Audit spin

[Shnatsel]

spin crate provides no_std spinlocks. It has 6000 downloads/day on crates.io and is a transitive dependency of lazy_static! in no_std mode.

[Shnatsel]

@64 has audited the RwLock implementation and discovered issues: zesterer/spin-rs#65
This is now a RustSec advisory: rustsec/advisory-db#132

They have also rewritten it based on Folly to fix those issues: zesterer/spin-rs#66
The new implementation has been reviewed by @xacrimon.

[64]

FWIW, I had a quick look over the other parts of spin and didn't see any glaring issues. If someone is going to take a proper look I'd recommend auditing Once because that's what lazy_static uses.

[Shnatsel]

Conversion of Once to MaybeUninit<T> is outstanding: zesterer/spin-rs#68

The rest of Once could still use a soundness audit.

[xacrimon]

I've looked through and tested Once as rigourusly as my knowledge allows and I have found no issues.

[Shnatsel]

Great! If the code isn't extensively commented already, it would be nice to add comments in the form of "This is sound because..."