Require T: NoUninit only for compare_exchange operation?

[hehaoqian]

Require T: NoUninit is too strict in many cases,
as many types from 3rdparty crates does not implement this trait,
significantly limits the usablility of ths crate.

I don't think get_mut, into_inner, load, store, swap needs T: NoUninit,
T: Copy should be just enough.

[hehaoqian]

For issues that fixed by v0.6 which introduces T: NoUninit

#36: compare_exchange issue
#35: Is this a real bug?
#23: Also compare_exhange issue

[hehaoqian]

get_mut, into_inner do not need T: Copy either

[hehaoqian]

Also, is the NoUninit from bytemuck is a proper choice?
T: NoUninit requires T: 'static,
but I don't think 'static is needed

I think compare_exchange operation for Atomic<&'a T> where 'a not 'static,
should be sound

[hehaoqian]

Still, this is not easy to use.
One option is to introduce unsafe version of compare_exchange,
where the user needs to ensure there is no internal padding.

The other option is to just fully remove the T: NoUninit,
and other require T: Copy for compare_exchange.

I don't think failure of compare_exchange,
is any of undefined bahavior defined in
https://doc.rust-lang.org/reference/behavior-considered-undefined.html

[Amanieu]

NoUninit is needed because we are transmuting values to integers before using them with atomics, and it's undefined behavior for there to be any uninitialized bytes in the resulting integer.

[hehaoqian]

At least, inner_ptr, get_mut , into_inner do not require T: NoUninit, because no conversion to int occurs

[hehaoqian]

I still don't think load, store, swap needs T: NoUninit,
T: Copy should be just enough.

[hehaoqian]

For compare_exchange,
An unsafe version of compare_exchange_unsafe could be introduced,
that does not require T: NoUninit

[hehaoqian]

Alternatively, remove all T: NoUninit requirement, but change Atomic::new() to unsafe function,
and add a safe version of Atomic::new() that requires T: NoUninit

[Amanieu]

We could remove the T: NoUninit for methods that don't involve atomic operations, but then the type won't be very useful. What exactly are you trying to achieve with this?