Set the default BreakTy to !

compiler/rustc_lint/src/types.rs

@@ -1135,6 +1135,8 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         };
 
         impl<'a, 'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueTypes<'a, 'tcx> {
+            type BreakTy = ();
+
             fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                 match ty.kind() {
                     ty::Opaque(..) => {

compiler/rustc_middle/src/ty/fold.rs

@@ -148,6 +148,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         pub struct Visitor<F>(F);
 
         impl<'tcx, F: FnMut(Ty<'tcx>) -> ControlFlow<()>> TypeVisitor<'tcx> for Visitor<F> {
+            type BreakTy = ();
             fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<()> {
                 self.0(ty)
             }
@@ -195,7 +196,7 @@ pub trait TypeFolder<'tcx>: Sized {
 }
 
 pub trait TypeVisitor<'tcx>: Sized {
-    type BreakTy = ();
+    type BreakTy = !;
 
     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> ControlFlow<Self::BreakTy> {
         t.super_visit_with(self)
@@ -331,6 +332,8 @@ impl<'tcx> TyCtxt<'tcx> {
         where
             F: FnMut(ty::Region<'tcx>) -> bool,
         {
+            type BreakTy = ();
+
             fn visit_binder<T: TypeFoldable<'tcx>>(
                 &mut self,
                 t: &Binder<T>,

compiler/rustc_mir/src/interpret/util.rs

@@ -18,6 +18,8 @@ where
     };
 
     impl<'tcx> TypeVisitor<'tcx> for UsedParamsNeedSubstVisitor<'tcx> {
+        type BreakTy = ();
+
         fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
             if !c.needs_subst() {
                 return ControlFlow::CONTINUE;

compiler/rustc_mir/src/monomorphize/polymorphize.rs

@@ -318,6 +318,8 @@ struct HasUsedGenericParams<'a> {
 }
 
 impl<'a, 'tcx> TypeVisitor<'tcx> for HasUsedGenericParams<'a> {
+    type BreakTy = ();
+
     fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         debug!("visit_const: c={:?}", c);
         if !c.has_param_types_or_consts() {

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -771,6 +771,8 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
     }
 
     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
+        type BreakTy = ();
+
         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
             match t.kind() {
                 ty::Param(_) => {

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -800,6 +800,8 @@ fn check_where_clauses<'tcx, 'fcx>(
                 params: FxHashSet<u32>,
             }
             impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
+                type BreakTy = ();
+
                 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                     if let ty::Param(param) = t.kind() {
                         self.params.insert(param.index);

src/tools/clippy/clippy_lints/src/redundant_clone.rs

@@ -563,6 +563,8 @@ impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
 struct ContainsRegion;
 
 impl TypeVisitor<'_> for ContainsRegion {
+    type BreakTy = ();
+
     fn visit_region(&mut self, _: ty::Region<'_>) -> ControlFlow<Self::BreakTy> {
         ControlFlow::BREAK
     }