Add CTFE function for enum types to return the number of variants.

[tikue]

This would allow for something like this:

enum Foo {
    A,
    B,
}

static two: uint = Foo::len();
assert!(two == 2u);

[thestinger]

This isn't going to happen because we aren't going to add special-cased CTFE functions. Rust doesn't have CTFE beyond syntax extensions. The number of enum variants can be retrieved by via reflection at runtime.

[metajack]

I don't feel super strongly about this, but it has come up at least twice already in Servo. One case was in the profiler where we have a enum for the categories and want to define an array of the size of the number of categories. I think a similar case occurs in rust-mozjs where there are a bunch of object types and we must make sure to get the number of reserved slots right or else things will crash.

How does retrieving the variants at runtime work?

[thestinger]

You can implement the TyVisitor trait and walk the primitives making up a type. It will pass you the number of struct fields or enum variants and their names too.

You can look at what std::repr does for a reasonably complex example. It's how println!("{:?}", x) works.

Although, we could also add intrinsics for this if there's a wide need for it.