My experience is that it will only autocomplete traits that you have imported. Which sucks because what if I don’t know what traits my type uses? It’s crazy

IntelliJ will suggest unimported trait methods and will automatically import the required trait if it is selected.

For example, on std:String, the + operator is hidden away in the Add trait implementation. (In this case it's reasonably obvious anyway.)

Ah, interesting. I haven't really noticed that sort of thing myself.