Add unique hashes to type descriptors

[brson]

For TLS and named service keys, making the keys types would be a very convenient way to make the interfaces type safe, e.g. let my_value: MyType = get_tls_value(). Unfortunately we don't have a way of comparing type descriptors right now, and type descriptors may be duplicated in each crate (I think). If type descriptors stored a unique hash, then we could compare types.

[brson]

@pcwalton and I were discussing another option for comparing types. If we can guarantee that every nominal type's type descriptor is represented by a distinct pointer (there are no duplicates across crates) then we can use traits to restrict type comparisons to only nominal types.

Maybe something like this:

trait TypeEq {
  static fn eq<A: TypeEq, B: TypeEq>() -> bool;
}

Then TlsKey could derive from TypeEq.

It is possible to compare structural types so we should probably consider that someday we'll inevitably want an interface for that too.

[bstrie]

I remember being excited about this idea, but it seems to have fallen off the radar. I'll nominate this for a milestone. Perhaps maturity 3?

[graydon]

I don't think comparing on types is ideal, since there might be multiple TLS keys of the same type. I wonder if we could just establish a special const expr or syntax extension that always evaluates to a new UUID or something?

[graydon]

filed as #7104, I think it's the right place for fixing TLS keys. Closing this one. Re-open if you disagree.