Unnecessary parentheses warnings with odd spans since 1.90

Code

# Cargo.toml

[package]
name = "repro"
version = "0.1.0"
edition = "2024"

[dependencies]

scryer-modular-bitfield = "0.11.4"

[lints.rust]
unused_parens = "deny"

// src/lib.rs

use scryer_modular_bitfield::bitfield;
use scryer_modular_bitfield::specifiers::*;

#[bitfield]
#[repr(u64)]
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub struct UntypedArenaPtr {
    #[allow(unused)]
    ptr: B61,
    m: bool,
    #[allow(unused)]
    padding: B2,
}

pub fn test() {
    dbg!(UntypedArenaPtr::new());
}

Current output

$ cargo +1.90 check
    Checking repro v0.1.0 (/home/bennet/git/tmp/repro)
error: unnecessary parentheses around type
  --> src/lib.rs:10:5
   |
10 |     #[allow(unused)]
   |     ^ help: remove these parentheses
   |
   = note: requested on the command line with `-D unused-parens`

error: unnecessary parentheses around type
  --> src/lib.rs:12:5
   |
12 |     m: bool,
   |     ^ help: remove these parentheses

error: unnecessary parentheses around type
  --> src/lib.rs:13:5
   |
13 |     #[allow(unused)]
   |     ^ help: remove these parentheses

warning: method `m` is never used
  --> src/lib.rs:12:5
   |
 7 | #[repr(u64)]
   | - method in this implementation
...
12 |     m: bool,
   |     ^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `repro` (lib) generated 1 warning
error: could not compile `repro` (lib) due to 3 previous errors; 1 warning emitted

Desired output

$ cargo +1.89 check
    Checking repro v0.1.0 (/home/bennet/git/tmp/repro)
warning: method `m` is never used
  --> src/lib.rs:12:5
   |
7  | #[repr(u64)]
   | - method in this implementation
...
12 |     m: bool,
   |     ^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `repro` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.60s

Rationale and extra context

It's rather of seeing a diagnostic for unnecessary parentheses that is pointing nowhere near parenthesis. Also as the offending code is suspected to be macro generated fixing it is non-actionable for the user and as such the diagnostic is confusing.

The involved types all use the #[bitfield] attribute macro, so I suspect it is generating code with unnecessary parenthesis and for some reason they weren't reported prior to 1.90 and for some reason starting with 1.90 they are now reported with odd spans.

Rust Version

$ rustc +1.90 --version --verbose
rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8

Anything else?

This was reported to the scryer-prolog project in mthom/scryer-prolog#3092

Reproduce the report with Cargo.toml and src/lib.rs using cargo +1.90 check, then compare the diagnostics with cargo +1.89 check. Investigate the unused_parens diagnostic handling for the #[bitfield] macro-generated types and their spans. Done means the compiler no longer reports misleading unnecessary-parentheses diagnostics at unrelated source locations.