# XO0536: a collection copied out of a list element, changed, and stored back

Reported by `xo vet`, not `xo check`. `var x = xs[i]` shares the element with xs, so the first change to x (a push, an element write, a var argument) copies all of it, and `xs[i] = x` stores the copy back. In a loop that copies the element on every pass: the PE linker copied a whole section at each relocation that way and linked xo.exe in quadratic time (2026-10-20). Change the element in place: `xs[i].push(v)`, `xs[i][j] = v`, or pass `xs[i]` to a `var` parameter (decision 0169); the store back is then not needed. Taking the element first (`xs[i] = []` right after the read) also avoids the copy and is not reported. See core 5.1 and decision 0294.

## Example

```xo
fn fill(var bufs: List[List[Int]], k: Int, n: Int) {
  for i in 0..n {
    var b = bufs[k]
    b.push(i)
    bufs[k] = b
  }
}
```

## Fix

```xo
fn fill(var bufs: List[List[Int]], k: Int, n: Int) {
  for i in 0..n {
    bufs[k].push(i)
  }
}
```

Run `xo explain XO0536` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).