Improve the warning messages for the #[diagnostic::on_unimplemented]

[weiznich]

This commit improves warnings emitted for malformed on unimplemented attributes by:

• Improving the span of the warnings
• Adding a label message to them
• Separating the messages for missing and unexpected options
• Adding a help message that says which options are supported

r? @compiler-errors

I'm happy to work on further improvements, so feel free to make suggestions.

[Urgau]

Diagnostics should not start with a Upper case.

There is some guidelines on the rustc-dev-guide https://rustc-dev-guide.rust-lang.org/diagnostics.html#suggestion-style-guide but unfortunatly not enough.

[compiler-errors]

Should be able to point at the if(Self = ()) here.

[weiznich]

I've pushed a "fix" to address this somewhat. The problem there is that if(Self = ()) doesn't parse as meta_item_list(). I'm not sure why that's the case or whether that's the right behavior. The pushed "fix" just turns the attribute into something that can be parsed (if(Self = "()") in that case).

[compiler-errors]

oh, ok

[compiler-errors]

@bors r+

[bors]

📌 Commit 3d03a8a has been approved by compiler-errors

It is now in the queue for this repository.