Write impl on Vec<u8> is easy to misuse

[mahkoh]

use std::io::{Write};

fn main() {
    let mut vec = b"a".to_vec();
    let _ = write!(&mut vec, "b");
    println!("{:?}", vec);
    // [97, 98]

    let _ = write!(vec, "b");
    println!("{:?}", vec);
    // [98, 98]
}

[sfackler]

For clarity, the second call to write! ends up using the Write impl on &mut [u8].

cc @erickt I had concerns about this kind of thing when these impls were being introduced but I thought I convinced myself that this kind of thing couldn't happen. Did something change in the way we designed these impls since then?

[sfackler]

Possible solutions are to either remove the impl for &mut [u8] or update write! to prevent it from invoking DerefMut. Seems to me like the second is the better approach.

[lilyball]

I believe the reason for the &mut *$dst was because it was considered a good thing to require callers to explicitly pass &mut references. I think we've actually gone back and forth on that a few times. Are we committed now to allowing callers to just say write!(someValue, "...")?