E0009 — Array index out of bounds

An indexing operation asked for a position outside the array’s length.

---

What triggers it

Resilient arrays are bounds-checked on every index read and write. A negative index or one >= len triggers this error at runtime; no silent out-of-bounds reads, no undefined behaviour.

Minimal example

fn main() {
    let xs = [1, 2, 3];
    return xs[5];
}

Output:

scratch.rs:3:12: error[E0009]: array index 5 out of bounds for length 3
   return xs[5];
          ^^^^^

Fix

Guard the index. len(xs) returns the current length; pair it with a runtime check or, better, a requires i < len(xs) contract so the bound is lifted to compile time.

fn main() {
    let xs = [1, 2, 3];
    let i = 1;
    if i >= 0 && i < len(xs) {
        return xs[i];
    }
    return 0;
}

Source

Emitted from the interpreter’s IndexExpression arm, the VM (Op::LoadIndex / Op::StoreIndex in resilient/src/vm.rs — produces VmError::ArrayIndexOutOfBounds), and the JIT via the res_array_get / res_array_set shims in resilient/src/jit_backend.rs.