add note about impossibility of an impl

Author: steveklabnik

_posts/2017-03-16-Rust-1.16.md

@@ -176,6 +176,8 @@ limit false positives.
 
 [we should remove the help message entirely]: https://github.com/rust-lang/rust/pull/37057
 
+> An aside: the above implementation is not possible; `Name` would need to use `String`, not `&str`.
+
 In other diagnostic changes, previous versions of Rust would helpfully attempt to suggest fixes for typos:
 
 ```rust