Function impact 

pub fn impact<F: Fs>(
    db: &GraphDb<F>,
    files: &[String],
    modified: &BTreeSet<String>,
    opts: &ImpactOptions,
) -> ImpactReport

What changing files reaches, one report per file.

modified is the caller’s own set — usually the whole diff — and decides only the modified flag; a partner in it is still reported. Paths are sorted and deduplicated, so the answer does not depend on the order the caller listed them in.