6845: Don't HirDisplay unknown types when displaying for source r=Veykril a=Veykril Was wondering why the add missing impl assist didn't do anything here:  Turns out me forgetting to set the Index::Idx type in the trait causes RA to panic due to it trying to to create an unparsable type in the `make` module. Now we get this instead which imo is definitely better to have.  Co-authored-by: Lukas Wirth <lukastw97@gmail.com>