Implement lint against ambiguous negative literals

[Urgau]

This PR implements a lint against ambiguous negative literals with a literal and method calls right after it.

ambiguous_negative_literals

(deny-by-default)

The ambiguous_negative_literals lint checks for cases that are confusing between a negative literal and a negation that's not part of the literal.

Example

-1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1

Explanation

Method calls take precedence over unary precedence. Setting the precedence explicitly makes the code clearer and avoid potential bugs.

Old proposed lint

ambiguous_unary_precedence

(deny-by-default)

The ambiguous_unary_precedence lint checks for use the negative unary operator with a literal and method calls.

Example

-1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1

Explanation

Unary operations take precedence on binary operations and method calls take precedence over unary precedence. Setting the precedence explicitly makes the code clearer and avoid potential bugs.

---

Note: This is a strip down version of #117161, without the binary op precedence.

Fixes #117155
@rustbot labels +I-lang-nominated
cc @scottmcm
r? compiler

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[traviscross]

@rustbot labels -I-lang-nominated

Let's remove the nomination here in favor of the one on #117161.

[traviscross]

@rustbot labels +I-lang-nominated

Thanks to discussion with @Urgau, I now notice that this is in fact proposing a subset of #117161 and so should be separately considered (though still probably should be discussed in the same breath). Let's renominate on that basis.

[scottmcm]

I'm (unsurprisingly) a fan of having a lint like this.

For triage discussion: do we want it called unary like this, or maybe ambiguous_literal_precedence?

[scottmcm]

We discussed this in the lang triage meeting today.

There was some unwillingness to phrase this in terms of unary operators, in the sense that there's lots of potentially-ambiguous things -- *foo.bar for example -- that we wouldn't necessarily want to lint on, expecting instead that people know those precedences.

There was broad agreement that the confusing case of negation with literals is particularly footgunny, however.

So the proposed decision is the following:

Deny-by-default lint ambiguous_negative_literal for cases that are confusing between a negative literal and a negation that's not part of the literal.

@rfcbot fcp merge

I'm checking tyler and josh's boxes per their in-meeting request.

(I think that's already the implemented semantic, just a different name?)

[rfcbot]

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot reviewed

[rfcbot]

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

[compiler-errors]

Some grammatical nits

[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.

[compiler-errors]

@bors r+

[bors]

📌 Commit c5e1a12 has been approved by compiler-errors

It is now in the queue for this repository.

[BurntSushi]

This completely broke Jiff once the change landed. Because it specifically supports negating spans like -1.hour(). In the case of Jiff, there is no ambiguity because -1.hour() and (-1).hour() and -(1.hour()) are all precisely equivalent.

[Urgau]

For info: T-lang decided to temporarily switch the lint to allow-by-default (done in #128449) until they have a bigger discussion about it.