Compiler is not complaining about the return types, why?

[fringe fulcrum]

I have this code https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=6729c9584282aed7a1a89786a49dd27c which compiles and I don't know why because the return types are different.

On one side I have fn stream_string() -> impl Stream<Item = String>
But on the other side I have:

    async fn get_log(&self) -> Pin<Box<dyn Stream<Item = Self::Item>>> {
        Box::pin(stream_string())
    }

Which returns stream_string() but they return different things (one is impl and the other is dyn)?

Why is the compiler not complaining about this?

[quasi topaz]

Box<impl Trait> can be coerced to Box<dyn Trait> just like any other Box<T> can be coerced to Box<dyn Trait> as long as T: Trait

[fringe fulcrum]

where in the documentation can I read about that?

[quasi topaz]

?play

use std::fmt::Display;

fn foo<'a>(s: impl Display + 'a) -> Box<dyn Display + 'a> {
    Box::new(s)
}

[quasi topaz]

https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions I guess

[fringe fulcrum]

so impl has nothing to do here, the coertion is from Sized to dyn ?