Mention the type in the label, to avoid confusion at the cost of redundancy

tests/run-make/missing-unstable-trait-bound/missing-bound.stderr

@@ -2,7 +2,7 @@ error[E0277]: `std::ops::Range<T>` is not an iterator
  --> missing-bound.rs:2:14
   |
 2 |     for _ in t {}
-  |              ^ not an iterator
+  |              ^ `Range<T>` is not an iterator
   |
   = note: `Range` only implements `Iterator` for select types in the standard library, particularly integers; to see the full list of types, see the documentation for the unstable `Step` trait
   = note: required for `std::ops::Range<T>` to implement `Iterator`