> Also, bytecode-based serde when?

Not in serde itself, but people have been experimenting with serde alternatives that are bytecode based. Nothing stable as far as I know, just experiments.

One early experiment is described in https://sdr-podcast.com/episodes/a-different-serde/ , which used a FORTH inspired stack based VM generated by derive macros at compile time (iirc).

Another experiment is https://lib.rs/crates/facet which is a more general derives to generate compile time introspection to generate metadata tables approach.