Document associated type equality constraint

[ehuss]

AFAIK, associated type equality constraints aren't documented at all (other than GenericArgsBinding in the grammar).

I'm not sure where exactly that would go or where to start. Seems related to trait bounds and the new section on constrained impls, though maybe it goes in Associated Types or maybe in the types chapter on impl trait and trait objects.

[Havvy]

Can you give example code for "associated type equality constraints"?

[ehuss]

The Item=u32 in:

fn f(iter: impl Iterator<Item=u32>) {}

Some questions that I think the docs should answer:

• How and when to use it and what does it mean?
• Where is it allowed to specify?
• Where is it required to specify?

For example, trait objects require the associated types to be specified, but impl trait does not.

[tlyu]

This syntax puzzled me for a while, too. I think there should be cross references with the (almost?) identical syntax used for default type parameters (#24).