[ironhaven]

That's right. Line 3 is undefined behaviour because you are creating mutable references to the uninit spare capacity of the vec. copy_to_slice only works with writing to initialized slices. The proper way for you example to mess with the uninitialized memory on a vec would be only use raw pointers or calling the newly added Vec::spare_capacity_mut function on the vec that returns a slice of MaybeUninit