Support `set_boxed_logger` in no_std environments with alloc

[TymanWasTaken]

I am writing a logging backend targeting https://vexide.dev, a project to support rust on a VEX V5 brain (an armv7a-none-eabihf processor), which provides alloc support without the rust standard library.

The log library docs mention the ability to use a Box to make dealing with runtime configuration easier, but it only supports this when the std feature is enabled, meaning it cannot be used on this platform. Would it be possible to modify the library to use alloc instead, allowing use of boxed loggers on some no_std platforms with alloc?

[KodrAus]

Hi @TymanWasTaken 👋

We did have plans at some point to replace this manual synchronization code with OnceLock, which requires std, but if we're happy to punt on that then I don't think there's any reason we shouldn't be able to add an alloc feature and gate set_boxed_logger on it.

Do you have any thoughts on this @Thomasdezeeuw?

[Thomasdezeeuw]

Removing the synchronisation code out of the crate would be nice. But I haven't seen movement to move std::sync::Once/OnceLock to core as it depends on OS primitive from what I can tell. So I don't see a clear path forward at the moment.

So I don't think it should block supporting the alloc crate.