One thing I don't see in this list (though maybe I'm missing it) is the ability to expose a Swift function as a function compatible with the C ABI. Something like extern "C" in C++ or no_mangle in Rust.

You can pass @convention(c) blocks around as C function pointers, but you can't currently call directly into a Swift library from C code. That makes it difficult to do things like expose platform-specific functionality to a shared C library using Swift.

Right now you have to implement the C function inside an Objective-C file, and then have your Objective-C code call your Swift code, or you have to change all your interfaces to allow injection of function pointers for all external dependencies.

It's the kind of thing that makes staying with Objective-C attractive for that type of stuff, but Swift in general has been a solid improvement, and I'd like to not have to give that up.