Function rust_tests_by_file 

pub fn rust_tests_by_file(
    files: &BTreeMap<String, Vec<(String, String)>>,
    ids: &[String],
) -> BTreeMap<String, Vec<String>>

Which file each test id belongs to: the longest binary id that prefixes it, then the longest module path of a file of that binary. files: path → (binary id, module path) as rust_reachable gives them.