I think it's `copy`:

```D
import std.algorithm;

void main()
{
    {
        int[] s = [1, 2, 3, 4, 5];
        s[1..3].copy(s[0..2]);
        assert(s == [2, 3, 3, 4, 5]);
    }
    {
        int[] s = [1, 2, 3, 4, 5];
        s[0..2] = s[1..3].dup;
        assert(s == [2, 3, 3, 4, 5]);
    }
}
```

This should avoid `.dup`'s memory allocation.