Using this crate in no_std and embedded environments

[itamarst]

I am wrapping the library with Python, using PyO3. PyO3's macros for wrapping classes aren't happy with having a lifetime parameter, so having a CDDL attribute is difficult (I don't want to parse the schema from scratch each time I validate a document).

More broadly, having a lifetime identifier on the CDDL struct just makes using it annoying since it percolates to everything.

I am very much not an expert, but I suspect this could be fixed simply by having the parser own the lexer and the original schema string?

[anweiss]

This is due to the fact that one of my original goals for the project was to implement this in as close to a zero-copy way as possible given CBOR's (and presumably CDDL's) use in constrained computing environments. I assume you're compiling to the WASM target? If so, I can just created owned versions of these structs which are used when targeting WASM. Would that work?

[anweiss]

Hey @itamarst, I need to think about this a bit more. Creating owned versions of everything will add quite a bit of duplication and extra work, which I don't feel is worth it just to work around PyO3's limitations for using lifetimes.

[itamarst]

1. Given Nicer API for repeat validation #105, mostly I just care about CDDL not having a livetime.
2. I can't really think of any use case for non-owning CDDL, so I would just design it to always own the schema bytes, and then you don't have two variants, you just have one. However, I am not really familiar with no_std world much, so maybe it makes sense there?
3. If you really want to support both, maybe Cow would do the trick?

[itamarst]

Oh, Cow has a lifetime too. Bleh.

[anweiss]

Yea, gonna move this to a Discussion and gauge interest for now. It's unclear, even to me, what the embedded/no_std folks actually require from this crate.

[itamarst]

What if CDDL was:

struct CDDL<B: AsRef<[u8]>> {...}

I think this might support all the options, looking at https://doc.rust-lang.org/std/convert/trait.AsRef.html#implementors?

[itamarst]

Hm, no, would still need a lifetime in order to store the underlying structs...

[itamarst]

OK, I figured out the solution (without requiring changes on your side): https://docs.rs/ouroboros/latest/ouroboros/attr.self_referencing.html allowed me to store both the String I get from Python and the resulting CDDL on the same object.

[anweiss]

Good find @itamarst! Will keep this discussion open nonetheless to facilitate further dialogue about CDDL in no_std environments.