Does anyone have a sense of how often matching against consts is used? While we now have good lints for the problem, the confusion between bindings and consts is well-documented. (It's even in SML criticisms from 1998, showing how must Rust is actually ML in C clothing 
)
I'd be really tempted to require const Bar => or const { 2 * N } => or similar for non-literal constants. Or {}, like in const generics. Or something.