remove `box_patterns` by cyrgani · Pull Request #156749 · rust-lang/rust

cyrgani · 2026-05-19T12:04:35Z

This PR removes the old feature box_patterns, tracked in #29641. As explained in #29641 (comment), this feature will not be stabilized since deref_patterns (#87121) is a better and more general version of this feature.

It was not possible to remove this feature until recently since the compiler itself still depended on this feature. This was changed with rust-lang/compiler-team#984.

This PR currently breaks crates that rely on box patterns using the incorrect pre-expansion feature gating (see #154045 for more details). Therefore, this needs a crater run.

Closes #29641.
Closes #105099.
Closes #156110.

rustbot · 2026-05-19T12:04:39Z

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

The Rustfmt subtree was changed

cc @rust-lang/rustfmt

The Clippy subtree was changed

cc @rust-lang/clippy

rustbot · 2026-05-19T12:04:43Z

r? @Kivooeo

rustbot has assigned @Kivooeo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 73 candidates
Random selection from 20 candidates

Kivooeo · 2026-05-19T23:25:26Z

Therefore, this needs a crater run

Which one? would check-only be sufficient for this?

tmandry · 2026-05-27T15:39:47Z

I don't expect the crater breakage to be significant; if it is, we should discuss again.

@rfcbot merge lang

rust-rfcbot · 2026-05-27T15:39:50Z

@tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

Concerns:

~~crater~~ resolved by remove box_patterns #156749 (comment)

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

traviscross · 2026-05-27T23:10:24Z

Looks good to me pending crater results; thanks @cyrgani.

@rfcbot reviewed
@rfcbot concern crater

Kivooeo · 2026-05-29T08:56:21Z

About to run crater. In my opinion check-only should be sufficient for this, do folks have other opinion?

fmease · 2026-05-29T10:27:44Z

About to run crater. In my opinion check-only should be sufficient for this, do folks have other opinion?

Yes, it's absolutely sufficient. This PR rejects previously valid code in the frontend of the compiler, it removes syntax. The only things that don't show up in check builds are monomorphization and codegen changes.

IINM one thing which you miss out on in check-only crater runs are potential doctest regressions but that's just something you have to accept if you want to have "faster" crater runs, otherwise craterbot check would never be an option for anyone. In any case, the beta crater runs will run in build-and-test mode and exercise doctests anyway, so we'd still catch such regressions eventually and still have a bit of time to either revert the change or to send (more) downstream patches. Anyway, it's not super likely that anybody is using box patterns in doctests (without also using them in the main crater), I'd say.

fmease · 2026-05-29T10:27:53Z

@bors try

remove `box_patterns`

rust-bors · 2026-05-29T12:37:35Z

☀️ Try build successful (CI)
Build commit: 1639375 (163937523761cc0def32f1ba9ed63fdf65ad3478, parent: dc375db7d8df0aa450e622c529147c95eee756f5)

Kivooeo · 2026-05-29T12:39:11Z

@craterbot run mode=check-only

rust-rfcbot · 2026-06-30T02:19:37Z

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

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

fmease · 2026-07-30T07:38:57Z

AFAICT this only needs a rebase and a formal "r=" blessing from @Kivooeo, then this can be shipped.

Seems like @cyrgani has been inactive lately, so we could rebase for them.

cyrgani · 2026-07-30T09:02:00Z

I rebased this, but right now this is still blocked on removing box pattern usage from clippy (rust-lang/rust-clippy#17008). Should I just include these changes in this PR as well?

fmease · 2026-07-30T10:28:01Z

I rebased this, but right now this is still blocked on removing box pattern usage from clippy (rust-lang/rust-clippy#17008). Should I just include these changes in this PR as well?

Ah, I missed that.

Yes, feel free to update the clippy subtree in this PR given that it's but a mechanical change that has to happen to make our CI pass -- the changes will get sync'ed anyway.

cyrgani · 2026-08-10T14:28:29Z

@rustbot ready

Kivooeo · 2026-08-11T12:08:27Z

#156749 (comment) is still unresolved. We should decide whether to tackle it in a follow-up or fix it in this PR

fmease · 2026-08-19T21:19:59Z

It would probably be beneficial to rename this file accordingly

View changes since the review

fmease · 2026-08-19T21:31:07Z

+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/removed-syntax-box-patterns.rs:3:19
+   |
+LL |     let _: char = x;
+   |                   ^ not found in this scope
+


_{(in reply to #156749 (comment))}

Instead of emitting a fatal parse error (what you are suggesting), we could parse box $pat as builtin # deref($pat) (PatKind::Deref) which would also naturally fix this subsequent name resolution error & basically any other pathological cases you could think of (for reference, the PR currently uses PatKind::Err).

fmease · 2026-08-19T21:35:33Z

    }

    /// Parses `box pat`
    fn parse_pat_box(&mut self) -> PResult<'a, PatKind> {


Should we add some sort of "// FIXME: Remove this entire function on YYYY-MM-DD!" (similarly for the struct field case)? Not sure

rustbot assigned Kivooeo May 19, 2026

cyrgani changed the title ~~remove box_patterrns~~ remove box_patterns May 19, 2026

This comment has been minimized.

Sign in to view

jakubadamw reviewed May 20, 2026

View reviewed changes

Comment thread tests/ui/match/issue-42679.rs

traviscross added the P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang label May 27, 2026

tmandry added the T-lang Relevant to the language team label May 27, 2026

rust-rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels May 27, 2026

traviscross added the I-lang-radar Items that are on lang's radar and will need eventual work or consideration. label May 27, 2026

This comment has been minimized.

Sign in to view

rust-bors Bot pushed a commit that referenced this pull request May 29, 2026

Auto merge of #156749 - cyrgani:unbox-full, r=<try>

1639375

remove `box_patterns`

Kivooeo added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jun 22, 2026

rustbot mentioned this pull request Jun 30, 2026

Tracking issue for release notes of #156749: remove box_patterns #158596

Open

fmease mentioned this pull request Jul 6, 2026

Tracking issue for migrating all remaining post-expansion feature gates to pre-expansion ones #154045

Open

17 tasks

traviscross removed the P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang label Jul 8, 2026

rust-cloud-vms Bot force-pushed the unbox-full branch from 89626b8 to 3105e80 Compare July 30, 2026 08:58

This comment has been minimized.

Sign in to view

rust-cloud-vms Bot force-pushed the unbox-full branch from 3105e80 to 09d89e2 Compare August 10, 2026 09:35

This comment has been minimized.

Sign in to view

cyrgani added 3 commits August 10, 2026 10:48

remove box_patterns feature

657b92e

remove box_patterns from remaining tests

116ac7e

clippy: replace box_patterns usage with deref_patterns

0ca8951

rust-cloud-vms Bot force-pushed the unbox-full branch from 09d89e2 to 0ca8951 Compare August 10, 2026 10:49

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 10, 2026

cyrgani mentioned this pull request Aug 13, 2026

replace box_patterns usage with deref_patterns rust-lang/rust-clippy#17008

Merged

fmease reviewed Aug 19, 2026

View reviewed changes

Uh oh!

Uh oh!

Conversation

cyrgani commented May 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented May 19, 2026

Uh oh!

rustbot commented May 19, 2026

Uh oh!

This comment has been minimized.

Kivooeo commented May 19, 2026

Uh oh!

Uh oh!

tmandry commented May 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rust-rfcbot commented May 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

traviscross commented May 27, 2026

Uh oh!

Kivooeo commented May 29, 2026

Uh oh!

fmease commented May 29, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

fmease commented May 29, 2026

Uh oh!

This comment has been minimized.

rust-bors Bot commented May 29, 2026

Uh oh!

Kivooeo commented May 29, 2026

Uh oh!

rust-rfcbot commented Jun 30, 2026

Uh oh!

fmease commented Jul 30, 2026

Uh oh!

cyrgani commented Jul 30, 2026

Uh oh!

This comment has been minimized.

fmease commented Jul 30, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

cyrgani commented Aug 10, 2026

Uh oh!

Kivooeo commented Aug 11, 2026

Uh oh!

fmease Aug 19, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

fmease Aug 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

fmease Aug 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

12 participants

cyrgani commented May 19, 2026 •

edited

Loading

tmandry commented May 27, 2026 •

edited

Loading

rust-rfcbot commented May 27, 2026 •

edited

Loading

fmease commented May 29, 2026 •

edited

Loading

fmease Aug 19, 2026 •

edited by rustbot

Loading

fmease Aug 19, 2026 •

edited

Loading

fmease Aug 19, 2026 •

edited

Loading