feat(bindings/swift): add Swift binding

[unixzii]

Don't merge! It's still working in progress.

This PR adds a new binding for Swift.

Implementation Details

Currently, the best way to integrate foreign-language libraries in Swift is via C APIs (FFI). We need to first bridge the Rust APIs to C APIs, and import the C header files in Swift with some proper linking settings.

We reused the C binding of OpenDAL, and import it directly in the Swift package. This can greatly share the same features that the C binding has already existed, and make code of the Swift binding as simple as possible (because it's just a wrapper).

To transfer large buffers between Swift and Rust code, we use the zero-copy initializer of Data type in Swift for receiving data from Rust. And use withUnsafeBytes to access the borrowed pointer to Data when sending data to Rust.

Future Directions

Asynchronous Operations

Swift Concurrency provides async-await feature and now it’s stable. We can embrace it to deliver a streamlined DX as the Rust crate per se does.

To make it, we have some prerequisites:

• Determine the async runtime in Rust (tokio works fine here and it’s able to provide an environment to be used in other languages).
• Expose async APIs in C binding.
• Transform callback-based APIs into async APIs in Swift.

Better Integration of Rust Binary

Swift package users typically don’t perform extra actions before using the package. But it’s required for us because we need to build the Rust crate as a static library. It’s better to eliminate this manual operation for users.

Some possible approaches are:

• Research Swift package plugins to see whether it‘s suitable for our use case.
• Use git submodule to make the Rust library a binary vendor. (Can be hard to maintain)
• Use xcframework to deliver a prebuilt package. (Only usable from Apple platforms and also hard to maintain)

Checklist

• Complete the binding APIs
• Refine package structure
• Add tests and CI actions

[Xuanwo]

Thanks a lot! Let's go! Are you willing to create a tracking issue for not finished tasks?

[unixzii]

Thanks a lot! Let's go! Are you willing to create a tracking issue for not finished tasks?

Sure, I will create one later.

[imWildCat]

uniffi (https://github.com/mozilla/uniffi-rs) could be a slightly better solution for Swift binding