`std::io::Empty` that is never read from

[Centri3]

What it does

If Empty is constructed yet its implementations of Read/BufRead are never required, we should suggest Sink instead which conveys this purpose better.

We also need to make sure some access does not require a trait outside of std that Sink doesn't implement.

Advantage

• This restricts it from being read from in the future, if that's undesirable.

Drawbacks

• This restricts it from being read from in the future, if that's desirable.

Example

use std::io::{prelude::*, empty};

fn b<T: Read>(t: &T) {}

fn a() {
    let mut empty_unused = empty();
    empty_unused.write(&[0]);

    let mut empty_used1 = empty();
    empty_used1.write(&[0]);
    empty_used1.read(&mut []);
    
    let mut empty_used2 = empty();
    b(&empty_used2);
}

Could be written as:

use std::io::{prelude::*, empty, sink};

fn b<T: Read>(t: &T) {}

fn a() {
    let mut empty_unused = sink();
    empty_unused.write(&[0]);

    let mut empty_used1 = empty();
    empty_used1.write(&[0]);
    empty_used1.read(&mut []);
    
    let mut empty_used2 = empty();
    b(&empty_used2);
}

[Centri3]

Write for Empty was only added (merged) last month and is still in nightly: rust-lang/rust#98154, as such we might want to wait until it's in stable to possibly implement this (just so we know it's here to stay and don't do unnecessary work).

There was also a PR to soft-deprecate Sink after this functionality was added but it doesn't seem it will ever be: rust-lang/rust#114384 (comment)