Generate WIT file from Rust module

[sviezypan]

This PR introduces capability of a rust macro to generate WIT file from annotated rust module. Some examples are in examples project and readme.

[vigoo]

I think you could just check if the target WIT's contents are the same, and not write to the file if so. That way although the macro would always run, it would not change the file times and make the WIT files dirty.

[vigoo]

I'm doing this in zio-k8s/zio-aws code generators, I think :)

[vigoo]

And for publishing, you can just copy the cargo settings and CI from for example https://github.com/golemcloud/golem-wasm-ast - there is an org secret that applies to this repo too for crates.io

[sviezypan]

I think you could just check if the target WIT's contents are the same, and not write to the file if so. That way although the macro would always run, it would not change the file times and make the WIT files dirty.

So you mean that macro would also read the file and compare the content with whatever it was trying to write and do not rewrite in case the content is the same?

[vigoo]

Yes that's what I suggest

[sviezypan]

Yes that's what I suggest

I had some difficulties making it work reliably so I opened issue about it - and also few others for visibility.
Other than that from my side I think this is ready to be released.