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.~~

Author: bors

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1498,7 +1498,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         impl<'tcx> ty::fold::TypeVisitor<'tcx> for OpaqueTypesVisitor<'tcx> {
-            fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<()> {
+            fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                 if let Some((kind, def_id)) = TyCategory::from_ty(t) {
                     let span = self.tcx.def_span(def_id);
                     // Avoid cluttering the output when the "found" and error span overlap:

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -474,7 +474,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 struct TraitObjectVisitor(Vec<DefId>);
 
 impl TypeVisitor<'_> for TraitObjectVisitor {
-    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<()> {
+    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
         match t.kind() {
             ty::Dynamic(preds, RegionKind::ReStatic) => {
                 if let Some(def_id) = preds.principal_def_id() {

compiler/rustc_infer/src/infer/mod.rs

@@ -1,5 +1,3 @@
-//! See the Book for more information.
-
 pub use self::freshen::TypeFreshener;
 pub use self::LateBoundRegionConversionTime::*;
 pub use self::RegionVariableOrigin::*;
@@ -1334,9 +1332,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        let mut r = resolve::UnresolvedTypeFinder::new(self);
-        value.visit_with(&mut r);
-        r.first_unresolved
+        value.visit_with(&mut resolve::UnresolvedTypeFinder::new(self)).break_value()
     }
 
     pub fn probe_const_var(

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -741,15 +741,18 @@ struct ScopeInstantiator<'me, 'tcx> {
 }
 
 impl<'me, 'tcx> TypeVisitor<'tcx> for ScopeInstantiator<'me, 'tcx> {
-    fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ControlFlow<()> {
+    fn visit_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: &ty::Binder<T>,
+    ) -> ControlFlow<Self::BreakTy> {
         self.target_index.shift_in(1);
         t.super_visit_with(self);
         self.target_index.shift_out(1);
 
         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 ScopeInstantiator { bound_region_scope, next_region, .. } = self;
 
         match r {

compiler/rustc_infer/src/infer/resolve.rs

@@ -111,19 +111,17 @@ impl<'a, 'tcx> TypeFolder<'tcx> for OpportunisticRegionResolver<'a, 'tcx> {
 /// involve some hashing and so forth).
 pub struct UnresolvedTypeFinder<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
-
-    /// Used to find the type parameter name and location for error reporting.
-    pub first_unresolved: Option<(Ty<'tcx>, Option<Span>)>,
 }
 
 impl<'a, 'tcx> UnresolvedTypeFinder<'a, 'tcx> {
     pub fn new(infcx: &'a InferCtxt<'a, 'tcx>) -> Self {
-        UnresolvedTypeFinder { infcx, first_unresolved: None }
+        UnresolvedTypeFinder { infcx }
     }
 }
 
 impl<'a, 'tcx> TypeVisitor<'tcx> for UnresolvedTypeFinder<'a, 'tcx> {
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<()> {
+    type BreakTy = (Ty<'tcx>, Option<Span>);
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         let t = self.infcx.shallow_resolve(t);
         if t.has_infer_types() {
             if let ty::Infer(infer_ty) = *t.kind() {
@@ -144,8 +142,7 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for UnresolvedTypeFinder<'a, 'tcx> {
                 } else {
                     None
                 };
-                self.first_unresolved = Some((t, ty_var_span));
-                ControlFlow::BREAK
+                ControlFlow::Break((t, ty_var_span))
             } else {
                 // Otherwise, visit its contents.
                 t.super_visit_with(self)

compiler/rustc_infer/src/traits/structural_impls.rs

@@ -69,7 +69,7 @@ impl<'tcx, O: TypeFoldable<'tcx>> TypeFoldable<'tcx> for traits::Obligation<'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.predicate.visit_with(visitor)
     }
 }

compiler/rustc_lint/src/types.rs

@@ -1131,16 +1131,14 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     fn check_for_opaque_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
         struct ProhibitOpaqueTypes<'a, 'tcx> {
             cx: &'a LateContext<'tcx>,
-            ty: Option<Ty<'tcx>>,
         };
 
         impl<'a, 'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueTypes<'a, 'tcx> {
-            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<()> {
+            type BreakTy = Ty<'tcx>;
+
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                 match ty.kind() {
-                    ty::Opaque(..) => {
-                        self.ty = Some(ty);
-                        ControlFlow::BREAK
-                    }
+                    ty::Opaque(..) => ControlFlow::Break(ty),
                     // Consider opaque types within projections FFI-safe if they do not normalize
                     // to more opaque types.
                     ty::Projection(..) => {
@@ -1159,9 +1157,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             }
         }
 
-        let mut visitor = ProhibitOpaqueTypes { cx: self.cx, ty: None };
-        ty.visit_with(&mut visitor);
-        if let Some(ty) = visitor.ty {
+        if let Some(ty) = ty.visit_with(&mut ProhibitOpaqueTypes { cx: self.cx }).break_value() {
             self.emit_ffi_unsafe_type_lint(ty, sp, "opaque types have no C equivalent", None);
             true
         } else {

compiler/rustc_macros/src/type_foldable.rs

@@ -35,7 +35,7 @@ pub fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::
             fn super_visit_with<__F: ::rustc_middle::ty::fold::TypeVisitor<'tcx>>(
                 &self,
                 __folder: &mut __F
-            ) -> ::std::ops::ControlFlow<()> {
+            ) -> ::std::ops::ControlFlow<__F::BreakTy> {
                 match *self { #body_visit }
                 ::std::ops::ControlFlow::CONTINUE
             }

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]

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())
             }
         }