Is it possible to use `impl Trait` as a function’s return type in a trait definition?

by

As trentcl mentions, you cannot currently place impl Trait in the return position of a trait method.

From RFC 1522:

impl Trait may only be written within the return type of a freestanding or inherent-impl function, not in trait definitions or any non-return type position. They may also not appear in the return type of closure traits or function pointers, unless these are themselves part of a legal return type.

  • Eventually, we will want to allow the feature to be used within traits […]

For now, you must use a boxed trait object:

trait A {
    fn new() -> Box<dyn A>;
}

See also:

Nightly only

If you wish to use unstable nightly features, you can use existential types (RFC 2071):

// 1.40.0-nightly (2019-11-05 1423bec54cf2db283b61)
#![feature(type_alias_impl_trait)]

trait FromTheFuture {
    type Iter: Iterator<Item = u8>;

    fn example(&self) -> Self::Iter;
}

impl FromTheFuture for u8 {
    type Iter = impl Iterator<Item = u8>;

    fn example(&self) -> Self::Iter {
        std::iter::repeat(*self).take(*self as usize)
    }
}

fn main() {
    for v in 7.example() {
        println!("{}", v);
    }
}

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