Lack of `mut` in bindings as a deny-by-default lint

[tczajka]

@trentj

But even so, what about the second example (I edited in), RefMut ? Semantically it should be essentially a &mut .

Right, I agree. To me that is a "const-correctness" problem with the API of Deref and DerefMut. The problem is that dereferencing RefMut requires &mut self rather than a weaker and more accurate &uniq self. It would be ideal if Rust had &uniq for unique, non-mutable references, and a corresponding DerefUniq.