Constraints for generics (generics details 3)

[josh11b]

This proposal describes where clauses that can add constraints on a type-of-type, for example define restrictions on its associated types. Example:

fn FindFirstPrime[T:! Container where .Element = i32]
    (c: T) -> Optional(i32) {
  // The elements of `c` have type `T.Element`, which is `i32`.
  ...
}

fn PrintContainer[T:! Container where .Element is Printable](c: T) {
  // The type of the elements of `c` is not known, but we do know
  // that type satisfies the `Printable` interface.
  ...
}

Some other constraints, such as Sized are defined as type-of-types directly, possibly parameterized.

[chandlerc]

I've not tried to do a complete or super detailed review, but I wanted to explicitly say I've gone through this at a high level and I'm really happy. The problems here are remarkably hard, and I feel like this is a really compelling set of solutions. I also think the analysis, explanation, comparisons, and alternatives are fantastic.

I've left a bunch of pretty minor comments as I went through. Many are really questions or ideas for the future, and completely not blocking. Others are optional or super minor. Don't wait on me for landing this. =D

[zygoloid]

I'm very happy with this.

@chandlerc, I think all the remaining unresolved comment threads are yours. I'm happy with you going ahead and approving once they're resolved.

[chandlerc]

Nice!