Ignore as_deref_mut in needless_option_as_deref

[Alexendoo]

Fixes #7846
Fixes #8047

It would be possible to only lint as_deref_mut where the Option is movable + unused later, but it wasn't clear to me how to test for that. So for now at least ignore it

changelog: [needless_option_as_deref]: No longer lints on as_deref_mut

[rust-highfive]

r? @Manishearth

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[Manishearth]

Currently still catching up on stuff from travel, might not be able to provide a timely review

r? @giraffate (feel free to reassign)

[surechen]

Hi, hope I won't disturb you. I 'm also interested in this issue and hope to discuss it together.

According to lint's comments, I think the author of lint wants to detect the situation "Option<&mut t> to Option<&mut t>", because this is not necessary, as_deref_mut is suitable for "&mut Option to Option<&mut t>", but because as_deref_mut has another meaning: "Leaves the original Option in-place, creating a new one containing a mutable reference to the inner type's Deref ::Target type.”, which caused a problem in the suggestion code in issue #8047.

So I think this lint is valuable for detecting unnecessary use of as_deref_mut.

For the example in issue #8047:

fn foo(_x: Option<&mut i32>) {}

fn main() {
     let mut y = 0;
     let mut x = Some(&mut y);
    
     foo(x.as_deref_mut()); // lint suggestion   `x`
     println!("{:?}", x); // use after move
}

Is it possible to scan to check the use after move situation in #8047 or just change the suggestion level to MaybeIncorrect?

[Alexendoo]

Yeah this is more of a quick fix, it introduces a false negative in return for fixing false positives. MaybeIncorrect would mean the suggestion isn't automatically applied by --fix but the warning would still be there

Detecting it properly would be preferable, but I wasn't sure how to go about that easily. That could always come as a later PR, or somebody may point out something I missed

[Manishearth]

Worth bringing up in the Clippy zulip channel and seeing what others think! https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy

[giraffate]

It has been discussed here: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/.60needless_option_as_deref.60.20with.20.60as_deref_mut.60.

[bors]

☔ The latest upstream changes (presumably #8196) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #8359) made this pull request unmergeable. Please resolve the merge conflicts.