An example code for `type_alias_impl_trait` feature

io_future.rs

#![feature(type_alias_impl_trait)]

use std::error;
use std::io;

use futures_core::TryFuture;
use futures_util::future::{MapErr, TryFutureExt};

#[allow(type_alias_bounds)] // The bound seems not to be removable.
pub type IoFuture<F: TryFuture> = MapErr<F, impl Fn(<F as TryFuture>::Error) -> io::Error>;

pub fn io_future<F>(f: F) -> IoFuture<F>
where
    F: TryFuture,
    F::Error: Into<Box<dyn error::Error + Send + Sync>>,
{
    f.map_err(into_io_error)
}

fn into_io_error<E>(e: E) -> io::Error
where
    E: Into<Box<dyn error::Error + Send + Sync>>,
{
    io::Error::new(io::ErrorKind::Other, e)
}

#[cfg(test)]
mod tests {
    use std::future;
    use std::mem;

    use super::*;

    type F = future::Ready<Result<(), &'static str>>;
    type FnIoFuture<F> = MapErr<F, fn(<F as TryFuture>::Error) -> io::Error>;

    #[test]
    fn size() {
        assert!(mem::size_of::<IoFuture<F>>() < mem::size_of::<FnIoFuture<F>>());
        assert_eq!(mem::size_of::<IoFuture<F>>(), mem::size_of::<F>());
    }
}