Rust: False positive for unused variable names

[brawer]

Description of the false positive

CodeQL seems to produce false positives for Rust variables in format strings.

Code samples or links to source code

On 2026-08-13, the folllowing Rust snippet got flagged by CodeQL on alltheplaces/osm-diffs#660. CodeQL posted a notice, claiming Variable 'name' is not used. However, the variable does get used in this snippet, via a format! macro. We also check for unused variables with clippy, which does not flag an unused variable for this code. So, this looks like a false positive from CodeQL.

        let producers: Vec<_> = sources
            .into_iter()
            .map(|(name, reader)| {
                let tx = tx.clone();
                s.spawn(move || -> Result<()> {
                    for record in reader.iter()? {
                        let bytes = record?;
                        let fti = FeatureToIndex::decode(bytes.as_slice()).with_context(|| {
                            format!("failed to decode a FeatureToIndex record from {name}")
                        })?;
                        tx.send(fti)?;
                        progress_bar.inc(1);
                    }
                    Ok(())
                })
            })
            .collect();

URL to the alert on GitHub code scanning (optional)

https://github.com/alltheplaces/osm-diffs/security/code-scanning/30

[hvitved]

Hi.

Thanks for reporting; I agree that this is a false positive.

Looking at the extraction log for the PR, there is a warning that rust-analyzer (which we use for extraction) is unable to expand the format invocation, which is what causes this.

Once #21714 is merged, this should hopefully be fixed.