[vgatherps]

Yes, this is the case that I ran into as well. You have to zero memory before reading and/or have some crazy combination of tracking what’s uninitialized capacity or initialized len, I think the rust stdlib write trait for &mut Vec got butchered over this concern.

It’s strictly more complicated and slower than the obvious thing to do and only exists to satisfy the abstract machine.

[Arnavion]

No. The correct way to write that code is to use .spare_capacity_mut() to get a &mut [MaybeUninit<T>], then write your Ts into that using .write_copy_of_slice(), then .set_len(). And that will not be any slower (though obviously more complicated) than the original incorrect code.

[vgatherps]

Oh this is very nice, I think it was stabilized since I wrote said code.

[nemothekid]

write_copy_of_slice doesn't look to be stable. I'll mess around with godbolt, but my hope that whatever incantation is used compiles down to a memcpy