mem::uninitialized is nearly always UB, switch to MaybeUninitialized or something else

[programmerjake]

mem::uninitialized is deprecated because it's basically impossible to use correctly:

From mem::uninitialized's docs:

The reason for deprecation is that the function basically cannot be used
correctly: the Rust compiler assumes that values are properly initialized.
As a consequence, calling e.g. mem::uninitialized::<bool>() causes immediate
undefined behavior for returning a bool that is not definitely either true
or false. Worse, truly uninitialized memory like what gets returned here
is special in that the compiler knows that it does not have a fixed value.
This makes it undefined behavior to have uninitialized data in a variable even
if that variable has an integer type.
(Notice that the rules around uninitialized integers are not finalized yet, but
until they are, it is advisable to avoid them.)

[novacrazy]

I've been meaning to do this, and will try to get to it soon. Thanks for reminding me.

[novacrazy]

Ugh, I hadn't touched the repo in a few months and wasn't paying attention to which remote I was pushing to, so now the initial implementation with MaybeUninit is at 289ee56. Oh well.

@fizyk20 Do you have any feedback for this implementation?

A few things of note:

1. I messed with the private transmute implementation to use ManuallyDrop rather than mem::forget. With any opt level >0 it compiles to the same machine code, and is slightly more efficient with no optimizations.
2. Similar to 1, I rewrote the sequence traits using MaybeUnint and ManuallyDrop instead of mem::forget
3. Replaced N::to_usize() with N::USIZE

Technically these are not breaking changes, and do not affect the external API.

[Robbepop]

From what I can tell this has already been fixed and merged to master so we could technically close this, right?

[novacrazy]

I was waiting on @fizyk20 's response, but they don't seem active on this repo anymore. I'll see about double-checking the code and pushing a version update when I have time in the next few days.

[Robbepop]

When @fizyk20 is no longer active on the repo, how can we effectively update the crate on crates.io?

[novacrazy]

I have the permissions to do that, so no worries.

[Robbepop]

That's good to hear because it would be really bad for this amazing crate!

[novacrazy]

This should be fixed in the 0.14.0 release. Please reopen if issues persist.

[RalfJung]

Is there any chance of a 0.13.x (semver-compatible) release with the fix? This bug breaks e.g. https://crates.io/crates/async-local-bounded-channel, and with rust-lang/rust#71274 the silent UB will turn into a loud panic.