rust-lang / rust-lang/rustfmt

rustfmt deletes comment beside macro identifier

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Dominant language
Rust
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Merged PRs (30d)
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Description

Rust allows placing comments inside macro names between the macro identifier and the exclamation/bang (playground link)

This comment is gobbled by rustfmt:

$ cat x.rs
fn main() {
    println/*test*/!("hi");
}

$ rustc x.rs && ./x
hi

$ rustfmt --check x.rs
Diff in /tmp/x.rs:1:
 fn main() {
-    println/*test*/!("hi");
+    println!("hi");
 }

$ rustc --version && rustfmt --version
rustc 1.85.0 (4d91de4e4 2025-02-17)
rustfmt 1.8.0-stable (4d91de4e48 2025-02-17)

Admittedly, this is a silly case and I don't expect anyone to actually be using this "feature" of Rust, but I thought I should document this low-priority issue that I found a little funny.

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Research direction

Reproduce the issue with the x.rs example and run rustfmt --check to confirm that the comment between println and ! is removed. Trace rustfmt's handling of comments around macro identifiers; done means formatting preserves the comment and the check reports no unwanted deletion.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
tooling
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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