I know I’ve read that part of the documentation before, but probably not for over five years. In cases where I’ve wanted something like that, I’ve tended to reach for macros, visual block editing, and s/\=. A few days ago I did a somewhat more complicated one that s/\U wouldn’t be sufficient for:

  :'<,'>s/.*/\="    pub const ".substitute(toupper(submatch(0)), "[^A-Z0-9]\+", "_", "g").": &'static str = \"".submatch(0)."\";"