Lower level write abstractions

[davidlattimore]

I wanted to see if you had any thoughts on possible lower-level abstractions for writing? In Wild, I was recently abstracting over 64/32 bit. The reading code from object has traits that we can use to aid this abstraction, but for writing, I ended up defining our own. Do you think there would be a place in the object crate for low-level write traits similar in nature to the existing read traits? i.e. just for setting fields on various ELF structs, but generic over ELF class, rather than writing the whole file.

[philipc]

I recently added a low level ELF Encoder in case you hadn't seen that. It's a bit different from what you've done though: it writes whole structs at once instead of fields. I think most of wild could still be converted to use it. The main sticking points I see are:

• wild does a lot of mutations of the symbol struct, so that would need to be restructured to mutate a object::write::elf::Sym instead and then write the symbol struct at the end.
• Encoder branches on is_64 instead of using static dispatch. I expect this should branch predict well, or maybe even optimise out, but not sure. Maybe we could add an ELF class type parameter to Encoder to support both.

Alternatively, adding the write traits for field mutation is probably okay too, since this would support additional use cases such as patching existing files (and maybe Encoder could be changed to use them itself). I'm not sure if these would be new traits or new methods on the current read traits. The problem with extending the read traits is they are gated by the read feature, and they have a few additional methods that do parsing beyond just reading fields so those methods would need to remain gated. On the other hand the problem with adding new traits is it's yet another set of similarly named types.