`this method is equivalent to` section of `Vec::extract_if` seems incorrect

[your-diary]

Location

https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.extract_if

Summary

The document says Vec::extract_if is equivalent to

let mut i = range.start;
while i < min(vec.len(), range.end) {
    if some_predicate(&mut vec[i]) {
        let val = vec.remove(i);
        // your code here
    } else {
        i += 1;
    }
}

, but I believe this is incorrect as the condition (i.e. i < min(vec.len(), range.end)) in while is re-evaluated in each iteration.

When vec.remove() is called, vec.len() is changed but range.end does not change, so the implementation can remove elements outside the given range.

Probably this is a correct version:

let mut i = range.start;
let mut end_exclusive = range.end;
while i < min(vec.len(), end_exclusive) {
    if some_predicate(&mut vec[i]) {
        end_exclusive -= 1;
        let val = vec.remove(i);
        // your code here
    } else {
        i += 1;
    }
}

Testing

playground

use std::{cmp::min, ops::Range};

fn main() {
    let l = vec![0, 1, 2, 3, 4, 5];

    //`Vec::extract_if`
    {
        let mut l = l.clone();
        l.extract_if(0..1, |_| true).for_each(|_| ());
        println!("{:?}", l); //=> [1, 2, 3, 4, 5];
    }

    //"equivalent" version (incorrect)
    {
        fn f<T>(
            vec: &mut Vec<T>,
            range: Range<usize>,
            mut some_predicate: impl FnMut(&mut T) -> bool,
        ) {
            //the body is copy&paste-ed from the doc as it is
            let mut i = range.start;
            while i < min(vec.len(), range.end) {
                if some_predicate(&mut vec[i]) {
                    let val = vec.remove(i);
                    // your code here
                } else {
                    i += 1;
                }
            }
        }

        let mut l = l.clone();
        f(&mut l, 0..1, |_| true);
        println!("{:?}", l); //=> []
    }

    //"equivalent" version (correct?)
    {
        fn f<T>(
            vec: &mut Vec<T>,
            range: Range<usize>,
            mut some_predicate: impl FnMut(&mut T) -> bool,
        ) {
            let mut i = range.start;
            let mut end_exclusive = range.end;
            while i < min(vec.len(), end_exclusive) {
                if some_predicate(&mut vec[i]) {
                    end_exclusive -= 1;
                    let val = vec.remove(i);
                    // your code here
                } else {
                    i += 1;
                }
            }
        }

        let mut l = l.clone();
        f(&mut l, 0..1, |_| true);
        println!("{:?}", l); //=> [1, 2, 3, 4, 5];
    }
}