Function check_crates_cover_rust 

fn check_crates_cover_rust(
    crates: &BTreeMap<String, bool>,
    rust: &BTreeMap<String, String>,
) -> Result<(), Error>

Fails if a Rust symbol’s page sits in a crate the map does not classify.

The resolver reads the crate out of the first segment of a rustdoc path and looks it up in the two lists. A path naming a crate in neither would silently take the /api/rust/ branch and look right, so it is an error here instead.