Optimize Ref<_, T> methods when T: Sized (#2752)

[bmberger]

Benchmarks are surprisingly showing minimal impact...

With optimization:
test r#ref::tests::bench_deref_mut_sized ... bench: 0.23 ns/iter (+/- 0.03)
test r#ref::tests::bench_deref_sized ... bench: 0.44 ns/iter (+/- 0.04)
test r#ref::tests::bench_from_bytes_sized ... bench: 4.64 ns/iter (+/- 0.03)
test r#ref::tests::bench_into_mut_sized ... bench: 0.23 ns/iter (+/- 0.03)
test r#ref::tests::bench_into_ref_sized ... bench: 0.44 ns/iter (+/- 0.00)

Without optimization:
test r#ref::tests::bench_deref_mut_sized ... bench: 0.22 ns/iter (+/- 0.02)
test r#ref::tests::bench_deref_sized ... bench: 0.44 ns/iter (+/- 0.03)
test r#ref::tests::bench_from_bytes_sized ... bench: 4.66 ns/iter (+/- 0.31)
test r#ref::tests::bench_into_mut_sized ... bench: 0.23 ns/iter (+/- 0.03)
test r#ref::tests::bench_into_ref_sized ... bench: 0.44 ns/iter (+/- 0.01)

Closes #2752

[gemini-code-assist]

Summary of Changes

Hello @bmberger, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces targeted optimizations for Ref methods within the zerocopy crate when dealing with Sized types. The goal is to improve performance by allowing direct pointer manipulation, avoiding unnecessary intermediate conversions. While initial benchmarks show only minimal performance changes, the update refines the internal handling of Sized types and includes new benchmarks to monitor these specific operations.

Highlights

• Optimization for Sized Types: Implemented a specialized code path for Ref<_, T> methods (into_ref, into_mut, deref, deref_mut) when the generic type T is Sized.
• Direct Pointer Manipulation: This optimization bypasses intermediate ByteSlice conversions by directly using Ptr for Sized types, aiming for improved efficiency.
• New Benchmarks: Added dedicated benchmark tests to measure the performance of these optimized Ref methods specifically for Sized types.
• Test Feature Enablement: Enabled the test feature in src/lib.rs to support the new benchmarks.

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[gemini-code-assist]

Code Review

This pull request introduces an optimization for Sized types in several methods of Ref<_, T>. The logic seems sound, and the addition of benchmarks is a great way to validate the performance impact. My main feedback is regarding significant code duplication across four methods, which could be refactored to improve maintainability. I've provided a suggestion to use a macro to deduplicate this logic.

[joshlf]

We're probably about to make a bit of noise for you on this PR, but also hopefully make your life a bit easier, with #2860. Let's hold off working on this for a few days until #2860 lands?