Tracking issue for VecDeque::rotate_{left|right} (feature vecdeque_rotate)

[DutchGhost]

Currently if you want to rotate a VecDeque n elements to the left or right, you'd have to write something like this:

use std::collections::VecDeque;

fn main() {
    let mut v = (0..10).collect::<VecDeque<_>>();
    
    println!("{:?}", v); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    
    // rotate left by 2,
    for _ in 0..2 {
        if let Some(popped_front) = v.pop_front() {
            v.push_back(popped_front);
        }
    }
    
    println!("{:?}", v); // [2, 3, 4, 5, 6, 7, 8, 9, 0, 1]
    
    // rotate right by 4
    for _ in 0..4 {
        if let Some(popped_back) = v.pop_back() {
            v.push_front(popped_back);
        }
    }
    
    println!("{:?}", v); //[8, 9, 0, 1, 2, 3, 4, 5, 6, 7]
    
}

I wonder if it would be possible to add 2 functions:

• rotate_left(mid: usize)
• rotate_right(mid: usize)

You then only have to write:

use std::collections::VecDeque;

fn main() {
    let mut v = (0..10).collect::<VecDeque<_>>();
    
    println!("{:?}", v); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    
    // rotate left by 2,
    v.rotate_left(2)
    println!("{:?}", v); // [2, 3, 4, 5, 6, 7, 8, 9, 0, 1]
    
    // rotate right by 4
    v.rotate_right(4);
    
    println!("{:?}", v); //[8, 9, 0, 1, 2, 3, 4, 5, 6, 7]
    
}

This would be equal to slice's rotate_left and rotate_right.
It could probably be implemented on a lower level than with pop's and pushes for a little more efficiency.

[scottmcm]

Ooh, I like this -- VecDeque is a circular buffer internally, so it should be doable with just a single at most two ptr::copy. EDIT: PR #56842

[scottmcm]

PR is r+'d, so I turned this into a tracking issue.