Function cargo::core::compiler::fingerprint::find_stale_file

fn find_stale_file<I>(
    mtime_cache: &mut HashMap<PathBuf, FileTime>,
    reference: &Path,
    paths: I
) -> Option<StaleItem>where
    I: IntoIterator,
    I::Item: AsRef<Path>,

The reference file is considered as “stale” if any file from paths has a newer mtime.