Vec::drain is unsound with range_bounds_assert_len feature

[KamilaBorowska]

Manually implementing assert_len can cause unsoundness in Vec::drain. This only happens in nightly, as overriding assert_len is only possible in nightly releases.

The following program will segfault.

#![feature(range_bounds_assert_len)]

use std::cell::Cell;
use std::ops::{Bound, Range, RangeBounds};

struct EvilRange(Cell<bool>);

impl RangeBounds<usize> for EvilRange {
    fn start_bound(&self) -> Bound<&usize> {
        unimplemented!()
    }
    fn end_bound(&self) -> Bound<&usize> {
        unimplemented!()
    }
    fn assert_len(self, _len: usize) -> Range<usize> {
        0..42
    }
}

fn main() {
    vec![1, 2, 3].drain(EvilRange(Cell::new(false)));
}

Related issues: #76393, #81154.

[dylni]

Thanks for noticing this. I think the best fix is to define the method on Range instead. I'll update this PR with that change unless anyone has a better idea.

[RalfJung]

Good catch!