Replace black_box with std::hint::black_box

[dsprenkels]

Rust version 1.66 now comes with a stabilized version of a black_box function that emits an "empty" assembly directive. This patch replaces the older optimization barrier, which was based on a volatile read with the newly standardized function.

Announcement from Mara:
https://hachyderm.io/@Mara/109518823276877083

Current implementation of black_box in rustc:
https://github.com/rust-lang/rust/blob/a803f313fdf8f6eb2d674d7dfb3694a2b437ee1e/compiler/rustc_codegen_llvm/src/intrinsic.rs#L341-L375

---

Review suggestions:

• This patch bumps the MSRV to the most recent version (1.66), which might be undesirable. One workaround is to wait for merging this until cfg_version is stabilized, and then write a fallback black_box based for rustc before 1.66.

[dsprenkels]

@tarcieri Thanks for the review! I added back the original implementation (based on the volatile read) and added the new barrier implementation under the core_hint_black_box feature flag.

I also updated the README file a bit to better represent how the barrier got updated over time

[tarcieri]

FYI: I don't actually have commit access to subtle 😦

I've asked about what to do with these PRs. Looks good to me now aside from bikeshedding the feature name.

[isislovecruft]

Mild preference for core-hint-black-box or core_hint_black_box since we've had at least three versions of the blackbox function so far, one of my assembly, one of my wat/wasm, and now the core::hint::pointer based one. It seems like a good reason to be explicit in the naming (albeit annoying verbose) about which "black box" users are opting into.

[isislovecruft]

@dsprenkels Thanks for updating the documentation as well!

The base branch was changed.

[tarcieri]

Heads up: the rustdoc for this function on the beta channel of rustc now contains a big scary warning saying black_box cannot be depended upon for cryptographic purposes:

https://doc.rust-lang.org/beta/core/hint/fn.black_box.html#when-is-this-useful

I think this should probably be reverted.

[dsprenkels]

I completely agree.

[dsprenkels]

The clarification docs were added in rust-lang/rust#108088. When I try to follow the rabbit hole I cannot seem to find where it was exactly specified first that black_box may not do anything useful. We should definitely revert the changes, but maybe consider keeping the feature flag to prevent a breaking change?