Function stamp 

pub fn stamp(root: &Path) -> String

A stamp over those files: size and modification time, in a fixed order.

Not a hash of the contents. Reading every byte of a build artefact on every entry into the repository would cost more than the work it avoids, and the question being asked is only “has anything changed since I last looked”, for which the filesystem’s own answer is enough. A file that is absent stamps as absent, so adding one is a change.