ything required to convert a portion of an IRI to a portion of
    a URI.

    RFC 3986 3.3:

       If a URI contains an authority component, then the path component
       must either be empty or begin with a slash ("/") character.  If a URI
       does not contain an authority component, then the path cannot begin
       with two slash characters ("//").  In addition, a URI reference
       (Section 4.1) may be a relative-path reference, in which case the
       first path segment cannot contain a colon (":") character.
    r