Add `RangeEnum` trait

[orenbenkiki]

Following the last few comments on #4926.

Add a RangeEnum trait for simple Enums (which implement FromPrimitive) to allow quick access to the total number of values, and provide an iterator over the values.

[nikomatsakis]

One thought I had in an e-mail conversation with @martindemello was that it might be useful to make a trait for working with enums which allows conversion not from the discriminant of an enum but rather from its index in the list of variants (which is usually, but not always, equal to the discriminant). This would allow us to guarantee for example that we can compact use bits from 0...n-1 to represent the values of an enum, and it would make it easier to define things like count() (total number of variants) and so forth. This might or might not be RangeEnum.

[sfackler]

Java defines the ordinal() method on enums to return the index of a variant. I find it dangerous as code using it is extremely fragile to modifications of the enum. It may be less dangerous here if it has to be manually specified to be derived, as that can serve as documentation that you need to be careful when modifying the enum. People might miss it if it's buried inside of Eq, Decodable, Encodable, Ord, ... though.

[nikomatsakis]

I'd rather say the onus is on the caller of ordinal() to use it in a context where order doesn't matter (e.g., an EnumSet).

[martindemello]

I think Ordinal would be a better trait name than RangeEnum

[nikomatsakis]

On Fri, Nov 15, 2013 at 10:34:02PM -0800, Martin DeMello wrote:

I think Ordinal would be a better trait name than RangeEnum

+1

[emberian]

This is still relevant and useful. Should this be baked into the compiler somehow?

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#824