The documentation for the URL type states that

The Parse function sets both Path and RawPath in the URL it returns, and URL's String method uses RawPath if it is a valid encoding of Path, by calling the EscapedPath method. [emphasis added]

which makes it sound like it will always be set but this is not the case.

In the parse() function called by Parse() and ParseRequestURI(), a comment states that it sets Path

and, optionally, RawPath. [...] We don't want to set it if the default escaping of Path is equivalent, to help make sure that people don't rely on it in general. [emphasis added]

For example:
Parsing "/foo" sets Path as "/foo" but does not set RawPath.
Parsing "%2Ffoo" sets the same Path, "/foo", but also sets RawPath as "%2Ffoo".
(see: https://play.golang.org/p/8R2_iCLDdrS)

This is unexpected behavior based on the public package documentation, which should be updated.