Code shared from the Rust Playground

playground.rs

#![allow(unused)]
fn main() {
}

// modifies v in place to drain all items for which the filter is true
// returns a vector with all elements which have been drained
fn drain_filter<T, F>(v: &mut Vec<T>, f: F) -> Vec<T>
where
    F: Fn(&T) -> bool,
{
    let mut drained_elements = Vec::new();
    let mut idx = 0;
    let mut len = v.len();

    while idx < len {
        if f(v.get(idx).unwrap()) {
            drained_elements.push(v.remove(idx));
            len -= 1;
        } else {
            idx += 1;
        }
    }

    drained_elements
}

#[test]
fn drain_filter_empty() {
    // Arrange
    let mut vec: Vec<i32> = vec![];

    // Act
    let drained_vec = drain_filter(&mut vec, |&x| x > 10);
    
    // Assert
    assert_eq!(vec, vec![]);
    assert_eq!(drained_vec, vec![]);
}

#[test]
fn drain_filter_all() {
    // Arrange
    let mut vec: Vec<i32> = vec![11, 12, 13];

    // Act
    let drained_vec = drain_filter(&mut vec, |&x| x > 10);
    
    // Assert
    assert_eq!(vec, vec![]);
    assert_eq!(drained_vec, vec![11, 12, 13]);
}

#[test]
fn drain_filter_none() {
    // Arrange
    let mut vec: Vec<i32> = vec![11, 12, 13];

    // Act
    let drained_vec = drain_filter(&mut vec, |&x| x < 10);
    
    // Assert
    assert_eq!(vec, vec![11, 12, 13]);
    assert_eq!(drained_vec, vec![]);
}

#[test]
fn drain_filter_some() {
    // Arrange
    let mut vec: Vec<i32> = vec![11, 12, 13, 20, 21, 5, 2, 50];

    // Act
    let drained_vec = drain_filter(&mut vec, |&x| x < 15);
    
    // Assert
    assert_eq!(vec, vec![20, 21, 50]);
    assert_eq!(drained_vec, vec![11, 12, 13, 5, 2]);
}