Preserve parentheses around `Fn` trait bounds in pretty printer by aytey · Pull Request #153483 · rust-lang/rust

aytey · 2026-03-06T10:09:27Z

The AST pretty printer was dropping parentheses around Fn trait bounds in dyn/impl types when additional + bounds were present. For example:

dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync

was pretty-printed as:

dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync

Without parens, + Send + Sync binds to the inner dyn ResourceLimiter instead of the outer type, producing invalid Rust.

The parser already tracks parentheses via PolyTraitRef.parens, but print_poly_trait_ref never checked this field. This adds popen() and pclose() calls when parens == Parens::Yes.

rustbot · 2026-03-06T10:09:32Z

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fmease

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tests/pretty/paren-trait-bound.rs

fmease · 2026-03-06T10:37:04Z

Thanks! Final request: Could you please squash the two commits into a single one?

The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example: dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync was pretty-printed as: dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust. The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`. Signed-off-by: Andrew V. Teylu <andrew.teylu@vector.com>

rustbot · 2026-03-06T10:39:01Z

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.

aytey · 2026-03-06T10:39:16Z

Thanks! Final request: Could you please squash the two commits into a single one?

Done!

fmease · 2026-03-06T10:47:24Z

Perfect! @bors r+ rollup

rust-bors · 2026-03-06T10:47:27Z

📌 Commit cb66c85 has been approved by fmease

It is now in the queue for this repository.

…fmease Preserve parentheses around `Fn` trait bounds in pretty printer The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example: dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync was pretty-printed as: dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust. The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`.

…uwer Rollup of 9 pull requests Successful merges: - #153466 (`rust-analyzer` subtree update) - #151280 (Fix incorrect trailing comma suggested in no_accessible_fields) - #152593 (Box in `ValTreeKind::Branch(Box<[I::Const]>)` changed to `List`) - #153174 (std: add wasm64 to sync::Once and thread_parking atomics cfg guards) - #153189 (refactor: move `check_align` to `parse_alignment`) - #153230 (Roll rustfmt reviewers for in-tree rustfmt) - #153445 (Consider try blocks as block-like for overflowed expr) - #153476 (bootstrap.py: fix typo "parallle") - #153483 (Preserve parentheses around `Fn` trait bounds in pretty printer)

…fmease Preserve parentheses around `Fn` trait bounds in pretty printer The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example: dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync was pretty-printed as: dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust. The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`.

…uwer Rollup of 14 pull requests Successful merges: - #153466 (`rust-analyzer` subtree update) - #151280 (Fix incorrect trailing comma suggested in no_accessible_fields) - #152593 (Box in `ValTreeKind::Branch(Box<[I::Const]>)` changed to `List`) - #153174 (std: add wasm64 to sync::Once and thread_parking atomics cfg guards) - #153485 (libcore float tests: replace macro shadowing by const-compatible macro) - #153495 (Fix ICE in `offset_of!` error recovery) - #152040 (Do not emit ConstEvaluatable goals if type-const) - #152741 (Suppress invalid suggestions in destructuring assignment) - #153189 (refactor: move `check_align` to `parse_alignment`) - #153230 (Roll rustfmt reviewers for in-tree rustfmt) - #153445 (Consider try blocks as block-like for overflowed expr) - #153452 (Cleanup unused diagnostic emission methods) - #153476 (bootstrap.py: fix typo "parallle") - #153483 (Preserve parentheses around `Fn` trait bounds in pretty printer)

Rollup merge of #153483 - aytey:dyn_paren_impl_fn_return, r=fmease Preserve parentheses around `Fn` trait bounds in pretty printer The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example: dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync was pretty-printed as: dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust. The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`.

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#153466 (`rust-analyzer` subtree update) - rust-lang/rust#151280 (Fix incorrect trailing comma suggested in no_accessible_fields) - rust-lang/rust#152593 (Box in `ValTreeKind::Branch(Box<[I::Const]>)` changed to `List`) - rust-lang/rust#153174 (std: add wasm64 to sync::Once and thread_parking atomics cfg guards) - rust-lang/rust#153485 (libcore float tests: replace macro shadowing by const-compatible macro) - rust-lang/rust#153495 (Fix ICE in `offset_of!` error recovery) - rust-lang/rust#152040 (Do not emit ConstEvaluatable goals if type-const) - rust-lang/rust#152741 (Suppress invalid suggestions in destructuring assignment) - rust-lang/rust#153189 (refactor: move `check_align` to `parse_alignment`) - rust-lang/rust#153230 (Roll rustfmt reviewers for in-tree rustfmt) - rust-lang/rust#153445 (Consider try blocks as block-like for overflowed expr) - rust-lang/rust#153452 (Cleanup unused diagnostic emission methods) - rust-lang/rust#153476 (bootstrap.py: fix typo "parallle") - rust-lang/rust#153483 (Preserve parentheses around `Fn` trait bounds in pretty printer)

rustbot assigned fmease Mar 6, 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 6, 2026

aytey changed the title ~~Preserve parentheses around Fn trait bounds in pretty printer~~ Preserve parentheses around Fn trait bounds in pretty printer Mar 6, 2026

fmease approved these changes Mar 6, 2026

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tests/pretty/paren-trait-bound.rs Outdated Show resolved Hide resolved

fmease 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-review Status: Awaiting review from the assignee but also interested parties. labels Mar 6, 2026

aytey force-pushed the dyn_paren_impl_fn_return branch from b73812e to cb66c85 Compare March 6, 2026 10:38

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 6, 2026

JonathanBrouwer mentioned this pull request Mar 6, 2026

Rollup of 9 pull requests #153490

Closed

This was referenced Mar 6, 2026

Rollup of 13 pull requests #153498

Closed

Rollup of 14 pull requests #153507

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rust-bors bot merged commit 03a8ae8 into rust-lang:main Mar 6, 2026
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rustbot added this to the 1.96.0 milestone Mar 6, 2026

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Preserve parentheses around `Fn` trait bounds in pretty printer#153483

Preserve parentheses around `Fn` trait bounds in pretty printer#153483
rust-bors[bot] merged 1 commit intorust-lang:mainfrom
aytey:dyn_paren_impl_fn_return

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