Add lint for impl-only-use RFC

[mathstuf]

Detecting the case where the impl-only-use RFC allows for use Trait as _ would be nice to have to find existing uses of use Trait as UnusedName.

[nk9]

I'm afraid I'm having trouble understanding what this issue is proposing. Can you please clarify?

@rustbot label S-waiting-on-author A-lint

[samueltardieu]

A trait can be made visible using any of the following construct:

use Trait;
use Trait as _;
use Trait as SomeOtherName;

If I understand this issue correctly, it proposes to lint against the third form if SomeOtherName is never used explictly.

@rustbot label -S-waiting-on-author

[y21]

This sounds a bit like https://rust-lang.github.io/rust-clippy/master/index.html#unused_trait_names ? (Both use Trait and use Trait as UnusedName would trigger the lint if the imported name is unused and it's only imported for its methods)

[samueltardieu]

It won't lint against the horrendous: (my bad, it does lint)

use std::io::Read as foo;

fn main() -> Result<(), std::io::Error> {
    let mut file = std::fs::File::open("/tmp/foo")?;
    let s = &mut vec![];
    println!("{}", file.read_to_end(s)?);
    Ok(())
}

[mathstuf]

Yes, @samueltardieu has it exactly right above. I'm interested in finding instances where I gave a name for a trait on import (IIRC, due to async/std Write traits conflicting) and can use the _ for one of them. Since it seems like it is already fixed, I'm guessing I'm using the renames somewhere in the case(s) I was worried about. Or they're in projects without active CI schedules :) .