Auto trait leakage on associated types of impl Trait return value

[benediktsatalia]

When using

rustc 1.86.0-nightly (419b3e2d3 2025-01-15)

What I tried to do

Return an impl Trait with an associated type and rely on the leakage of auto traits like Send and Sync on the associated type.

Simplified example:

trait Inner {}

trait Outer {
    type I: Inner;
    
    fn get_inner(&self) -> Self::I;
}

impl Inner for () {}

impl Outer for () {
    type I = ();
    
    fn get_inner(&self) -> Self::I { 
        ()
    }
}

fn outer1() -> impl Outer {
    ()
}

fn outer2() -> impl Outer<I = impl Inner> {
    ()
}

fn requires_send(_: impl Send) {}

fn main() {
    
    // uncommenting the following does not compile!
    // requires_send(outer1().get_inner());
    
    // this does compile!
    requires_send(outer2().get_inner());
}

What happened

The version using outer2 compiles fine and does the leakage, while the version with outer1 does not leak and therefore the associated type does not implement Send.

What I expected

I would have expected that both versions behave the same with regards to auto trait leakage. Maybe this is intended behavior, but it was surprising to me and I also couldn't find any documentation about this behavior.

PS: I first posted this on the user forum: https://users.rust-lang.org/t/impl-trait-return-value-with-associated-type-that-should-be-send-sync/124089 where it got suggested that I post an issue here.