Function process_doc 

pub fn process_doc(
    env: &mut BuildEnvironment,
    docname: &str,
    doctree: &Doctree,
    srcdir: &Path,
    parse_dependencies: &[String],
)

Record every file docname’s doctree pulls in, replacing whatever the previous read of that document left behind.

parse_dependencies are the parse-recorded srcdir-relative paths (include targets); the doctree walk contributes the image uris.

Mirrors Sphinx’s collector contract: the entry is dropped entirely when the document depends on nothing, so dependencies holds only documents that have dependencies (Sphinx’s defaultdict(set) behaves the same way after clear_doc).