Function join 

fn join(directory: &str, page: &str) -> String

A rustdoc page path, under a module directory or at a tree’s own root.

A re-exported crate has no enclosing directory — pub use atune_core; is atune_core/index.html, the root of that crate’s own tree — and the separator has to be dropped rather than emitted, because the leading / of an absolute-looking path would make the crate segment empty and check_crates_cover_rust unable to classify it.