enum discriminant should only be as big as needed

[thestinger]

I don't think there's any need to make these 64-bit, or even require that they be 32-bit. It's somewhat surprising how big stuff like Option<()> is at the moment. It's very painful for stuff like SmallIntMap and Deque that use ~[Option<T>], and for implementing set types as wrappers around map types.

For compatibility with C code, a #[discriminant = c_int] attribute can be offered to choose whatever primitive integer type is desired (C++11 has this feature too, as class enum foo: uint8_t)

[sanxiyn]

See #1647 for the past discussion.

[graydon]

Yeah this is a dupe. We have at least 4 bugs open on this.