Function rustdoc::config::parse_extern_html_roots

fn parse_extern_html_roots(
    matches: &Matches
) -> Result<BTreeMap<String, String>, &'static str>

Extracts --extern-html-root-url arguments from matches and returns a map of crate names to the given URLs. If an --extern-html-root-url argument was ill-formed, returns an error describing the issue.