Function classify_baseline_file 

pub fn classify_baseline_file(
    json: &str,
    expected: BaselineKind,
) -> BaselineFileKind

Which command a saved baseline file belongs to, read as expected’s format.

kind decides whenever the file carries one, which is every baseline saved from this version onward. A file without one predates the member, so the keys it carries decide instead, exactly as declares_baseline_format documents.

Reads the raw file rather than a deserialized struct, because the one format with required fields (dead-code) has to answer this before its own parse error hides the answer.