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.

[nicoburns]

This seems like important functionality, but will require an MSRV bump to 1.83. We should consider how best to handle this (possibly a new minor version?)

[zachs18]

It could also be implemented as an opt-in cargo feature (that is documented in the README or elsewhere to have a higher MSRV than the "base" crate), e.g.

impl<T> ThinVec<T> {
    #[cfg(not(feature = "const_new")]
    pub fn new() -> ThinVec<T> {
        ThinVec::with_capacity(0) // existing impl
    }

    #[cfg(feature = "const_new")]
    pub const fn new() -> ThinVec<T> {
         // const impl using &EMPTY_HEADER
    }
}