Regression: proc-macro inner attribute via `cfg_attr` on out-of-line module fails with "unclosed delimiter"

[JurajSadel]

Code

I tried this code:

A proc-macro attribute used as an inner attribute on an out-of-line (file) module, gated behind cfg_attr, fails to parse when the cfg is active.

macros/src/lib.rs (proc-macro crate):

use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn identity(_attr: TokenStream, input: TokenStream) -> TokenStream {
    input
}

Cargo.toml:

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

[dependencies]
macros = { path = "macros" }

src/lib.rs:

#![feature(custom_inner_attributes, proc_macro_hygiene)]

pub mod demo;

src/demo.rs:

#![cfg_attr(enabled, macros::identity)]
//! docs

Build with the cfg active:

RUSTFLAGS='--cfg enabled' cargo +nightly build

I expected to see this happen: the crate compiles, exactly as it does on nightly-2026-07-19 (and as it still does when the cfg is inactive so the attribute is stripped).

Instead, this happened: the file fails to parse.

error: this file contains an unclosed delimiter
 --> src/demo.rs:1:22
  |
1 | #![cfg_attr(enabled, macros::identity)]
  |   -        -         ^
  |   |        |
  |   |        unclosed delimiter
  |   unclosed delimiter

error: could not compile `reproducer` (lib) due to 1 previous error

The failure is specific to the cfg_attr-wrapped proc-macro inner attribute on an out-of-line module. These all still work:

• direct #![macros::identity] (no cfg_attr)
• inline mod demo { #![cfg_attr(enabled, macros::identity)] /* … */ }
• outer #[cfg_attr(enabled, macros::identity)] pub mod demo;
• same file with the cfg inactive (attribute stripped)

Fails on both cargo build and cargo doc.

Real-world impact: broke esp-hal nightly CI, which uses #![cfg_attr(docsrs, procmacros::doc_replace)] on ~45 module files.

Version it worked on

It most recently worked on: nightly-2026-07-19 (rustc eff8269f7 2026-07-18)

Version with regression

rustc --version --verbose:

rustc 1.99.0-nightly (87e5904f5 2026-07-20)
binary: rustc
commit-hash: 87e5904f5eb6398af6b22eac2802c78934260c48
commit-date: 2026-07-20
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly

Backtrace

Backtrace

No backtrace — parse error, not an ICE.

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged +A-proc-macros +A-parser +F-custom_inner_attributes

[mejrs]

Possibly #158046?

cc @Dnreikronos @petrochenkov

[Dnreikronos]

Possibly #158046?

cc @Dnreikronos @petrochenkov

I will work it!
Ty!