Deserialize arrays: avoid leaking partially allocated memory when failing

[acerone85]

Closes #811

[Short description of the changes.]

• Minor changes to the logic for deserializing arrays, using a [MaybeUninit; N] instead of MaybeUninit<[T]; N>.
• Dropping initialized values when failing and the array has been partially initialized

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

[Add or remove entries as needed]

• Rust SDK
• Sway compiler
• Platform documentation (for out-of-organization contributors, the person merging the PR will do this)
• Someone else?

[acerone85]

This is ugly, but mem::transmute cannot be used for arrays with generic length.

See rust-lang/rust#61956
(The solution advocated there also mentions calling mem::forget(uninit), but I don't think in this case is necessary and apparently it causes a compilation error.

[Dentosal]

May I suggest this pattern instead:

fuel-vm/fuel-compression/src/impls.rs

Lines 128 to 154 in dfab064

	// which do not require initialization.
	let mut tmp: [MaybeUninit<T>; S] = unsafe { MaybeUninit::uninit().assume_init() };
	let mut i = 0;
	while i < c.len() {
	match T::decompress_with(&c[i], ctx).await {
	Ok(value) => {
	// SAFETY: MaybeUninit can be safely overwritten.
	tmp[i].write(value);
	}
	Err(e) => {
	// Drop the already initialized elements, so we don't leak the memory
	for initialized_item in tmp.iter_mut().take(i) {
	// Safety: First i elements have been initialized successfully.
	unsafe {
	initialized_item.assume_init_drop();
	}
	}
	return Err(e);
	}
	}
	i += 1;
	}
	// SAFETY: Every element is initialized.
	let result = tmp.map(|v| unsafe { v.assume_init() });
	Ok(result)

[acerone85]

Done in 741007f

[xgreenx]

Could you revert all changes related to ; please?:)

[acerone85]

This happens automatically when I run cargo +nightly fmt --all.
Is there anything I need to add to the rustfmt configuration?

[acerone85]

Done (it also caught a trailing semicolon in another file, I will keep that change). See 81ebf29