Tracking issue for bindings in sub-patterns after @s (feature(bindings_after_at)) · Issue #65490 · rust-lang/rust (original) (raw)

This is the tracking issue for #![feature(bindings_after_at)], which allows patterns of form binding @ pat to have bindings in pat, for example ref x @ Some(y).

Implementation history

• On 2019-12-24, Initial implementation of #![feature(bindings_after_at)] #66296, containing the initial implementation, landed. The PR was written by @Centril and reviewed by @pnkfelix and @matthewjasper.
• On 2020-02-10, Correctly parse mut a @ b #68992 landed. The PR was written by @matthewjasper and reviewed by @Centril. The PR fixed a parser recovery bug wherein mut x @ y would make y a mutable binding, manifesting as issues Wrong bindings_after_at warnings #67861 and bindings_after_at: outer mutability annotation affects inner bindings #67926.
• On 2020-02-19, parse: recover mut (x @ y) as (mut x @ mut y). #69236 landed. The PR was written by @Centril and reviewed by @estebank. The PR adjusted the parser changes in Correctly parse mut a @ b #68992 to retain good error messages on e.g. mut (x @ y) patterns, suggesting mut x @ mut y as opposed to mut x @ y.
• On 2020-03-08, test(pattern): add tests for combinations of pattern features #69690 landed. The PR was written by @thekuom and reviewed by @Centril and added run-pass tests for the combination of bindings-after-at, or-patterns, box-patterns, slice-patterns.
• On 2020-03-08, test(bindings_after_at): add dynamic drop tests for bindings_after_at #69810 landed. The PR was written by @thekuom and reviewed by @Centril and added dynamic-rop run-pass tests for bindings-after-at.
• On 2020-03-10, test(patterns): add patterns feature tests to borrowck test suite #69817, written by @thekuom and reviewed by @Centril, landed. The PR added borrow checker tests in combination with or-patterns & other pattern features.
• On 2020-03-26, borrowck: prefer "value" over "_" in diagnostics #70389, written by @Centril and reviewed by @mark-i-m, landed. The PR fixed some bugs in diagnostics where we would end up using _ as the name of the place when matching on a temporary. Now it says "value" in the diagnostic instead.

Old description

Pre-1.0 Rust used to support writing code like:

match x { y @ z => { ... } }

match a { b @ Some(c) => { ... } }

PR #16053 disabled the ability to write such code. More precisely, it disabled the ability to introduce any kind of binding in subpattern P in the context of any binding-pattern ident @ P

The reason we did this was because it was too hard to handle them soundly in the AST-borrowck.

However, now that we have migrated 100% to NLL and thus MIR-borrowck, we should be able to soundly re-enable this feature, without too much effort (I hope).

• Back when the feature was removed, we made an explicit note that the long term intention was to reenable it.
• And this is a useful coding pattern at times. (The whole reason I went looking for this issue, and then decided to file it after discovering it did not seem to exist, was because I wanted to write code that required this feature, namely a pattern like: ty::Ref(_, adt_ty @ ty::TyS { kind: ty::Adt(adt_def, _), .. }, _))

Note that any attempt to re-enable the feature should do a careful survey of the various cases that can arise (copying, moving, mixing ref and non-ref, match ergonomics, ...)

Also, any attempt to re-enable the feature should also attempt to re-add all the tests that were removed with PR #16053