Function rustfmt_format_diff::scan_diff

pub(crate) fn scan_diff<R>(
    from: R, 
    skip_prefix: u32, 
    file_filter: &str
) -> Result<(HashSet<String>, Vec<Range>), FormatDiffError> where
    R: Read, 

Scans a diff from from, and returns the set of files found, and the ranges in those files.