Generating strong ordinal values independent of discriminants

[fralalonde]

[Enhancement proposal]

Context: I'm developing an enum-indexed structure that is similar to a HashMap<Enum, V> but that is backed by an array of size Enum::COUNT (where COUNT is strum-generated EnumCount). Indexing currently uses the enum's discriminant (as usize) which is brittle because discriminants outside of the array's indexing range could inadvertently be assigned to values of the Enum, breaking hell loose.

While strum's enum_iter() could be used to work with ordinal values, it would incur O(n) performance, negating the speed advantage of array indexing. I could not find any other strum mechanism of help, but maybe I overlooked a feature.

To fix this, I am thinking of developing a macro generating an fn ordinal(&self) -> usize for an Enum where returned index is guaranteed between 0..Enum::COUNT-1. The reciprocal fn from_ordinal(usize) -> E is not required but could easily be generated too.

Is this mechanism something that you believe would be within strum's scope - i.e. should I make a PR out of it? Would the EnumIndexArray generation also be of interest?