Question mark lint

[gnieto]

Lint which suggest the usage of the question mark operator when it can be used. At the moment, it checks for calls to is_none, followed by a return None;.

This PR (partially) solves: #2358

I've coded only the case described on the linked issue, but it should be possible to lint for other pieces of code that may use the question mark, like the following one:

use std::io;
use std::io::Read;
use std::fs::File;

fn read_file() -> Result<String, io::Error> {
    let f = File::open("file.txt");
    let mut f = match f {
        Ok(file) => file,
        Err(e) => return Err(e),
    };

    let mut s = String::new();
    match f.read_to_string(&mut s) {
        Ok(_) => Ok(s),
        Err(e) => Err(e),
    }
}

[oli-obk]

That's not the correct span. It should hilight the entire if a.is_none() { return None; }

[gnieto]

I've fixed the span. Can you check it now?

Thanks!

[oli-obk]

Jup, this is perfect now.