Uplift clippy::double_neg lint as double_negations

[kadiwa4]

Warns about cases like this:

fn main() {
    let x = 1;
    let _b = --x; //~ WARN use of a double negation
}

The intent is to keep people from thinking that --x is a prefix decrement operator. ++x, x++ and x-- are invalid expressions and already have a helpful diagnostic.

I didn't add a machine-applicable suggestion to the lint because it's not entirely clear what the programmer was trying to achieve with the --x operation. The code that triggers the lint should always be reviewed manually.

Closes #82987

[rustbot]

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[kadiwa4]

@rustbot author

[nnethercote]

This needs T-lang approval. The appropriate labels have been applied to summon them.

[bors]

☔ The latest upstream changes (presumably #127174) made this pull request unmergeable. Please resolve the merge conflicts.

[kadiwa4]

@rustbot ready

[nnethercote]

Still waiting on T-lang, AFAICT.

[kadiwa4]

@rustbot label: -S-waiting-on-review +S-waiting-on-team

[bors]

☔ The latest upstream changes (presumably #128985) made this pull request unmergeable. Please resolve the merge conflicts.

[traviscross]

I looked around on GitHub for anyone using this. The main use case I found is that people who have implemented Neg for a type then tend to write a test or fuzzing code that looks like this:

fn test_neg_is_own_inverse() {
    let x = MyType::new(42);
    assert_eq!(x, --x);
}

It's probably fine to use parentheses there.

@rfcbot reviewed

[nikomatsakis]

@rfcbot reviewed

I see no harm in writing -(-x) -- I would definitely prefer that.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[scottmcm]

Makes sense to me! It's not something that people would normally write, so worth linting since x = --y won't trip unused.

(I guess they could use it as a really obscure way to debug_assert!(y != MIN), but how about no.)

@rfcbot reviewed

[kadiwa4]

These changes were necessary to get the clippy dev tool to compile

[kadiwa4]

This file is supposed to be auto-generated (it has a comment at the top saying so), but most people seem to edit it manually. I used the generator though, that's why so many lines are changed in this file.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[bors]

☔ The latest upstream changes (presumably #136070) made this pull request unmergeable. Please resolve the merge conflicts.

[kadiwa4]

@rustbot label: -S-waiting-on-team +S-waiting-on-review -I-lang-nominated

[nnethercote]

Updated code looks fine to me. I think/hope this has gone through all the appropriate hoops and is ready to be merged.

@bors r+

[bors]

📌 Commit c1dcbeb has been approved by nnethercote

It is now in the queue for this repository.