It's not really like there's a budget for it, and "!" is already a type people have seen in the return position of extremely common macros (panic!, todo!, etc.)

It's not really like there's a budget for it, and "!" is already a type people have seen in the return position of extremely common macros (panic!, todo!, etc.), functions (std::process::exit, etc.), and expressions (return, break, continue). The question is more whether it's unambiguous (it is), and whether there's something else you would rather use it for (there isn't).