This was suggested by @alexcrichton way back in 2015:

mod std::io {
    fn copy<R: Read, W: Write>(r: R, w: W) -> io::Result<u64> { ... }
}

but it wasn't done as it regressed 3 (root) crates in a crater run. Still, ideally if the API breakage policy is ever revisited, it would be better to take Read and Write by-value, as a type impl'ing Read transmits this property to &mut references of it.

@rustbot label T-libs-api rust-2-breakage-wishlist