Impl only use

[max-sixty]

Would close #2887

• This is the only meaningful change
• Is this too hacky? I can't find a type of the relevant '_' Symbol or anything more than its name to match on. Its span is Span { lo: BytePos(0), hi: BytePos(0), ctxt: #0 }
• Would there have been a more focused way to test this rather than the system test? The existing unit tests do no rewriting - they don't access a Context object (which I didn't easily see how to create)
• I cleaned up some other bits when working my way through the code. LMK if you want to retain

[max-sixty]

The integration tests have a couple of failures; I think unrelated. For example:

Diff in /Users/maximilian/workspace/rustfmt/rand/src/rngs/entropy.rs at line 290:
         all(target_arch = "wasm32", feature = "stdweb"),
         all(target_arch = "wasm32", feature = "wasm-bindgen"),
     ),
-),))]
+)))]
 type Os = NoSource;

 type Custom = NoSource;

...which I think is https://github.com/rust-random/rand/blob/master/src/rngs/entropy.rs?utf8=%E2%9C%93#L261 (although not exactly the same file...)

[topecongiro]

Thank you for the PR!

I think that this is good enough. The compiler does not always provide enough information for rustfmt.