Extend a list by transferring another list

items.extend(other) returns Result[(), AllocError]. It moves all elements from another list of the same type after reserving space:

def combined() -> Result[list[i64], AllocError]:
    mut items = list[i64]()?
    more = [10, 20, 30]?
    items.extend(more)?
    Ok(items)

def main() -> Result[(), AllocError | IoError]:
    print(combined()?)?
    Ok(())

[10, 20, 30]

The source is consumed even when reservation fails; its elements are then cleaned up and the destination stays unchanged. To retain a source, explicitly write items.extend(copy(source)?). References and general iterables are not accepted as sources yet. The destination needs mutable access. Existing capacity is reused, and moving elements does not clone or allocate their contents. The source literal also returns an allocation result; its ? propagates that failure before extend runs.