MaybeUninit::uninit().assume_init() is undefined behavior

[ExpHP]

pyo3/src/gil.rs

Lines 303 to 306 in 83d0ac4

	if next_idx == 0 {
	self.inner
	.push_back(unsafe { mem::MaybeUninit::uninit().assume_init() });
	}

This is explicitly noted in the MaybeUninit docs as undefined behavior (std::mem:;unintialized was deprecated for a reason!). The correct solution is to store [MaybeUninit<T>; BLOCK_SIZE] in the type, and to only call assume_init() on individual items that are known to be initialized.

(also unrelated: the current implementation of ArrayList<T> is not safe for T that have destructors due to its use of indexed assignment instead of ptr::write. It should have a T: Copy bound to reflect this)

[ExpHP]

To make matters even worse, you're using it for NonZero<*mut T>, a type which has invalid bit patterns!

(not that that makes a difference; mem::MaybeUninit::uninit().assume_init() is already undefined behavior for all types! This fact merely makes it... deadlier)

[kngwyu]

Good catch, thanks.

[kngwyu]

Closed via #537 for now