New lint: manual_ok_err

[samueltardieu]

changelog: [manual_ok_err]: new lint

Detect manual implementations of .ok() or .err(), as in

let a = match func() {
    Ok(v) => Some(v),
    Err(_) => None,
};
let b = if let Err(v) = func() {
    Some(v)
} else {
    None
};

which can be replaced by

let a = func().ok();
let b = func().err();

This pattern was detected in the wild in the Rust reimplementation of coreutils: uutils/coreutils#6886 (review)

[rustbot]

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[samueltardieu]

Putting it to WIP as there is a case that is incorrectly handled.
Done

[samueltardieu]

(the latest push is a rebase on master)

[samueltardieu]

r? @y21

[samueltardieu]

More maintainer roulette

r? @blyxyas

[blyxyas]

Everything seems to be correct, and it seems to be a very useful lint. I think it has some bright future! (Some clarity nits were found)

[blyxyas]

A refactoring or comment would help the clarity of this fragment!

[samueltardieu]

Comment added

[blyxyas]

The FCP just opened, we'll wait a few days and check that everything's alright. I like the idea of this lint 👍

[pitaj]

Does the current implementation consider const code? It probably shouldn't lint in those cases.

[samueltardieu]

Does the current implementation consider const code? It probably shouldn't lint in those cases.

It doesn't lint in const code (checked in matches/mod.rs). I have added a new test for checking this because of your remark.