Add if len > CAP { unreachable_unchecked() }

[barakugav]

First of all, love the library! use it all the time 😄

This PR add a if len > CAP { unreachable_unchecked() } call in ArrayVec::len().
We know this assumption, and telling the compiler about it let it generate better code and eliminate some bound checks.

It also include a small benchmark for ArrayVec::clone() that demonstrate the (massive) speed up.

# before
> cargo bench clone
running 2 tests
test clone_512 ... bench:         345 ns/iter (+/- 20) = 1484 MB/s
test clone_8   ... bench:           4 ns/iter (+/- 0) = 2000 MB/s

# after
> cargo bench clone
running 2 tests
test clone_512 ... bench:         205 ns/iter (+/- 15) = 2497 MB/s
test clone_8   ... bench:           2 ns/iter (+/- 0) = 4000 MB/s

Clone is my specific use case, but im sure other generated code is more optimized when the compiler knows this.

Thanks for the support in advance ❤️

[bluss]

Two big questions for the maintainer here: should we add a build script? Going from no build script to having one is a step back.

Second, reviewing the function: this function rustc_version must be removed. Use a maintained crate to do this - preferably autocfg - don't copy/code your own version check. The reason is long-term maintainability.

Suggested resolution: Wait until the MSRV allows this without trouble. Publish a new 0.8.x soon with updated MSRV (and a slowly sliding MSRV policy), use this without build.rs.

[barakugav]

i found a workaround that doesn't require build.rs, tell me what you think

[barakugav]

@bluss I replaced hint::assert_unchecked(len <= CAP) with if len > CAP { hint::unreachable_unchecked() }, which is stable in 1.51, and got the same speedup/asm. build.rs removed.

[barakugav]

oh, unreachable_unchecked is stable since 1.27 but as const fn only from 1.57... at least is lower than assert_unchecked'a 1.81 requirement.
This can wait until we bump msrv to 1.57, or i can revert to the build.rs solution. Either way, i think this optimization can have a large impact for all users, so please consider it 🙏

[bluss]

We're fixing clone in the other PR, so we need one more example for when this helps in this case. I thought "codegen tests" are a good way to look at this, so if it's possible to make one like for clone it would be good.

Alt solution, add .len_dynamic() which is non-const and use it in critical places in the crate. It would be internal, so not in the API, so only help internal methods.