Function test_files 

fn test_files(files: &BTreeMap<PathBuf, File>) -> BTreeSet<PathBuf>

The files counted as test code: test paths (see test_path: test, bench and example targets, tests.rs and *_tests.rs), #![cfg(test)] files, and every file that an out-of-line mod declared in test code loads, transitively. A declaration is test code when it or an enclosing inline module is #[cfg(test)], or when its file is test code.