Parse and accept type equality constraints in where clauses

[jroesch]

Implement the missing type equality constraint specified in RFC 135.

Examples

fn sum<I: Iterator>(_: I) -> i32
where
    I::Item == i32 // or `I::Item = i32`
{
    // ...    
}

[kennytm]

Edit: Obsoleted, see the next few comments.

Now that associated type for common traits has been landed on nightly while this issue is still open, it becomes impossible to use an iterator of a concrete type. Before this is fixed, here is a workaround (but I think this could be simplified):

#![feature(associated_types)]

struct CustomStruct {
    this: int,
    that: int,
}

fn do_something(i: int) {
    println!("{}", i);
}

// Old code
#[cfg(target_os="none")]
fn foo_old<I>(mut iter: I) where I: Iterator<CustomStruct> {
    for obj in iter {
        do_something(obj.this + obj.that);
    }
}

// New code, but doesn't work due to #20041.
/*
fn foo_new<I>(mut iter: I) where I: Iterator, <I as Iterator>::Item = CustomStruct {
    for obj in iter {
        do_something(obj.this + obj.that);
    }
}
*/

// Workaround code, inspired by http://redd.it/2r2fbl
trait Is<Sized? A> { fn this(&self) -> &A; }
impl<Sized? A> Is<A> for A { fn this(&self) -> &A { self } }
fn workaround_20041<A, B: Is<A>>(a: &B) -> &A { a.this() }

fn foo_workaround<I>(mut iter: I) where I: Iterator, <I as Iterator>::Item: Is<CustomStruct> {
    for obj in iter {
        let obj = workaround_20041::<CustomStruct, _>(&obj);
        do_something(obj.this + obj.that);
    }
}

fn main() {
    foo_workaround(vec![CustomStruct { this: 11111, that: 22222 }].into_iter());
}

[japaric]

@kennytm You can use "associated type bindings":

fn foo<I>(it: I) where I: Iterator<Item=Foo> {}

[kennytm]

@japaric : Oh nice, thanks. Found this buried deeply in https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md#constraining-associated-types (hintupdate guidehint)

[jroesch]

@kennytm @steveklabnik would the one to talk to about the docs. It is probably a good idea to do that. Full equality constraints should be coming soon after 1.0.

[steveklabnik]

Yes, we don't have any associated type documentation, I plan on tackling that soon.