1. swiftc main.swift -emit-module-path main.swiftmodule -emit-executable -enable-private-imports -Xfrontend -enable-implicit-dynamic
2. ./main -> prints From original bar()
3. swiftc -emit-library inject.swift -o inject.dylib -I . -Xlinker -undefined -Xlinker suppress -Xlinker -flat_namespace -Xfrontend -disable-access-control
4. DYLD_INSERT_LIBRARIES=inject.dylib ./main -> prints From replacement bar()

• Passing -Xfrontend -enable-implicit-dynamic removes you from having to add dynamic to everything you want to be replacable
• Passing -enable-private-imports to the main.swift build, and then -Xfrontend -disable-access-control to the dylib build stops you from having to make things public unnecessarily. Depending on the use case if the symbols are public already you could remove these flags
• The dylib compilation depends on the main module. I was hoping I could use a string or something instead and specify main.Foo.bar explicitly instead of having to depend main directly for building
• There must be a better way than passing -undefined suppress -flat_namespace to the linker for the dylib compilation. You could pass -U SYMBOL but I think that's a bit more annoying to track down and maintain

Useful reference: https://tech.guardsquare.com/posts/swift-native-method-swizzling