This type uses MaybeInit with the expectation that assignment code will set values in every cell. Moreover, the matrix is usually larger than requested in new(…) because it is padded to a power-of-two size. All in all that is error-prone and hard to debug (been there, #597).

Rewrite this with a safe and easy API.

One idea is to use Vec::with_capacity and extend. Another way could be based on iterators and use collect_vec.

Regarding padding, the type should take the responsibility to fill the padding rows. It is not actually necessary to allocate that padding space. The padding can be represented with a single value to repeat, and the padding is done at the last possible time in de_interleaving. It could look like a method set_padding_strategy(…), see the existing enum InstancePaddingStrategy.