net/url: Why make struct Userinfo's username and password immutable?

[shiduanguang]

go/src/net/url/url.go

Line 374 in b41eee4

// The Userinfo type is an immutable encapsulation of username and

I need to hide username and password of an url when log it.
So, I use u, err := url.Parse(rawurl), and want to change u.User.
But url.Userinfo username and password is not exported.

[shiduanguang]

solution: https://github.com/golang/go/issues/24572