Implement Read/Write on Cursor over &mut Vec

[dbrodie]

It is currently impossible to use a std::io::Cursor over a Vec in a smart pointer (Mutex, RefCell, ...) in a sane way. Please implement Read/Write/... over Cursor<&'a mut Vec>.

Discussed on users.rust-lang.org.

[est31]

Good workaround is to call the as_mut_slice function on the mutable Vec reference when creating the Cursor. Then you'll have Cursor<&'a mut[..]> and Read/Write is implemented.

[nox]

AFAICT, adding such an impl is a breaking change:

use std::marker::PhantomData;

struct Cursor<T>(PhantomData<T>);

impl<T> Cursor<T> {
    fn new(_: T) -> Self {
        Cursor(PhantomData)
    }
}

trait Write {
    fn flush(&mut self) {}
}

impl<'a> Write for Cursor<&'a mut [u8]> {}
impl Write for Cursor<Vec<u8>> {}

// This code doesn't type check if this is uncommented:
// impl<'a> Write for Cursor<&'a mut Vec<u8>> {}

fn main() {
    let mut vec = vec![0u8];
    Cursor::new(vec.as_mut()).flush();
}

[sfackler]

New implementations are allowed to cause inference breakage.

[dtolnay]

Seems reasonable. I would be interested in seeing a PR that implements this.