fix: fix the capture behavior of `if let` in closures by Embers-of-the-Fire · Pull Request #154210 · rust-lang/rust

Embers-of-the-Fire · 2026-03-22T12:12:15Z

Closes #153982.
TL;DR This patch adds the missing capture behavior change for if let statements introduced in RFC 2229.

This patch converts

self.walk_local(init, pat, None, || self.borrow_expr(init, BorrowKind::Immutable))?;

into

self.walk_local(init, pat, None, || Ok(()))?;

so that if let now behaves like let.

rustbot · 2026-03-22T12:12:20Z

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meithecatte · 2026-03-22T12:25:49Z

Could you add a test case that's a bit more like #153982, so that there's also a field that doesn't get captured in the first place?

Kivooeo · 2026-03-22T12:30:22Z

cc @Nadrieril

meithecatte

So, this is technically a breaking change, much like #138961, right? It'd be nice to have a proof-of-concept test on which the borrow checker will start erroring, at which point we'd probably wanna do a crater run...

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Kivooeo · 2026-03-22T12:40:54Z

@bors try

…e, r=<try> fix: fix the capture behavior of `if let` in closures

rust-bors · 2026-03-22T14:53:26Z

☀️ Try build successful (CI)
Build commit: dec9417 (dec9417b8611e34e787a3e4c37686b5131f9e5c5, parent: 562dee4820c458d823175268e41601d4c060588a)

Kivooeo · 2026-03-22T15:24:01Z

I'm wondering which mode we should run crater in. Would cargo check be sufficient, or should we use build or build and test instead?

Embers-of-the-Fire · 2026-03-22T15:35:42Z

Would cargo check be sufficient

AFAIK check should be sufficient since this patch modifies something rather high in the compiler, and in theory, strong dependencies on the size of closures shouldn't be very common. The most likely result is that we may break the unit tests for some of the lower level libraries, but it’s unlikely that this will affect functionality.

meithecatte · 2026-03-22T15:37:39Z

Since this PR changes the drop order in some cases, it is technically conceivable that some code would have an observable change in behavior after this PR gets merged. It is also technically possible that a crate's test suite would notice this. I would however be extremely surprised if someone has written such code and made a test suite good enough to be able to detect this – previous experience with crater runs for similar adjustments confirms this.

If it was my call to make, I'd run a check run and assume that in the exceedingly unlikely case where someone does depend on this, it'll get caught by the full crater run for the beta.

But my call to make it is not.

theemathas · 2026-03-22T15:38:31Z

It can technically change run time behavior by changing drop order. I think that's kind of unlikely to be relied on though.

Kivooeo · 2026-03-22T16:06:09Z

Based on the discussion above, I'm going with just check.

@craterbot run mode=check-only

craterbot · 2026-03-22T16:06:47Z

👌 Experiment pr-154210 created and queued.
🤖 Automatically detected try build dec9417
⚠️ Try build based on commit 66777ad, but latest commit is 4d78fd1. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

jieyouxu · 2026-03-23T05:58:33Z

It can technically change run time behavior by changing drop order. I think that's kind of unlikely to be relied on though.

If this can change runtime behavior (ergo, a stable breaking change), then IMO this definitely justifies a full build-and-test not just check-only. I imagine lang would likewise want as much of the full picture as possible on ecosystem impact when making the accept/reject call.

Nadrieril · 2026-03-24T00:50:40Z

@craterbot cancel

craterbot · 2026-03-24T00:51:05Z

🗑️ Experiment pr-154210 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Nadrieril · 2026-03-24T00:51:52Z

@craterbot run mode=build-and-test

craterbot · 2026-03-24T00:52:15Z

👌 Experiment pr-154210 created and queued.
🤖 Automatically detected try build dec9417
⚠️ Try build based on commit 66777ad, but latest commit is 4d78fd1. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Signed-off-by: Embers-of-the-Fire <stellarishs@163.com>

rustbot · 2026-05-21T09:43:49Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Add focused Miri coverage for the if-let closure capture change, demonstrating UB at closure construction when partial pattern capture reborrows dangling references. Assisted-by: OpenAI:gpt-5.5 Signed-off-by: Embers-of-the-Fire <stellarishs@163.com>

RalfJung · 2026-05-24T15:53:15Z

Nice tests, thanks :)

Nadrieril · 2026-08-18T13:42:20Z

Apologies, this slipped under my radar! Crater got reviewed, FCP approved, tests added, so everything looks good.

@bors r+

rust-bors · 2026-08-18T13:42:24Z

📋 This PR cannot be approved because it currently has the following labels: needs-fcp, needs-crater.

Nadrieril · 2026-08-18T13:42:54Z

@bors r+

rust-bors · 2026-08-18T13:42:56Z

📌 Commit f491a6c has been approved by Nadrieril

It is now in the queue for this repository.

…full-capture, r=Nadrieril fix: fix the capture behavior of `if let` in closures Closes rust-lang#153982. **TL;DR** This patch adds the missing capture behavior change for `if let` statements introduced in RFC 2229. This patch converts ```rust self.walk_local(init, pat, None, || self.borrow_expr(init, BorrowKind::Immutable))?; ``` into ```rust self.walk_local(init, pat, None, || Ok(()))?; ``` so that `if let` now behaves like `let`.

…uwer Rollup of 8 pull requests Successful merges: - #154210 (fix: fix the capture behavior of `if let` in closures) - #156176 (Initial implementation of `FnPtr` trait) - #160767 (Unify E0117 foreign-trait label for ADT/primitive types with existing Slice/Array/Tuple handling) - #160489 (Adding diagnostic item markers for multiple fs functions and structs) - #161220 (bootstrap: Allow `./x fix --allow-dirty`) - #161291 (Rename `ProjectionPredicate` and `TraitPredicate`) - #161296 (Enable overflow checks in `rustc_thread_pool`) - #161304 (Rename test so it matches the issue)

…full-capture, r=Nadrieril fix: fix the capture behavior of `if let` in closures Closes rust-lang#153982. **TL;DR** This patch adds the missing capture behavior change for `if let` statements introduced in RFC 2229. This patch converts ```rust self.walk_local(init, pat, None, || self.borrow_expr(init, BorrowKind::Immutable))?; ``` into ```rust self.walk_local(init, pat, None, || Ok(()))?; ``` so that `if let` now behaves like `let`.

Rollup of 18 pull requests Successful merges: - #154210 (fix: fix the capture behavior of `if let` in closures) - #156176 (Initial implementation of `FnPtr` trait) - #160767 (Unify E0117 foreign-trait label for ADT/primitive types with existing Slice/Array/Tuple handling) - #161297 (std: use UNIX's `Instant` and `SystemTime` on Hermit) - #160489 (Adding diagnostic item markers for multiple fs functions and structs) - #160643 (Require windowed (and exception) for Xtensa ABI) - #161088 (suppress projection errors already covered by a trait error) - #161114 (Remove fields from TypeKind: Struct, Enum, Union and Tuple) - #161115 (Assorted allocator nitpicks) - #161220 (bootstrap: Allow `./x fix --allow-dirty`) - #161296 (Enable overflow checks in `rustc_thread_pool`) - #161298 (remove rustc_error_messages dependency) - #161304 (Rename test so it matches the issue) - #161309 (`allow(non_camel_case_types)` in `minicore.rs`) - #161315 (Relax codgen test variable regex) - #161318 (Doc: clarify how `Read::bytes` handling Interrupted errors) - #161321 (Update books) - #161335 ([compiletest] Use the correct rustc lib directory for query_rustc_output)

Rollup merge of #154210 - Embers-of-the-Fire:feat/if-let-no-full-capture, r=Nadrieril fix: fix the capture behavior of `if let` in closures Closes #153982. **TL;DR** This patch adds the missing capture behavior change for `if let` statements introduced in RFC 2229. This patch converts ```rust self.walk_local(init, pat, None, || self.borrow_expr(init, BorrowKind::Immutable))?; ``` into ```rust self.walk_local(init, pat, None, || Ok(()))?; ``` so that `if let` now behaves like `let`.

Rollup of 18 pull requests Successful merges: - rust-lang/rust#154210 (fix: fix the capture behavior of `if let` in closures) - rust-lang/rust#156176 (Initial implementation of `FnPtr` trait) - rust-lang/rust#160767 (Unify E0117 foreign-trait label for ADT/primitive types with existing Slice/Array/Tuple handling) - rust-lang/rust#161297 (std: use UNIX's `Instant` and `SystemTime` on Hermit) - rust-lang/rust#160489 (Adding diagnostic item markers for multiple fs functions and structs) - rust-lang/rust#160643 (Require windowed (and exception) for Xtensa ABI) - rust-lang/rust#161088 (suppress projection errors already covered by a trait error) - rust-lang/rust#161114 (Remove fields from TypeKind: Struct, Enum, Union and Tuple) - rust-lang/rust#161115 (Assorted allocator nitpicks) - rust-lang/rust#161220 (bootstrap: Allow `./x fix --allow-dirty`) - rust-lang/rust#161296 (Enable overflow checks in `rustc_thread_pool`) - rust-lang/rust#161298 (remove rustc_error_messages dependency) - rust-lang/rust#161304 (Rename test so it matches the issue) - rust-lang/rust#161309 (`allow(non_camel_case_types)` in `minicore.rs`) - rust-lang/rust#161315 (Relax codgen test variable regex) - rust-lang/rust#161318 (Doc: clarify how `Read::bytes` handling Interrupted errors) - rust-lang/rust#161321 (Update books) - rust-lang/rust#161335 ([compiletest] Use the correct rustc lib directory for query_rustc_output)

rustbot assigned mati865 Mar 22, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 22, 2026

theemathas added T-lang Relevant to the language team needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. needs-crater This change needs a crater run to check for possible breakage in the ecosystem. labels Mar 22, 2026

meithecatte reviewed Mar 22, 2026

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rust-bors Bot pushed a commit that referenced this pull request Mar 22, 2026

Auto merge of #154210 - Embers-of-the-Fire:feat/if-let-no-full-captur…

dec9417

…e, r=<try> fix: fix the capture behavior of `if let` in closures

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 22, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Mar 24, 2026

craterbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 24, 2026

Embers-of-the-Fire added 2 commits May 21, 2026 15:14

feat: remove callback param for walk_local method

2aadcee

test: add if let test for partial capture

1f81a55

Signed-off-by: Embers-of-the-Fire <stellarishs@163.com>

Embers-of-the-Fire force-pushed the feat/if-let-no-full-capture branch from 4da8000 to 12dd438 Compare May 21, 2026 09:43

RalfJung reviewed May 24, 2026

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Comment thread src/tools/miri/tests/fail/match/closures/if-let-partial-pattern.rs

Embers-of-the-Fire force-pushed the feat/if-let-no-full-capture branch from 12dd438 to f491a6c Compare May 24, 2026 15:38

Nadrieril removed needs-crater This change needs a crater run to check for possible breakage in the ecosystem. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. labels Aug 18, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 18, 2026

JonathanBrouwer mentioned this pull request Aug 18, 2026

Rollup of 8 pull requests #161311

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JonathanBrouwer mentioned this pull request Aug 18, 2026

Rollup of 13 pull requests #161322

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This was referenced Aug 19, 2026

Rollup of 19 pull requests #161337

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Rollup of 18 pull requests #161338

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rust-bors Bot merged commit b1b8729 into rust-lang:main Aug 19, 2026
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rustbot added this to the 1.100.0 milestone Aug 19, 2026

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