(self)
     }
 }
@@ -1025,11 +1025,11 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Region<'tcx> {
         folder.fold_region(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_region(*self)
     }
 }
@@ -1040,11 +1040,11 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
         folder.tcx().reuse_or_mk_predicate(self, new)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ty::PredicateKind::super_visit_with(&self.inner.kind, visitor)
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_predicate(*self)
     }
 
@@ -1062,7 +1062,7 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_predicates(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|p| p.visit_with(visitor))
     }
 }
@@ -1072,7 +1072,7 @@ impl<'tcx, T: TypeFoldable<'tcx>, I: Idx> TypeFoldable<'tcx> for IndexVec<I, T>
         self.map_id(|x| x.fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
@@ -1092,12 +1092,12 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::Const<'tcx> {
         folder.fold_const(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.ty.visit_with(visitor)?;
         self.val.visit_with(visitor)
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_const(self)
     }
 }
@@ -1117,7 +1117,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match *self {
             ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
             ty::ConstKind::Param(p) => p.visit_with(visitor),
@@ -1135,7 +1135,7 @@ impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
         self
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 5aa312d7c29..5d1b976ae97 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -160,7 +160,7 @@ impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
             GenericArgKind::Type(ty) => ty.visit_with(visitor),
@@ -392,7 +392,7 @@ impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<()> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }