False positive warning in unused_parens lint with dyn Trait returned from a closure #143653
Description
Code
I tried this code:
#![deny(unused_parens)] #![allow(warnings)] trait MyTrait {} fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {} fn main() {}I expected compile with no warnings, but instead, the current nightly yields:
error: unnecessary parentheses around type --> src/main.rs:5:43 | 5 | fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {} | ^ ^ | note: the lint level is defined here --> src/main.rs:1:9 | 1 | #![deny(unused_parens)] | ^^^^^^^^^^^^^ help: remove these parentheses | 5 - fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {} 5 + fn foo(_: Box<dyn FnMut(&mut u32) -> &mut dyn MyTrait + Send + Sync>) {} | error: aborting due to 1 previous error Version with regression
Bisecting via cargo-bisect-rustc yields:
searched nightlies: from nightly-2025-07-02 to nightly-2025-07-08
regressed nightly: nightly-2025-07-04
searched commit range: 6677875...da58c05
regressed commit: 25face9
bisected with cargo-bisect-rustc v0.6.10
Host triple: x86_64-unknown-linux-gnu
Reproduce with:
cargo bisect-rustcwhere my guess is that #142237 is the cause (cc @benschulz as the author)