Function missing_unit_tests 

pub fn missing_unit_tests(
    root: impl AsRef<Path>,
    language: Language,
) -> Result<Vec<PathBuf>>

Walk root recursively and return every source file (for language) that has no colocated unit test, sorted for deterministic output.

A file that is itself a test is never treated as a subject; every other source file must have its colocated test sibling. Returns an error if the tree under root cannot be read.