# XO0510: use of a moved Buf, StrBuilder, or BytesBuilder

`Buf[T]`, `StrBuilder`, and `BytesBuilder` are mutable in place and move only. `freeze()`, assigning, passing to a function, sending on a channel, or capturing in a spawned task moves the buffer, and the old name may not be used again (until it is reassigned). Finish all writes before moving, or start a new buffer. See core 5.3.

## Example

```xo
fn f() -> List[Int] {
  var b = Buf[Int].with_capacity(8)
  b.push(1)
  let xs = b.freeze()
  b.push(2)
  xs
}
```

## Fix

```xo
fn f() -> List[Int] {
  var b = Buf[Int].with_capacity(8)
  b.push(1)
  b.push(2)
  b.freeze()
}
```

Run `xo explain XO0510` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).