speedy VS postcard

I would look at speedy which is, as the name suggests, pretty fast. It encodes varints in a way that's different from LEB128, and is much quicker to both encode and decode. The format they use is at the top of this file -- you can do it just with a couple of shifts and masks, and decoding is similarly fast. Speedy also hard-branches in the code with a switch statement based on how many bytes you need, which in my tests is something like 2x faster than the alternative, despite it being a branch (it's just turns into a lookup table in assembly).

I've been enjoying postcard as a lightweight efficient binary serialization.

That said, postcard is not a self describing format (so everyone needs the same schema, no changes allowed), but I do have a tracking issue open for ways to handle this in the future.

For example, look at the Postcard serializer for inspiration.

Serde should be no_std, I have used it embedded with crates such as postcard. You can see how postcard disables default features of serde here : https://github.com/jamesmunns/postcard/blob/main/Cargo.toml

Another option that may work for some is SerDe on rust. You just write a struct with the fields you want #[derive(Serialize, Deserialize)] above it, and if codegens the functions to deserialize that struct from JSON and serialize it back. Example looks like this https://docs.rs/serde_json/latest/serde_json/#creating-json-by-serializing-data-structures (but you have to use serde-json-core if you don’t have an allocator). Can also easily reserialize to something small like postcard that’s meant for embedded storage.

https://github.com/jamesmunns/postcard

postcard seems like it would be particularly strong for the wasm use case as it produces small messages that are light in memory.