x509-cert: Creating x509 certificates without allocations

Hello,

I'm interested in using x509-cert in an environment where no heap allocation is possible. I'm still new to Rust but if I understand correctly, x509-cert is working well in a no_std setting but currently require an implementation for alloc (mostly to use Vec).

Do you think that having a "no_alloc" version of x509-cert is possible and desirable? The main drawback I see is that I think it will force the developper to set a maximum size for its x509 certifcate at compilation time.

If you think it is a good idea to provide a version of x509-cert which does not require alloc, do you have a prefered approach for that (for example, replacing the use of Vec with heapless::Vec or tinyvec::ArrayVec)? If needed, I can work on it myself but I'll be happy to follow any advice you can give me.

Start by reviewing x509-cert's current uses of alloc and Vec, then compare the proposed heapless::Vec and tinyvec::ArrayVec approaches. Define whether a no-alloc version is feasible and how certificate size limits would be configured; done requires an agreed design and implementation scope.