Yes, we could have added support for other calling conventions in the original @ccall PR. But we skipped that because it was a lot of discussion already to get to the point of merging the current version. And it seemed better to merge it than to get stuck working out notation for calling conventions or other attributes (Ccallmacro by ninjaaron · Pull Request #32748 · JuliaLang/julia · GitHub)
If someone wanted to figure out the nitty gritty detail of calling convention for FFI calls, I’m sure that would be a great PR. Last time we weren’t sure if we needed a general annotations syntax for defining side effects, but now we have @assume_effects ... @ccall. I’m still not sure whether there’s other annotations which might be needed.