no_std, no alloc serialization about capnproto-rust HOT 10 CLOSED

You can get the bytes by calling message.get_segments_for_output(), which will give you a single-segment OutputSegments.

Currently there is no way to use the packed encoding in no-alloc mode. I think we could add support for that and for plain serialize_packed::write_message() by putting the multi-segment support behind a #[cfg] attribute.

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Thanks for help.
Is adding support for packed encoding in no-alloc mode something I could help with?

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The big obstacle I see is on the reading side of things. Currently serialize_packed::read_message() allocates a vector on the heap:

It's unclear to me how we should make that work in the no-alloc case. I suppose we need to add a way to make that allocation configurable, but I don't see an obvious nice way to do it.

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Usually in the situations like that the user should decide of the size of the buffer used to serialize/deserialize data.
I used in the past this library for the c++/protobuf: https://embeddedproto.com/ and this is written in an elegant way. But using the same approach would require a lot of work here. And of course this library is only targeting embedded development.

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We could add a serialize::read_message_into_buffer_segments() function which is like serialize::read_message(), but additionally takes a &mut [u8] to use as the backing storage, and returns a message::Reader<BufferSegments<&[u8]>> (instead of a message::Reader<OwnedSegments>).

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I can try to play a little bit with it in the upcoming days if you want, but be aware I am not very familiar with the code and CapN proto library 😄

Off topic question:
I'm testing the library a little bit and I added AnyPointer and List(Uint16) to one struct. But when I call init_ method to initialize the field the struct itself is borrowed so I cannot make another call to initialize another field. Is this a limitation? Of is there another way?

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But when I call init_ method to initialize the field the struct itself is borrowed so I cannot make another call to initialize another field.

You cannot have a mutable reference to both child fields at the same time, but you can initialize them sequentially using the reborrow() method: https://dwrensha.github.io/capnproto-rust/2014/12/27/custom-mutable-references.html

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#451 adds no-alloc serialize_packed::write_message() and serialize_packed::read_message_no_alloc().

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Fixed in #451, and released in capnp-v0.18.5.

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Wow, nicely done 🎉 I will test it for sure

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