Migrate to core::hint::black_box()

[BD103]

Objective

Many of our benchmarks use criterion::black_box(), which is used to prevent the compiler from optimizing away computation that we're trying to time. This can be slow, though, because criterion::black_box() forces a point read each time it is called through ptr::road_volatile().

In Rust 1.66, the standard library introduced core::hint::black_box() (and std::hint::black_box()). This is an intended replacement for criterion's version that uses compiler intrinsics instead of volatile pointer reads, and thus has no runtime overhead. This increases benchmark accuracy, which is always nice 👍

Note that benchmarks may appear to improve in performance after this change, but that's just because we are eliminating the pointer read overhead.

Solution

• Deny criterion::black_box in clippy.toml.
• Fix all imports.

Testing

• cargo clippy -p benches --benches