The tracking issue for this feature is: #34511

The conservative_impl_trait feature allows a conservative form of abstract return types.

Abstract return types allow a function to hide a concrete return type behind a trait interface similar to trait objects, while still generating the same statically dispatched code as with concrete types.

#![feature(conservative_impl_trait)]

fn even_iter() -> impl Iterator<Item=u32> {
    (0..).map(|n| n * 2)
}

fn main() {
    let first_four_even_numbers = even_iter().take(4).collect::<Vec<_>>();
    assert_eq!(first_four_even_numbers, vec![0, 2, 4, 6]);
}

In today's Rust, you can write function signatures like:

fn consume_iter_static<I: Iterator<Item=u8>>(iter: I) { }

fn consume_iter_dynamic(iter: Box<Iterator<Item=u8>>) { }

In both cases, the function does not depend on the exact type of the argument. The type held is "abstract", and is assumed only to satisfy a trait bound.

• In the _static version using generics, each use of the function is specialized to a concrete, statically-known type, giving static dispatch, inline layout, and other performance wins.
• In the _dynamic version using trait objects, the concrete argument type is only known at runtime using a vtable.

On the other hand, while you can write:

fn produce_iter_dynamic() -> Box<Iterator<Item=u8>> { }

...but you cannot write something like:

fn produce_iter_static() -> Iterator<Item=u8> { }

That is, in today's Rust, abstract return types can only be written using trait objects, which can be a significant performance penalty. This RFC proposes "unboxed abstract types" as a way of achieving signatures like produce_iter_static. Like generics, unboxed abstract types guarantee static dispatch and inline data layout.