rust-lang/rustfmt

rustfmt eats attributes while formatting functions with variadic parameters

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#6561 opened on May 11, 2025

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C-buggood first issue

Description

If you format...

#[allow()]
unsafe extern "C" {
    #[allow()]
    pub fn foo(#[allow()] arg: *mut u8, #[allow()]...);
}

Then it will format into

#[allow()]
unsafe extern "C" {
    #[allow()]
    pub fn foo(#[allow()] arg: *mut u8, ...);
}

(The choice of attribute seems irrelevant, I've used allow because it compiles in any position).

Note that this position is explicitly documented at https://doc.rust-lang.org/nightly/reference/attributes.html#r-attributes.allowed-position

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic Rust knowledgeUnderstanding of rustfmt internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the formatting logic for variadic parameters in rustfmt, likely in the file `src/formatting/items.rs` or similar, where attributes are parsed and attached. The issue shows that attributes on the variadic parameter (`...`) are dropped. Check the code that handles function parameter attributes, particularly in the context of variadic arguments. The documentation at https://doc.rust lang.org/nightly/reference/attributes.html#r attributes.allowed position confirms that attributes are allowed on variadic parameters. Look for tests in the `tests/` directory that cover variadic functions to understand the expected behavior.