Naming impl trait in return types

Return position impl Trait introduces types that do not have proper names. If you find yourself frequently giving that type a name, your best bet is to introduce a type alias. However, in a pinch, it is possible to access those types by getting the type for the surrounding function and extracting its FnOnce::Output associated type. Given a function like...

#![allow(unused)]
fn main() {
fn make_iter() -> impl Iterator<Item = u32> {
    0 .. 100
}
}

...one could use the type make_iter::Output to access the inferred impl Iterator (which will be a Range<u32>, in this case).

Function types can generally be named via their path:

• For function items, simply the name of the function (fn_name)
• For inherent methods, the fully qualified syntax of Type::fn_name
• For trait methods defined in an impl, use the fully qualified syntax <Type as Trait>::fn_name (as if fn_name were an associated type)

In each case, the type for the function is a zero-sized type that implements the Fn, FnMut, and FnOnce traits. This type is considered to be defined in its surrounding module and it is also possible to use it in other contexts, such as to implement the Default trait:

#![allow(unused)]
fn main() {
fn foo() -> u32 { 22 }

impl Default for foo {
    fn default() -> Self {
        foo
    }
}
}