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Stabilize fixed-length slice patterns (excluding ..) #48836
Description
This is a sub-issue of the larger tracking issue for slice patterns (#23121). It specifically tracks the plans to stabilize fixed-length slice patterns -- that is, excluding the .. operator (which is entangled with some syntactic questions). There is an implementation PR in #48516.
Summary
Using this feature, a pattern like [a, b, c] can be used to match against fixed-length arrays:
let arr = [1, 2, 3];
let [a, b, c] = arr; // where arr: [T;3]
let [a, ref b, c] = arr; // where arr: [T;3]One can also use such a pattern to match against slices:
let arr = &[1, 2];
match arr {
[a, b, c] => { /* this arm will not apply */ }
[a, b] => { /* this arm will */ }
_ => { /* this wildcard is required, since we don't know length statically */
}Notable points and tests
Here are some of the things I think we should be testing. I didn't have time to find if we have all these tests, perhaps someone can help (cc @petrochenkov, @mikhail-m1 ?)
- test showing irrefutable matches (
let [a, b] =) applied to fixed-length to move things out - test showing irrefutable matches (
let [a, b] =) applied to create reference - test showing attempt to move from a borrowed array or slice gives error
- move-out-of-slice1.rs sort of tests that, but for an oddly special case involving box patterns
- test that wildcard is required when matching slice with fixed-length arrays
match-vec-fixedtests that we get warnings for two slice patterns of same length- other cases?
Things not stabilized
- The main thing not stabilized is the ability to create a slice of unmatched items.
Activity
nikomatsakis commentedon Mar 8, 2018
@rfcbot fcp merge
I propose we merge the subset above. @petrochenkov has a PR here:
#48516
I'd like to see us document the tests better, but I think that we have them =)
rfcbot commentedon Mar 8, 2018
Team member @nikomatsakis has proposed to merge this. The next step is review by the rest of the tagged teams:
No concerns currently listed.
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!
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petrochenkov commentedon Mar 8, 2018
There are many tests testing slice patterns (at least 67 of them contain
feature(slice_patters)), it's hard to find specific examples.compile-fail/borrowckcontains tests for moving out elements in array patterns erroneously.match-vec-*/match-slice-*tests incompile-failtest various pattern properties like exhaustiveness and number of elements.#47926 added tests for legally moving out elements from fixed-size arrays and dynamic drops.
irrefutable-slice-patterns.rscreates references with irrefutable slice patterns.rfcbot commentedon Mar 8, 2018
🔔 This is now entering its final comment period, as per the review above. 🔔
nikomatsakis commentedon Mar 9, 2018
@petrochenkov good enough, thanks
rfcbot commentedon Mar 18, 2018
The final comment period is now complete.
Auto merge of #48516 - petrochenkov:stabsl, r=nikomatsakis
Rollup merge of rust-lang#67712 - Centril:stabilize-slice_patterns, r…
Rollup merge of rust-lang#67712 - Centril:stabilize-slice_patterns, r…