Comment after associated type bound is incorrectly discarded
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A-associated-type-bounds
A-comments
C-bug
I-unexpectedly-removes-code
- Dominant language
- Rust
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- Forks
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- Avg merge
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- Merged PRs (30d)
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Description
Summary
Forked out of https://github.com/rust-lang/rustfmt/issues/6234.
I tried to format this code:
trait Foo {
type Bar: Baz1 // what
;
}
Expected behavior
I expected to see this happen:
- Comment is preserved
Actual behavior
Instead, this happened:
- Comment is removed
trait Foo {
type Bar: Baz1;
}
Configuration
rustfmt cli options used (if applicable):
$ rustfmt repro.rs --check
rustfmt configuration file (e.g. rustfmt.toml, if applicable): N/A
Reproduction Steps
Trivial.
Meta
rustfmt --version:
rustfmt 1.8.0-stable (01f6ddf758 2026-02-11)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the minimal Rust example in repro.rs and run rustfmt repro.rs --check using the reported configuration. Trace how rustfmt handles the associated type bound and its trailing comment. Done means formatting preserves // what while producing valid, checked output.
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
- 50/100