Consider using `once_cell` instead of `lazy_static`

[lopopolo]

Hi @BurntSushi, I don't currently use the unicode feature but I am looking to do so. bstr would be the only crate in my lockfile that uses lazy_static. There has been a lot of movement in the ecosystem to port over to once_cell. I personally think the API is nicer and the maintenance status is in a better spot.

once_cell has the added benefit of being slated for inclusion into std, which means it has the potential to simply re-export those std types once they are stabilized.

once_cell's MSRV is in the 1.36 range, which is compatible with the 1.60 MSRV bstr now has.

rustc is also suggesting once_cell for statics:

• suggest once_cell::Lazy for non-const statics rust-lang/rust#100507

Here's some examples throughout the ecosystem:

• Replace lazy_static with once_cell wasm-bindgen/wasm-bindgen#2962
• Migrate from lazy_static crate to std::sync::LazyLock tikv/rust-prometheus#448
• Replace lazy_static with once_cell tokio-rs/tokio#3187
• Migrate from lazy_static to once_cell tokio-rs/tracing#2147
• Replace lazy_static with once_cell clap-rs/clap#3828
• use std::lazy::SyncLazy instead of lazy_static rust-lang/rustfmt#5251
• Switch lazy_static to once_cell zeromq/zmq.rs#169
• Replace lazy_static with once_cell rust-onig/rust-onig#169
• Replace lazy_static! with once_cell::sync::Lazy immunant/c2rust#617
• Replace lazy_static with once_cell rust-lang/crates.io#5044
• Replace lazy_static with once_cell uutils/coreutils#3704
• Migrate most of wasmtime from lazy_static to once_cell bytecodealliance/wasmtime#4368
• Migrate lazy_static to once_cell seanmonstar/reqwest#1565

[lopopolo]

Now that I've taken a peek at this it looks like this update would need to be made to ucd-generate.

[BurntSushi]

Right. The main reason why I haven't done this yet is that I just don't think it matters much. And with it coming to std, doing a migration to once_cell before std just seemed like churn. So I left well enough alone.

With that said, if someone wanted to submit patches to ucd-generate and then bstr, I'd do my best to get them merged.

I do expect that eventually, Real Soon Now, once regex-automata 0.3 is out, it will have its own built-in synchronization mechanism for initializing DFAs lazily (and in a way that works without std). So if you wait long enough, the lazy_static dependency should just disappear (and won't even be replaced by once_cell). But I don't know when that will be. Maybe within the calendar year?

[lopopolo]

Awesome. The changes to regex-automata sound exciting! I'm happy to wait this one out.