Seems I keep finding new ways to be surprised by how Swift works. Here I thought that the Swift SDK/Toolchain on linux provided a Linux version of both BlocksRuntime and libobjc4. But that's not the case. The objective-C bridging support isn't built into the toolchain or the Swift compiler at all on non-Darwin! At least we get blocks though 😅
So I guess, @bugaevc if you're looking for a new project, there's unfinished porting work to add Swift interop to GNUStep :)