pub fn register_widget_tag_owned(tag: &str)Expand description
Register a tag whose name is only known at run time, so <tag ...>
markup is accepted.
The derive knows its tag at compile time and passes a &'static str;
a name read out of a file does not exist until the file is read. This
takes the name by reference, checks the set first, and leaks the string
only when the tag is new - a bounded, once-per-tag cost, because the
registry outlives every caller and the parser consults it for the life
of the process.