Tiny-const-generics

[huitseeker]

This is an easy initial fragment of #186 aimed at breaking off one easy part out of that big PR, and get us started on the path to min_const_generics. Props to @jon-chuang for the actual work.

(#186 needs a bit of bespoke specialization to achieve performance parity, so this just aims for code simplification)

bench on master:
https://gist.github.com/413013073c2c81e8999e77c1cb37c5b8
bench on this PR:
https://gist.github.com/1f1cc7a70d02709889b9a0dfa180f2cd

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[daira]

Untested ACK

[ChenxingLi]

Have you planned to replace the macro bigint_impl! with the const-generics trait?

[huitseeker]

@ChenxingLi The work in that direction is ongoing in #186, I believe.

[Pratyush]

@ValarDragon are we okay with bumping the next release to require a minimum supported rust version of 1.51?

[ValarDragon]

Isn't rust 1.51 only a week old? imo, its overly prohibitive for a library to be backwards incompatible with versions that aren't even a month old. cref #215 #219.

Perhaps we should only depend on 1.51 after 6 weeks or so, when 1.52 comes out? And until then maintain a 1.51 branch?

[ChenxingLi]

@ChenxingLi The work in that direction is ongoing in #186, I believe.

Thanks! That's cool.

[ValarDragon]

I made a compiler_1.51 branch, lets switch this to point at that branch, and merge into there? (And then plan to merge that branch into master once the rust compiler hits 1.53)?

[weikengchen]

Dev, you mentioned 1.53 in the last reply, is it 1.51? https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html Or we just want to wait until 1.53 so it is more commonly used?

[ValarDragon]

The 1.51 branch was meant to indicate targeting that MSRV, and then we'd want to hold off on merging that into master until 1.53.

I think this is good to merge into the 1.51 branch now.

[weikengchen]

Sure, we can do it soon. I am looking at the code. Do you think the mentioning of 1.51+ in README.md and the version check necessary then?

[ValarDragon]

I think so, it will make the MSRV clearly documented, and if a compilation error occurs from someone using an insufficient version, it will provide a clear error message.