Commented function parameter causes wrapping which breaks function declaration

[illicitonion]

Summary

I tried to format this code:

pub async fn foo(
    // OriginalUri(original_uri): OriginalUri,
) -> Option<Option<Option<Option<String>>>> {
    None
}

Expected behavior

I expected the code to continue to successfully compile after rustfmt.

Actual behavior

It gets reformatted to:

pub async fn foo(// OriginalUri(original_uri): OriginalUri,)
 -> Option<Option<Option<Option<String>>>> {
    None
}

where the closing ) ends up being commented out.

This only happens if both the comment and the return type are sufficiently long/complex.

Configuration

$ rustfmt +nightly --edition 2024 src/lib.rs

Meta

rustfmt --version:

rustfmt 1.9.0-nightly (0c68443b0a 2026-03-10)

[jieyouxu]

Triage: doesn't need nightly, just need to specify edition.

$ rustfmt repro.rs --check --edition=2024

rustfmt 1.8.0-stable (4a4ef493e3 2026-03-02)

[ytmimi]

Very similar to #6520, but I don't think this is a straight duplicate since the function parameters are empty in this example.

[aznszn]

@rustbot claim