feat(rustc_middle): Ty::contains_error method

Add a new method on `rustc_middle::Ty` to detect if a type contains any
`TyKind::Error`.

This is more specific than `TypeVisitableExt::references_error`, which
is true in many other situations.

Author: onkoe

compiler/rustc_middle/src/ty/sty.rs

@@ -1315,6 +1315,29 @@ impl<'tcx> Ty<'tcx> {
         cf.is_break()
     }
 
+    /// Checks whether a type recursively contains some `TyKind::Error(_)`.
+    ///
+    /// Example: The (erroneous) type `Vec<{type error}, {type error}>` returns
+    /// true.
+    pub fn contains_error(self) -> bool {
+        struct ContainsErrorVisitor;
+
+        impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ContainsErrorVisitor {
+            type Result = ControlFlow<()>;
+
+            fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
+                if let ty::Error(..) = t.kind() {
+                    ControlFlow::Break(())
+                } else {
+                    t.super_visit_with(self)
+                }
+            }
+        }
+
+        let cf = self.visit_with(&mut ContainsErrorVisitor);
+        cf.is_break()
+    }
+
     /// Returns the type and mutability of `*ty`.
     ///
     /// The parameter `explicit` indicates if this is an *explicit* dereference.