Add new matches_instead_of_eq lint

[GuillaumeGomez]

Fixes #14688.

r? @samueltardieu

changelog: Add new pedantic matches_instead_of_eq lint

[github-actions]

Lintcheck changes for b5f52cd

Lint	Added	Removed	Changed
clippy::matches_instead_of_eq	38	0	0

This comment will be updated if you push new changes

[GuillaumeGomez]

With dogfood errors, I was able to strenghten quite a lot the lint and handle suggestions with dereferencing.

One of the nice changes that came with this lint is:

-        if cx
-            .tcx
-            .crate_types()
-            .iter()
-            .any(|t: &CrateType| matches!(t, CrateType::ProcMacro))
-        {
+        if cx.tcx.crate_types().contains(&CrateType::ProcMacro) {

[lukaslueg]

Maybe a nit: If the type being compared is PartialEq but manually implements that, there may be a subtle difference in behavior, as matches! only does the pattern-matching, while the custom PartialEq-impl may go beyond that; even if one might consider such a PartialEq-impl hazardous for exactly this reason, we may want to not lint if PartialEq is custom-made, as the lint might cause a change in behavior.

[GuillaumeGomez]

Very good point. I'll handle this case as well, thanks!

[GuillaumeGomez]

Added suggested change.

[samueltardieu]

Suggested change

	/// Checks for the use of `matches!` with types that implement `PartialEq` (not manually).
	/// Checks for the use of `matches!` with types that automatically derive `PartialEq`.

View changes since the review

[samueltardieu]

I think this won't catch types whose components have a non-automatically-derived PartialEq, for example here:

enum E { … }

impl PartialEq for E {
    …
}

#[derive(PartialEq)]
struct S(E);

fn f(s: &S) {
    if matches!(s, S(E::…)) { … }
}

In this case, we have multiple choices:

• not lint
• lint with MaybeIncorrect and a note saying that E's PartialEq may not behave as pattern matching

While in other cases, I think we can lint with MachineApplicable.

View changes since the review

[GuillaumeGomez]

I think we shouldn't link in this case to follow the "do not lint if manual PartialEq impl" rule.

[samueltardieu]

The check will have to be carefully placed as to not analyze all types recursively every time there is a `matches!` macro call. You might also want to cache the analysis results.

[rustbot]

☔ The latest upstream changes (possibly #17053) made this pull request unmergeable. Please resolve the merge conflicts.