Issues porting function to trait with impl trait type alias

[choco]

Hello everyone,
I have a function that takes a generic stream and returns another stream, like for example a transformation on top of the previous one. So for example this works:

use futures::Stream;
use futures::stream;
use bytes::Bytes;
use futures::StreamExt;

fn transform(input: impl Stream<Item = Bytes> + Send + Sync + 'static) -> impl Stream<Item = Bytes> + Send + Sync + 'static {
    input.map(|el| Bytes::from(el))
}

#[tokio::main]
async fn main() {
    let mut transformed_stream = transform(
        stream::iter(vec![Bytes::from("hello")])
    );
    println!("{:?}", transformed_stream.next().await);
}

Rust Playground

but if I try to move it into a trait, using the type alias feature for impl trait, then it doesn't:

#![feature(type_alias_impl_trait)]
use futures::Stream;
use futures::stream;
use bytes::Bytes;
use futures::StreamExt;

trait CoolTrait {
    type OutputStream: Stream<Item = Bytes> + Send + Sync + 'static;
    
    fn transform(&self, input: impl Stream<Item = Bytes> + Send + Sync + 'static) -> Self::OutputStream;
}

struct CoolTraitImpl;

impl CoolTrait for CoolTraitImpl {
    type OutputStream = impl Stream<Item = Bytes>;
    
    fn transform(&self, input: impl Stream<Item = Bytes> + Send + Sync + 'static) -> Self::OutputStream {
        input.map(|el| Bytes::from(el))
    }
}

#[tokio::main]
async fn main() {
    let cool_trait_impl = CoolTraitImpl;
    let mut transformed_stream = cool_trait_impl.transform(
        stream::iter(vec![Bytes::from("hello")])
    );
    println!("{:?}", transformed_stream.next().await);
}

Rust Playground

and I get an error that I don't really understand. Is what I'm trying to do above possible? What am I missing?
Thanks for your help.

[jonh]

Missing GAT.

fn transform(&self, input: impl Stream<Item = Bytes> + Send + Sync + 'static) -> Self::OutputStream;
is like
fn transform<S>(&self, input: S) -> Self::OutputStream where S: Stream<Item = Bytes> + Send + Sync + 'static

Without GATs you move the 'S' to make the trait generic.

[choco]

Ah, that makes perfect sense. Thanks!