Stabilise exhaustive integer pattern matching

[varkor]

Extend Rust's pattern matching exhaustiveness checks to cover the integer types: u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize and char.

Rendered.

fn matcher_full(x: u8) {
  match x { // ok
    0 ..= 31 => { /* ... */ }
    32 => { /* ... */ }
    33 ..= 255 => { /* ... */ }
  }
}

fn matcher_incomplete(x: u8) {
  match x { //~ ERROR: non-exhaustive patterns: `32u8..=255u8` not covered
    0 ..= 31 => { /* ... */ }
  }
}

This feature has already been implemented behind the feature flag exhaustive_integer_patterns, so this RFC is viewed as a motion to stabilise the feature. See rust-lang/rust#50912 (comment) for the previous motion to (unstably) approve this feature.

[Centril]

@rfcbot fcp merge

Feature name: #![feature(exhaustive_integer_patterns)]
Version target: 1.32 (2019-01-18)
Tracking issue: rust-lang/rust#50907

The feature exhaustive_integer_patterns feature has baked in the nightly compiler for approximately 12 weeks which is sufficient amount of time. Originally, the feature did not pass through an RFC but it was accepted by an FCP-merge over at rust-lang/rust#50912 (comment).

This RFC serves as:

• An introduction for people who were unaware of this new capability,
• A stabilization report,
• As assurance that the proper RFC process is followed.

Given the clear motivation in improving code correctness and to fix bugs in the language, I propose that we accept this RFC and stabilize exhaustive_integer_patterns as per the stabilization report given in this RFC. Once this RFC is accepted, and if no further issues are found, the feature will move to immediate stabilization without further delay.

[rfcbot]

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

• @Centril
• @aturon
• @cramertj
• @eddyb
• @joshtriplett
• @nikomatsakis
• @nrc
• @pnkfelix
• @scottmcm
• @withoutboats

Concerns:

• isize-usize-exhaustiveness resolved by Stabilise exhaustive integer pattern matching #2591 (comment)

Once a majority of reviewers approve (and none object), 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!

See this document for info about what commands tagged team members can give me.

[scottmcm]

I love the more specific error messages that come with this!

[Centril]

@rfcbot concern isize-usize-exhaustiveness

Seems like a valid concern re. usize and isize for reasons aforementioned (e.g. in #2591 (comment)); I'd like to see RFC amended to extract assumptions about the max value of usize and min+max values of isize out to a separate feature gate exhaustive_usize_patterns as a conservative choice that can be changed in the future if so desired. The extraction can be done as part of the stabilization PR.

[Centril]

@varkor Thoughts on / mind making the adjustments per #2591 (comment)?

[varkor]

Exhaustive integer pattern matching will be very much complimented by #947, so it's probably ripe time to start a new RFC for open-ended range patterns.

[Centril]

Given the latest changes...

@rfcbot resolve isize-usize-exhaustiveness

[rfcbot]

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

[rfcbot]

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

[Centril]

🎉 Huzzah! This RFC has been merged! 🎉

Tracking issue: rust-lang/rust#50907
Stabilization PR: rust-lang/rust#56362