Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery by fmease · Pull Request #162008 · rust-lang/rust

fmease · 2026-08-30T11:36:38Z

Follow-up to #156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary.

Re. box expression removal, see #162008 (comment) (TL;DR: it's been 3 years).

cc @cyrgani

_{(No LLM was or will be used by me during the entire creation process of this PR)}

rustbot · 2026-08-30T11:36:44Z

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cyrgani · 2026-08-30T12:13:41Z

Maybe it's also time to just remove the suggestion for box_syntax? The removal PR (#108471) was merged in 1.70, three years ago, so everyone should have had plenty of time to move away from it.

fmease · 2026-08-30T12:32:27Z

        || [
            kw::Async,
            kw::Do,
-            kw::Box,


Modifying can_begin* usually affects stable(!) macro rules matching behavior. In this case, it leads to code like the following to go from fail to pass:

macro_rules! mk { ($e:expr) => { compile_error!("expr"); }; (box $e:expr) => {}; } mk!(box 0);

Strictly speaking that's a new guarantee (in the sense that re-adding box exprs would be breaking). It's unclear if lang FCP #108471 (comment) covers this. If you don't want to involve T-lang at this time, we could also keep kw::Box in can_begin_expr for now and add a FIXME to revisit it in the future.

View changes since the review

I would prefer us to split this out of the suggestion / expression recovery remove, and nominate lang for it, this PR should just add a FIXME.

However, let's document that changing this function can change stable macro rules behavior too.

fee1-dead

LGTM, r=me

View changes since this review

fee1-dead · 2026-08-30T20:36:24Z

        || [
            kw::Async,
            kw::Do,
-            kw::Box,


I would prefer us to split this out of the suggestion / expression recovery remove, and nominate lang for it, this PR should just add a FIXME.

However, let's document that changing this function can change stable macro rules behavior too.

rustbot · 2026-09-02T09:19:40Z

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.

fmease · 2026-09-02T09:21:01Z

Applied suggestions found in #162008 (comment).

LGTM, r=me

@bors r=fee1-dead rollup

rust-bors · 2026-09-02T09:21:04Z

📌 Commit 2f8aad1 has been approved by fee1-dead

It is now in the queue for this repository.

rust-bors · 2026-09-02T09:24:18Z

⚠️ A new commit 7a71a7440f6244d302ae4ffb161724b34f5ba75b was pushed.

This pull request was unapproved.

fmease · 2026-09-02T09:25:02Z

Oops, forgot to git add something. Added.

@bors r=fee1-dead

rust-bors · 2026-09-02T09:25:05Z

📌 Commit 7a71a74 has been approved by fee1-dead

It is now in the queue for this repository.

…ng behavior

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

…uwer Rollup of 23 pull requests Successful merges: - #162164 (Revert "Implement Debug for C-like enums with a concatenated string") - #160564 (volatile: allow accesses to non-AM memory to trap) - #161579 (suggest calling a fn item used as the iterator of a `for` loop) - #162044 (coverage: Resolve spans to file-coordinates in a separate step) - #162120 (Introduce `PerOwnerLoweringState`) - #162151 (Test itanium mangling of `f16` and `f128`) - #162162 (Don't special-case `!` in stability checks anymore) - #162181 (Remove wrong UnusedBraces lint for iterator loop in edition 2024 ) - #162187 (Rename `thir::ExprKind::Use` to `ValueExpr`) - #158401 (mgca: Don't ICE when evaluating ValTrees that contain error constants) - #159873 (fuchsia: Add safestack as a supported sanitizer for x86_64 fuchsia) - #161847 (Preserve visibility in nested macro import suggestions) - #161972 (Improve tests for `#[track_caller]` in async) - #162008 (Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery) - #162065 (std: don't reference `libc::O_NOFOLLOW` on VxWorks in `set_perm_nofollow`) - #162076 (docs(num): clarify conditions under which error occurs in `impl TryFrom<int> for int`) - #162152 (Revert "retrieve supported GCC targets from the sysroot") - #162153 (Prefer `LLVMGetVersion` for runtime info) - #162168 (fix ICE in project_goals/inherent) - #162171 (Explain LoongArch f16 NaN-boxing in inline asm) - #162173 (fix supposedly unreachable `bug!` being reachable) - #162191 (core: mark float `ClampBounds` methods as `#[inline]`) - #162199 (docs(time): clarify exact seconds for hour and minute)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

Rollup of 23 pull requests Successful merges: - #157808 (sanitizers: Implement support for the sanitize ignorelist) - #160564 (volatile: allow accesses to non-AM memory to trap) - #161579 (suggest calling a fn item used as the iterator of a `for` loop) - #162044 (coverage: Resolve spans to file-coordinates in a separate step) - #162120 (Introduce `PerOwnerLoweringState`) - #162151 (Test itanium mangling of `f16` and `f128`) - #162162 (Don't special-case `!` in stability checks anymore) - #162181 (Remove wrong UnusedBraces lint for iterator loop in edition 2024 ) - #162187 (Rename `thir::ExprKind::Use` to `ValueExpr`) - #158401 (mgca: Don't ICE when evaluating ValTrees that contain error constants) - #159873 (fuchsia: Add safestack as a supported sanitizer for x86_64 fuchsia) - #161847 (Preserve visibility in nested macro import suggestions) - #161972 (Improve tests for `#[track_caller]` in async) - #162008 (Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery) - #162065 (std: don't reference `libc::O_NOFOLLOW` on VxWorks in `set_perm_nofollow`) - #162076 (docs(num): clarify conditions under which error occurs in `impl TryFrom<int> for int`) - #162152 (Revert "retrieve supported GCC targets from the sysroot") - #162153 (Prefer `LLVMGetVersion` for runtime info) - #162168 (fix ICE in project_goals/inherent) - #162171 (Explain LoongArch f16 NaN-boxing in inline asm) - #162173 (fix supposedly unreachable `bug!` being reachable) - #162191 (core: mark float `ClampBounds` methods as `#[inline]`) - #162199 (docs(time): clarify exact seconds for hour and minute)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

Rollup of 29 pull requests Successful merges: - #161694 (add `Complex` ABI run-make test) - #162014 (Move more `rustdoc-html` tests using `--test` into the right folder) - #162164 (Revert "Implement Debug for C-like enums with a concatenated string") - #160564 (volatile: allow accesses to non-AM memory to trap) - #161579 (suggest calling a fn item used as the iterator of a `for` loop) - #162044 (coverage: Resolve spans to file-coordinates in a separate step) - #162120 (Introduce `PerOwnerLoweringState`) - #162132 (std: improve safety documentation in UNIX stack overflow code) - #162151 (Test itanium mangling of `f16` and `f128`) - #162162 (Don't special-case `!` in stability checks anymore) - #162181 (Remove wrong UnusedBraces lint for iterator loop in edition 2024 ) - #162187 (Rename `thir::ExprKind::Use` to `ValueExpr`) - #158401 (mgca: Don't ICE when evaluating ValTrees that contain error constants) - #159873 (fuchsia: Add safestack as a supported sanitizer for x86_64 fuchsia) - #161847 (Preserve visibility in nested macro import suggestions) - #161951 (Windows: add fallback if `canonicalize` fails) - #161972 (Improve tests for `#[track_caller]` in async) - #162008 (Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery) - #162065 (std: don't reference `libc::O_NOFOLLOW` on VxWorks in `set_perm_nofollow`) - #162076 (docs(num): clarify conditions under which error occurs in `impl TryFrom<int> for int`) - #162152 (Revert "retrieve supported GCC targets from the sysroot") - #162153 (Prefer `LLVMGetVersion` for runtime info) - #162168 (fix ICE in project_goals/inherent) - #162171 (Explain LoongArch f16 NaN-boxing in inline asm) - #162173 (fix supposedly unreachable `bug!` being reachable) - #162180 (remove outdated next-solver FIXMEs) - #162191 (core: mark float `ClampBounds` methods as `#[inline]`) - #162195 (docs(time): clarify exact seconds for week and day) - #162199 (docs(time): clarify exact seconds for hour and minute)

@cyrgani

…fee1-dead Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery Follow-up to rust-lang#156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary. Re. `box` expression removal, see rust-lang#162008 (comment) (TL;DR: it's been 3 years). cc @cyrgani (No LLM was or will be used by me during the entire creation process of this PR)

rustbot assigned fee1-dead Aug 30, 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 Aug 30, 2026

fmease changed the title ~~Render the box pattern removal diagnostic more actionable~~ Render the box pattern removal diagnostic more actionable & fully remove box expression recovery Aug 30, 2026

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fmease force-pushed the actionable-box-pat-diag branch from 6933264 to e292f3c Compare August 30, 2026 13:29

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fmease force-pushed the actionable-box-pat-diag branch from e292f3c to 2f8aad1 Compare September 2, 2026 09:19

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fmease added 2 commits September 2, 2026 11:21

Render the box pattern removal diagnostic more actionable

7ee0fba

Remove parse recovery for box expressions

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fmease force-pushed the actionable-box-pat-diag branch from 2f8aad1 to 7a71a74 Compare September 2, 2026 09:24

rust-bors Bot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 2, 2026

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Add warnings to can_begin_* methods that affect stable macro matchi…

7a71a74

…ng behavior

fmease changed the title ~~Render the box pattern removal diagnostic more actionable & fully remove box expression recovery~~ Render the box pattern removal diagnostic more actionable & remove box expression recovery Sep 2, 2026

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Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery - #162008

Render the `box` pattern removal diagnostic more actionable & remove `box` expression recovery#162008
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