Transmutes through usize do not preserve provenance, use AtomicPtr instead

[saethlin]

It looks to me like this crate has copied a strategy from the standard library that I patched out of the standard library in rust-lang/rust#95621. Since Rust has Provenance this pattern causes this crate to execute UB.

I encountered this issue in a crater run for rust-lang/rust#121282.

[spearman]

Thanks for reporting. Made an attempt here to use AtomicPtr 1cdde6b
but tests are failing with:

tcache_thread_shutdown(): unaligned tcache chunk detected

[saethlin]

I suggest using Miri to work on code like this. Run these to install Miri:

rustup toolchain add nightly
rustup component add --toolchain=nightly miri

Then run your tests with Miri using cargo +nightly miri test.

When I do that on your branch, Miri reports a data race. I suspect that's the cause of the allocator corruption.