Doc landing page and the 404 page could have the library search box

http://doc.rust-lang.org/master/ and http://doc.rust-lang.org/master/doesntexist could hook into the javascript that runs the search on http://doc.rust-lang.org/master/std/ to all show a search box themselves.

(The 404 page could even be intelligently auto-filled based on the URL, as an additional enhancement.)
