Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) by fmease · Pull Request #162652 · rust-lang/rust

fmease · 2026-09-11T16:23:43Z

Follow-up to #162269. Addresses fmease/rasur#7 (item 6).

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

rustbot · 2026-09-11T16:23:48Z

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fmease · 2026-09-11T16:24:13Z

This is so niche that it does not need a crater run IMO and it might not even require an FCP.

fmease · 2026-09-11T16:43:11Z

Context for T-lang:

We generally only support parenthesizing trait bounds, not however any other kind of bound (namely, outlives-bounds and use-bounds) and as it stands there would be no use in generalizing this part of the grammar. So we allow bounds like (Trait), (for<'a> path::to::Trait<'a>) and (Fn() -> i32) but we don't allow ('a) or (use<N>)...

...Well, when precise capturing lists were introduced & stabilized, something was overlooked: While we correctly reject bare trait object types¹ like Trait + (use<T>) (where the use-bound doesn't come first) before this PR we would incorrectly syntactically accept bare trait object types like (use<T>) + Trait or (use<>)+ (where the use-bounds come first).

Note that all of this only concerns the grammar of Rust as use-bounds are semantically invalid in trait object types anyway. This is just part of my ongoing efforts to deep clean Rust's grammar + rustc's parser.

Arguably this is super niche and thus I actually think that merging this w/o a T-lang FCP would be fine but I wanted to at least inform you of this.

The Reference doesn't need updating as it already conforms to what I expect & propose here. See also TraitObjectType, Bound etc.

Which are syntactically legal in all editions by the way; they're merely semantically illegal in Rust >=2021. ↩

… bare trait object types (`(use<…>)+`)

tmandry · 2026-09-16T17:38:41Z

@rfcbot fcp merge lang

rust-rfcbot · 2026-09-16T17:38:45Z

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

No concerns currently listed.

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 · 2026-09-16T17:39:10Z

Thanks @fmease.

@rfcbot reviewed

rust-rfcbot · 2026-09-23T15:41:18Z

🔔 This is now entering its final comment period, as per the review above. 🔔

nikomatsakis · 2026-09-23T15:41:21Z

@rfcbot reviewed

adwinwhite

@bors r+

View changes since this review

rust-bors · 2026-10-09T01:23:59Z

📌 Commit fc6fa6d has been approved by adwinwhite

It is now in the queue for this repository.

…r=adwinwhite Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to rust-lang#162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

…uwer Rollup of 7 pull requests Successful merges: - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163956 (Pass the unremapped path to the `rustc` invocation for doctests) - #163634 (move overflow lint computation into decorator) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`)

Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to #162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

rust-bors · 2026-10-09T08:03:00Z

⌛ Testing commit fc6fa6d with merge 5458ca3...

Workflow: https://github.com/rust-lang/rust/actions/runs/37902581796

…r=adwinwhite Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to rust-lang#162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

JonathanBrouwer · 2026-10-09T08:05:51Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-10-09T08:05:54Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/37902581796

The next pull request likely to be tested is #164024.

…uwer Rollup of 11 pull requests Successful merges: - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163956 (Pass the unremapped path to the `rustc` invocation for doctests) - #163634 (move overflow lint computation into decorator) - #163666 (Updates the expect message library/core/src/time.rs) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`) - #164000 (When mentioning that closure doesn't implement trait, point at closure) - #164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - #164017 (cg_llvm: Avoid some explicit casts to `*const c_char`)

…r=adwinwhite Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to rust-lang#162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

…uwer Rollup of 14 pull requests Successful merges: - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163193 (cfi: mangle `f128` as `e` rather than `g` on platforms without `_Float128`) - #163634 (move overflow lint computation into decorator) - #163666 (Updates the expect message library/core/src/time.rs) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`) - #163912 (Fix debug assert failure in `note_obligation_cause_code_inner`) - #163972 (const-eval: ICE when we hit a non-const fn) - #164000 (When mentioning that closure doesn't implement trait, point at closure) - #164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - #164017 (cg_llvm: Avoid some explicit casts to `*const c_char`) - #164025 (Less `CanonicalVarValues`)

…r=adwinwhite Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to rust-lang#162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

…uwer Rollup of 14 pull requests Successful merges: - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163193 (cfi: mangle `f128` as `e` rather than `g` on platforms without `_Float128`) - #163634 (move overflow lint computation into decorator) - #163666 (Updates the expect message library/core/src/time.rs) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`) - #163912 (Fix debug assert failure in `note_obligation_cause_code_inner`) - #163950 (don't treat inherited opaques as defining) - #164000 (When mentioning that closure doesn't implement trait, point at closure) - #164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - #164017 (cg_llvm: Avoid some explicit casts to `*const c_char`) - #164025 (Less `CanonicalVarValues`)

…uwer Rollup of 24 pull requests Successful merges: - #161998 ( Support type-relative assoc item paths in generic param defaults & const param types) - #162106 (Helpful suggestions for incorrect address-of mutability (2)) - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163938 (-Zassumptions-on-binders: rewrite alias outlives constraints more goodly) - #163939 (Better debug impls for some assumptions on binders types) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163956 (Pass the unremapped path to the `rustc` invocation for doctests) - #164042 (Allow testing cg-gcc on any target) - #162443 (Do not retain `Normalization` goal errors in nested goals for `BestObligationVisitor:: non_trivial_candidates `) - #162908 (Fix - const parameters rejected when identical) - #163193 (cfi: mangle `f128` as `e` rather than `g` on platforms without `_Float128`) - #163634 (move overflow lint computation into decorator) - #163666 (Updates the expect message library/core/src/time.rs) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`) - #163912 (Fix debug assert failure in `note_obligation_cause_code_inner`) - #163950 (don't treat inherited opaques as defining) - #163972 (const-eval: ICE when we hit a non-const fn) - #164000 (When mentioning that closure doesn't implement trait, point at closure) - #164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - #164008 (properly ignore the current goal's usages) - #164017 (cg_llvm: Avoid some explicit casts to `*const c_char`) - #164025 (Less `CanonicalVarValues`)

Rollup merge of #162652 - fmease:reject-bare-obj-paren-use, r=adwinwhite Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`) Follow-up to #162269. Addresses fmease/rasur#7 (item 6). (No LLM was or will be used by me during the entire creation process of this PR)

fmease added I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Sep 11, 2026

rustbot assigned adwinwhite Sep 11, 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 Sep 11, 2026

fmease changed the title ~~Syntactically reject parenthesized precise capturing lists in bare trait object types ((use<…>)+)~~ Syntactically reject leading parenthesized precise capturing lists in bare trait object types ((use<…>)+) Sep 11, 2026

fmease force-pushed the reject-bare-obj-paren-use branch from 1c3974f to b41ef23 Compare September 11, 2026 16:25

fmease mentioned this pull request Sep 11, 2026

Potential issues with Rust's grammar I might upstream to r-l/r at some point fmease/rasur#7

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fmease force-pushed the reject-bare-obj-paren-use branch from b41ef23 to fc6fa6d Compare September 11, 2026 16:51

Syntactically reject leading parenthesized precise capturing lists in…

fc6fa6d

… bare trait object types (`(use<…>)+`)

fmease added waived-reference-pr This language change does not need a Reference PR. S-waiting-on-t-lang Status: Awaiting decision from T-lang labels Sep 11, 2026

fmease mentioned this pull request Sep 11, 2026

Reject leading grouped use-bounds in bare trait object types fmease/rasur#41

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traviscross added P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang T-lang Relevant to the language team labels Sep 16, 2026

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rust-rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Sep 23, 2026

traviscross removed I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Sep 23, 2026

rustbot mentioned this pull request Oct 3, 2026

Tracking issue for release notes of #162652: Syntactically reject leading parenthesized precise capturing lists in bare trait object types ((use<…>)+) #163704

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fmease removed the S-waiting-on-t-lang Status: Awaiting decision from T-lang label Oct 4, 2026

adwinwhite approved these changes Oct 9, 2026

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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 Oct 9, 2026

JonathanBrouwer mentioned this pull request Oct 9, 2026

Rollup of 7 pull requests #164015

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JonathanBrouwer mentioned this pull request Oct 9, 2026

Rollup of 11 pull requests #164024

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Rollup of 14 pull requests #164026

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Rollup of 14 pull requests #164027

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Rollup of 14 pull requests #164039

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Rollup of 24 pull requests #164052

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rust-bors Bot merged commit 79c674c into rust-lang:main Oct 9, 2026
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rustbot added this to the 1.101.0 milestone Oct 9, 2026

fmease deleted the reject-bare-obj-paren-use branch October 10, 2026 04:01

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