Remove TypeSuper{Foldable,Visitable} impls for Region.

These traits exist so that folders/visitors can recurse into types of
interest: binders, types, regions, predicates, and consts. But `Region`
is non-recursive and cannot contain other types of interest, so its
methods in these traits are trivial.

This commit inlines and removes those trivial methods.

Author: nnethercote

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -105,7 +105,7 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
             if let ty::RegionKind::ReEarlyBound(ebr) = r.kind() {
                 self.variances[ebr.index as usize] = ty::Invariant;
             }
-            r.super_visit_with(self)
+            ControlFlow::Continue(())
         }
 
         #[instrument(level = "trace", skip(self), ret)]

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

@@ -13,7 +13,7 @@ use rustc_hir::intravisit::Visitor;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::print::RegionHighlightMode;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
 use rustc_span::Span;
 
 use std::ops::ControlFlow;
@@ -81,7 +81,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                     self.highlight.highlighting_region(r, self.counter);
                     self.counter += 1;
                 }
-                r.super_visit_with(self)
+                ControlFlow::Continue(())
             }
         }
 

compiler/rustc_middle/src/ty/fold.rs

@@ -37,7 +37,8 @@ where
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        let r = r.super_fold_with(self);
+        // This one is a little different, because `super_fold_with` is not
+        // implemented on non-recursive `Region`.
         (self.lt_op)(r)
     }
 

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2518,7 +2518,7 @@ impl<'tcx> FmtPrinter<'_, 'tcx> {
                     self.used_region_names.insert(name);
                 }
 
-                r.super_visit_with(self)
+                ControlFlow::Continue(())
             }
 
             // We collect types in order to prevent really large types from compiling for

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -583,24 +583,6 @@ impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::Region<'tcx> {
     }
 }
 
-impl<'tcx> TypeSuperFoldable<TyCtxt<'tcx>> for ty::Region<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(self)
-    }
-}
-
-impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Region<'tcx> {
-    fn super_visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(
-        &self,
-        _visitor: &mut V,
-    ) -> ControlFlow<V::BreakTy> {
-        ControlFlow::Continue(())
-    }
-}
-
 impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,

compiler/rustc_middle/src/ty/visit.rs

@@ -364,7 +364,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ValidateBoundVars<'tcx> {
             _ => (),
         };
 
-        r.super_visit_with(self)
+        ControlFlow::Continue(())
     }
 }
 

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -450,7 +450,7 @@ fn is_impossible_method(tcx: TyCtxt<'_>, (impl_def_id, trait_item_def_id): (DefI
             {
                 return ControlFlow::Break(());
             }
-            r.super_visit_with(self)
+            ControlFlow::Continue(())
         }
         fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
             if let ty::ConstKind::Param(param) = ct.kind()

compiler/rustc_traits/src/chalk/lowering.rs

@@ -998,7 +998,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for BoundVarsCollector<'tcx> {
             _ => (),
         };
 
-        r.super_visit_with(self)
+        ControlFlow::Continue(())
     }
 }
 
@@ -1048,7 +1048,7 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for NamedBoundVarSubstitutor<'a, 'tcx> {
             _ => (),
         };
 
-        r.super_fold_with(self)
+        r
     }
 }
 
@@ -1142,7 +1142,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ParamsSubstitutor<'tcx> {
                 }
             },
 
-            _ => r.super_fold_with(self),
+            _ => r,
         }
     }
 }
@@ -1223,6 +1223,6 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for PlaceholdersCollector {
             _ => (),
         };
 
-        r.super_visit_with(self)
+        ControlFlow::Continue(())
     }
 }

compiler/rustc_type_ir/src/fold.rs

@@ -16,8 +16,10 @@
 //!   - Types of interest, for which the methods delegate to the folder.
 //!   - All other types, including generic containers like `Vec` and `Option`.
 //!     It defines a "skeleton" of how they should be folded.
-//! - `TypeSuperFoldable`. This is implemented only for each type of interest,
-//!   and defines the folding "skeleton" for these types.
+//! - `TypeSuperFoldable`. This is implemented only for recursive types of
+//!   interest, and defines the folding "skeleton" for these types. (This
+//!   excludes `Region` because it is non-recursive, i.e. it never contains
+//!   other types of interest.)
 //! - `TypeFolder`/`FallibleTypeFolder`. One of these is implemented for each
 //!   folder. This defines how types of interest are folded.
 //!
@@ -72,9 +74,9 @@ pub trait TypeFoldable<I: Interner>: TypeVisitable<I> {
 
 // This trait is implemented for types of interest.
 pub trait TypeSuperFoldable<I: Interner>: TypeFoldable<I> {
-    /// Provides a default fold for a type of interest. This should only be
-    /// called within `TypeFolder` methods, when a non-custom traversal is
-    /// desired for the value of the type of interest passed to that method.
+    /// Provides a default fold for a recursive type of interest. This should
+    /// only be called within `TypeFolder` methods, when a non-custom traversal
+    /// is desired for the value of the type of interest passed to that method.
     /// For example, in `MyFolder::try_fold_ty(ty)`, it is valid to call
     /// `ty.try_super_fold_with(self)`, but any other folding should be done
     /// with `xyz.try_fold_with(self)`.
@@ -118,11 +120,11 @@ pub trait TypeFolder<I: Interner>: FallibleTypeFolder<I, Error = !> {
         t.super_fold_with(self)
     }
 
-    fn fold_region(&mut self, r: I::Region) -> I::Region
-    where
-        I::Region: TypeSuperFoldable<I>,
-    {
-        r.super_fold_with(self)
+    // The default region folder is a no-op because `Region` is non-recursive
+    // and has no `super_visit_with` method to call. That also explains the
+    // lack of `I::Region: TypeSuperFoldable<I>` bound on this method.
+    fn fold_region(&mut self, r: I::Region) -> I::Region {
+        r
     }
 
     fn fold_const(&mut self, c: I::Const) -> I::Const
@@ -167,11 +169,11 @@ pub trait FallibleTypeFolder<I: Interner>: Sized {
         t.try_super_fold_with(self)
     }
 
-    fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, Self::Error>
-    where
-        I::Region: TypeSuperFoldable<I>,
-    {
-        r.try_super_fold_with(self)
+    // The default region folder is a no-op because `Region` is non-recursive
+    // and has no `super_visit_with` method to call. That also explains the
+    // lack of `I::Region: TypeSuperFoldable<I>` bound on this method.
+    fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, Self::Error> {
+        Ok(r)
     }
 
     fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Self::Error>
@@ -216,10 +218,7 @@ where
         Ok(self.fold_ty(t))
     }
 
-    fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, !>
-    where
-        I::Region: TypeSuperFoldable<I>,
-    {
+    fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, !> {
         Ok(self.fold_region(r))
     }
 

compiler/rustc_type_ir/src/visit.rs

@@ -13,8 +13,11 @@
 //!   - Types of interest, for which the methods delegate to the visitor.
 //!   - All other types, including generic containers like `Vec` and `Option`.
 //!     It defines a "skeleton" of how they should be visited.
-//! - `TypeSuperVisitable`. This is implemented only for each type of interest,
-//!   and defines the visiting "skeleton" for these types.
+//! - `TypeSuperVisitable`. This is implemented only for recursive types of
+//!   interest, and defines the visiting "skeleton" for these types. (This
+//!   excludes `Region` because it is non-recursive, i.e. it never contains
+//!   other types of interest.)
+//!
 //! - `TypeVisitor`. This is implemented for each visitor. This defines how
 //!   types of interest are visited.
 //!
@@ -62,12 +65,13 @@ pub trait TypeVisitable<I: Interner>: fmt::Debug + Clone {
     fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
 }
 
+// This trait is implemented for types of interest.
 pub trait TypeSuperVisitable<I: Interner>: TypeVisitable<I> {
-    /// Provides a default visit for a type of interest. This should only be
-    /// called within `TypeVisitor` methods, when a non-custom traversal is
-    /// desired for the value of the type of interest passed to that method.
-    /// For example, in `MyVisitor::visit_ty(ty)`, it is valid to call
-    /// `ty.super_visit_with(self)`, but any other visiting should be done
+    /// Provides a default visit for a recursive type of interest. This should
+    /// only be called within `TypeVisitor` methods, when a non-custom
+    /// traversal is desired for the value of the type of interest passed to
+    /// that method. For example, in `MyVisitor::visit_ty(ty)`, it is valid to
+    /// call `ty.super_visit_with(self)`, but any other visiting should be done
     /// with `xyz.visit_with(self)`.
     fn super_visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
 }
@@ -92,11 +96,11 @@ pub trait TypeVisitor<I: Interner>: Sized {
         t.super_visit_with(self)
     }
 
-    fn visit_region(&mut self, r: I::Region) -> ControlFlow<Self::BreakTy>
-    where
-        I::Region: TypeSuperVisitable<I>,
-    {
-        r.super_visit_with(self)
+    // The default region visitor is a no-op because `Region` is non-recursive
+    // and has no `super_visit_with` method to call. That also explains the
+    // lack of `I::Region: TypeSuperVisitable<I>` bound.
+    fn visit_region(&mut self, _r: I::Region) -> ControlFlow<Self::BreakTy> {
+        ControlFlow::Continue(())
     }
 
     fn visit_const(&mut self, c: I::Const) -> ControlFlow<Self::BreakTy>

src/librustdoc/clean/auto_trait.rs

@@ -1,7 +1,7 @@
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
-use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable, TypeSuperFoldable};
+use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable};
 use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};
 use thin_vec::ThinVec;
 
@@ -740,10 +740,9 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for RegionReplacer<'a, 'tcx> {
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        (match *r {
-            ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
-            _ => None,
-        })
-        .unwrap_or_else(|| r.super_fold_with(self))
+        match *r {
+            ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned().unwrap_or(r),
+            _ => r,
+        }
     }
 }