すでに可変参照されている変数を可変参照している。

fn main() {
    let mut value = 0;
    let mut mr1 = &mut value;
    let mut mr2 = &mut value;
    println!("{mr1}");
    println!("{mr2}");
}

error[E0499]: cannot borrow `value` as mutable more than once at a time
 --> src/main.rs:4:19
  |
3 |     let mut mr1 = &mut value;
  |                   ---------- first mutable borrow occurs here
4 |     let mut mr2 = &mut value;
  |                   ^^^^^^^^^^ second mutable borrow occurs here
5 |     println!("{mr1}");
  |               ----- first borrow later used here

過去のループで保存した可変参照と競合している。

fn main() {
    let mut value = 0;
    let mut refs = vec![];
    for _ in 0..2 {
        refs.push(&mut value);
    }
}

error[E0499]: cannot borrow `value` as mutable more than once at a time
 --> src\main.rs:5:19
  |
5 |         refs.push(&mut value);
  |         ----      ^^^^^^^^^^ `value` was mutably borrowed here in the previous iteration of the loop
  |         |
  |         first borrow used here, in later iteration of loop

詳しくは『スライスの分割と split_at_mut 系の関数』を参照。

fn main() {
    let mut obj = [0, 0];
    let a = &mut obj[0];
    let b = &mut obj[1];
    *a += 1;
    *b += 1;
    assert_eq!(obj[0], obj[1]);
}

error[E0499]: cannot borrow `obj[_]` as mutable more than once at a time
 --> src\main.rs:4:13
  |
3 |     let a = &mut obj[0];
  |             ----------- first mutable borrow occurs here
4 |     let b = &mut obj[1];
  |             ^^^^^^^^^^^ second mutable borrow occurs here
5 |     *a += 1;
  |     ------- first borrow later used here
  |
  = help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

2025 年現在の借用チェッカー NLL の偽陽性が原因になっている。

詳しくは『借用チェッカーの制限 - 偽陽性の例 2』を参照。