rustc should warn about unused traits.

```rust
trait A{}

fn main() {}
```

This won't yield a compiler warning. However the equivalent with a struct will:

```rust
struct A{}

fn main() {}
```

https://play.rust-lang.org/?gist=0787e43a11bb28969e9f4978954c3fdc

rustc 1.19.0-nightly (d3abc80b3 2017-05-09)