Comments to clarify the working of HasTypeFlagsVisitor.

Author: nnethercote

compiler/rustc_middle/src/ty/visit.rs

@@ -33,14 +33,6 @@ pub trait TypeVisitableExt<'tcx>: TypeVisitable<TyCtxt<'tcx>> {
     }
 
     fn has_type_flags(&self, flags: TypeFlags) -> bool {
-        // N.B. Even though this uses a visitor, the visitor does not actually
-        //      recurse through the whole `TypeVisitable` implementor type.
-        //
-        //      Instead it stops on the first "level", visiting types, regions,
-        //      consts and predicates just fetches their type flags.
-        //
-        //      Thus this is a lot faster than it might seem and should be
-        //      optimized to a simple field access.
         let res =
             self.visit_with(&mut HasTypeFlagsVisitor { flags }).break_value() == Some(FoundFlags);
         trace!(?self, ?flags, ?res, "has_type_flags");
@@ -485,11 +477,16 @@ impl std::fmt::Debug for HasTypeFlagsVisitor {
     }
 }
 
+// Note: this visitor traverses values down to the level of
+// `Ty`/`Const`/`Predicate`, but not within those types. This is because the
+// type flags at the outer layer are enough. So it's faster than it first
+// looks, particular for `Ty`/`Predicate` where it's just a field access.
 impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for HasTypeFlagsVisitor {
     type BreakTy = FoundFlags;
 
     #[inline]
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // Note: no `super_visit_with` call.
         let flags = t.flags();
         if flags.intersects(self.flags) {
             ControlFlow::Break(FoundFlags)
@@ -500,6 +497,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for HasTypeFlagsVisitor {
 
     #[inline]
     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // Note: no `super_visit_with` call, as usual for `Region`.
         let flags = r.type_flags();
         if flags.intersects(self.flags) {
             ControlFlow::Break(FoundFlags)
@@ -510,6 +508,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for HasTypeFlagsVisitor {
 
     #[inline]
     fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // Note: no `super_visit_with` call.
         let flags = FlagComputation::for_const(c);
         trace!(r.flags=?flags);
         if flags.intersects(self.flags) {
@@ -521,6 +520,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for HasTypeFlagsVisitor {
 
     #[inline]
     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // Note: no `super_visit_with` call.
         if predicate.flags().intersects(self.flags) {
             ControlFlow::Break(FoundFlags)
         } else {