[oneshtein]

  fn f() {
    let mut v = vec![1, 2, 3, 4, 5];
    let (header, tail) = v.split_at_mut(1);
    b(&header[0], &mut tail[0]);
  }

[loeg]

split_at_mut is just unsafe code (and sibling comment mentioned it hours before you did). The borrow checker doesn't natively understand that.

[Cyph0n]

It is safe btw. The difference is that it returns two mutable references vs. one shared ref and one mutable ref. But as they noted, a mutable ref can always be “downgraded” into a shared ref.