Equality constraints on associated constants #70256
Description
We currently do not support any way to constrain an associated constant as needing to be equal to some constant value. The natural way to achieve this would be to, by analogy with e.g., Foo<0>, also allow Foo<ID = 0>. The same disambiguation rules would apply as for Foo<X> in terms of when a block { ... } is needed, e.g., Foo<ID = { N + 1 }>.
In compiler internal terms, we would redefine ast::AssocTyConstraintKind as:
/// The kinds of an `AssocConstraint`.
#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
pub enum AssocConstraintKind {
/// E.g., `A = Bar` in `Foo<A = Bar>`.
EqualityTy { ty: P<Ty> },
/// E.g., `A = 0` in `Foo<A = 0>`.
EqualityConst { ct: AnonConst },
/// E.g. `A: TraitA + TraitB` in `Foo<A: TraitA + TraitB>`.
Bound { bounds: GenericBounds },
}