Add example of Swift <--> C++ interop

[jiawen]

In the last year, Swift has gradually gained the ability to interop with C++. rules_swift should add a minimal example on how that would work. It would provide at baseline a means by which we can easily reproduce issues (module naming, static and dynamic linking, ODR violations, etc) as they are discovered.

[gkoreman]

Yes please!

I just tried this out and was pleasantly surprised at how easy it was. Here are some quick notes based on my experience in case anyone else sees this before the examples come out.

BUILD

swift_library(
    name = "mySwiftLib",
    srcs = [ "lib.swift" ], #note - if you make this main.swift then it will have an extra _main symbol which conficts with your app
    generates_header = True,
    copts = ["-cxx-interoperability-mode=default"], #needed to export c++ bindings
)

cc_binary(
...
    deps = [":mySwiftLib"]
)

SWIFT lib.swift

public func printWelcomeMessage(_ name: String) {
  print("Welcome \(name)")
}

C++

#include "full/path/to/module/mySwiftLib-Swift.h"

int main(int, char**)
{
    // Namespace appears to be the full path to the module
    full_path_to_module::printWelcomeMessage("friend");
    return 0;
}

[davidzchen]

It would be great to have an example of the other way around: calling C++ code from Swift. This will make it easier for developers to utilize C++ libraries in Swift code.

[kersson]

Just put up a PR to add a basic example to show how to invoke C++ code from Swift: #1280

[jiawen]

@brentleyjones I just whipped up a short example of calling Swift from C++ (the first part of the Mountain Peak example). Should I put up a PR to include it in the examples?

[brentleyjones]

Does it add anything more than 321d2c3? If so, yes.

[jiawen]

Does it add anything more than 321d2c3? If so, yes.

Sent #1454.

If there's any interest, I also have a slightly more complex example that has a C++ class conform to a Swift protocol. Nothing that's not in the documentation, but potentially useful since it's a common use case.