Tracking issue for RFC 2086: Allow Irrefutable Patterns in if-let and while-let statements

[aturon]

This is a tracking issue for the RFC "Allow Irrefutable Patterns in if-let and while-let statements" (rust-lang/rfcs#2086).

Steps:

• Implement the RFC (mentoring instructions here)
• Adjust documentation (Document irrefutable_let_patterns reference#507)
• Stabilization PR (Stabilise irrefutable if-let and while-let patterns #57535)

[arielb1]

Mentoring Instructions

This is actually an easy one:

The code that emits the error message is the check_arms function here:

rust/src/librustc_const_eval/check_match.rs

Line 317 in 325ba23

fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,

It nicely contains branches for if let and while let, so we just need to adapt them to emit lints instead of errors. We emit lints using the lint_node function - you can see an example calls of it below in the function. Remember to add a feature gate.

You'll also want to declare the new lints you are using, which is done in:

rust/src/librustc/lint/builtin.rs

Line 19 in 325ba23

declare_lint! {

Just add another declare_lint! for each new lint.

Feature Gates

I should add some version of this to the README.md, but until then:

Also, you'll want to feature gate the new feature, so that it can't be used accidentally until its stabilized. For that, add a feature gate to the list of feature gates here:

rust/src/libsyntax/feature_gate.rs

Line 388 in 325ba23

);

And link this tracking issue (#44495). You can test the feature in tcx.sess (e.g. tcx.sess.features.borrow().specialization). If it's not present, you should maintain the old behavior.

Don't forget to add tests, both for the new behavior and the old one (the test that the old behavior remains without a feature gate should be in src/test/compile-fail/feature-gate-YOUR-FEATURE.rs).

[Nokel81]

I will now start on the implementation of this

[nikomatsakis]

@Nokel81 just checking in -- how's it going? Any questions?