Make ThinVec::new constant

[schungx]

I am not sure if there are any technical blockers on this... can ThinVec::new be made const?

Having to reinitialize every instance causes a very slight regression when I moved from Vec to ThinVec. I'd much rather it copies from pre-generated bits based on const.

[schungx]

Any chance to do this?

[zachs18]

Since a single, shared static header is used for empty ThinVecs, having the current ThinVec::new be a const fn is only possible once Rust's const_refs_to_static feature becomes stable.

[sivadeilra]

It looks like const_refs_to_static is stable now (as of December 2024). Could this be done now? I realize it would require raising the minimum supported Rust version, unfortunately.

[schungx]

Currently ThinVec breaks many const functions if replacing Vec's because Vec::new is const. It is a hassle and a roadblock towards adoption of this crate.