Prevent optimizing away public interfaces needed by an XCFramework (aka dyld: Symbol not found)

[bnickel (Brian Nickel)]

I've found a solution, based on the following lead:

What are the tradeoffs of -cross-module-optimization?

-cross-module-optimization (CMO) is still in an "experimental" phase. Eventually we want to enable it by default (unless a module is compiled with -enable-library-evolution). And yes, CMO has some tradeoffs. On the one hand it can improve performance significantly, e.g. if a module calls a generic function in another module. On the other hand this can lead to a code size increase. The concept of CMO is comparable to LTO, but it's much more powerful, because it enables the swift (SIL) optimizer…

While I don't know exactly what's causing the cross-module symbol culling, enabling library evolution does appear to fix the problem:

        .target(
            name: "HeapSwiftCore",
            dependencies: [
                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
            ],
            swiftSettings: [
                .unsafeFlags([
                    "-enable-library-evolution",
                    "-emit-module-interface"
                ]),
            ]),

The only problem I have now is warning about swift-protobuf not being compiled with evolution support, but I believe I can fix this with @_implementationOnly import attributes, as described in below since I'm not exposing this outside of testing:

Risks of compiler warning: 'Module was not compiled with library evolution support'

Stable module interfaces (-emit-module-interface flag) requires library evolution support (-enable-library-evolution). If you enable the former but not the latter, the compiler will produce an invalid module interface file that will cause runtime crashes when used. If a library A depends on a library B, and A is built with stable module interfaces and library evolution, B must also be built with stable module interfaces and library evolution, unless A only imports B using an @_implementationOnl…

I'll probably end up with something like:

#if BUILD_FOR_DEVELOPMENT
import SwiftProtobuf
#else
@ _implementationOnly import SwiftProtobuf
#endif