support explicit discriminant numbers on tag variants

[graydon]

Currently tags always use 0 ... N, where N is the number of tag variants, as the discriminant values.

This makes them useless for modeling C enums, which they are the natural analogue of. We should support a form of variant-declarator that provides a discriminant value, as (say) an uint const expression or something.

Ideal sort of example:

tag color {
red = 0xff;
blue = 0xfe;
}

It'd also be nice to be able to cast back and forth against uint values, and similar "like in C" enum tricks. But one thing at a time.

[kevina]

I'll look into this. Expect something in the next couple of days.

[graydon]

see Issue #1397.

[nikomatsakis]

Should it be legal to have both associated data and an explicit discriminant value?

[nikomatsakis]

My opinion: why not? Also, the discriminant value ought to increase by 1 from the previous discriminant, just as in an enum. So:

tag t {
a = 1; // discriminant == 1
b = 10; // discriminant == 10
c(uint); // discriminant == 11
d(uint) = 15; // discriminant == 15
}

[kevina]

I'm not sure on the value of having both data and discriminant values, and even less sure on the syntax, for now I'll disallow it by not supporting the syntax for it.

Also, my patch does increase by 1 when not specified just like the C enum does as it is trivial to implement.

[brson]

@kevina - This is done?

[brson]

I'm told it's as done as it's going to get. Yay!