Support no_std

[dtolnay]

We have been putting this off because nobody has needed it so far.

But in dtolnay/dtoa#8:

<pierre-rouanet> I'm currently trying to use serde_json on a microcontroller

[japaric]

FWIW, this morning I wrote a serde-json-core crate that provides JSON (de)serialization on no_std. It's certainly not on par with the real serde-json as I don't plan to add any stuff that requires dynamic memory allocation to it, or add support for floats. I also made some adjustments to avoid having the binary size of my firmware increase by several KBs. Anyway, you can check the documentation for details about what works and what doesn't work.

Also, I don't plan to send a PR to serde-json as I doubt all my changes are going to be welcomed, but anyone else is free to try.

cc @pierre-rouanet @perlindgren @MrBuddyCasino

[dtolnay]

Nice! I am happy to have @japaric own a separate no_std JSON library. The API is sufficiently different that I think both std and no_std users will be better served if we keep these as separate libraries. I have retagged this issue as a docs issue to drop a link to serde-json-core in our readme and rustdoc.

[dtolnay]

I added a link to serde-json-core in our readme and rustdoc, as well as https://serde.rs/no-std.html. Thanks!

[dtolnay]

Reopening because there continues not to be a no_std JSON crate that can support types from alloc (String, Vec). rust-embedded-community/serde-json-core#4
I would be willing to consider a PR that adds a std feature on by default, and exposes the same or a subset of the current API of serde_json by using the alloc crate in no_std mode.