I meant the low-level dispatch of methods; the application binary interface that Objective-C uses to communicate with the APIs that make up almost the entire OS X GUI API. You can't mix that with the ABI for Windows API or any Linux toolkit (except GNUStep).

> Most OSX apps and services still aren't even written in Objective-C

Most OS-X apps aren't written in Objective-C? Really? Come on. If it was written specifically for OS X or iOS then it's almost guaranteed to be in Objective-C.

> You can compile Objective-C using the gnu compiler and have been able to for ages.

Nobody says you can't. But the only real useful thing you can with that is link to GNUStep.

> Swift is an amalgam of many different programming languages and styles, including Rust. None of which have anything specific to do with OSX/iOS about them.

At the high-level you are right but on the low-level it's specifically designed to interface with Objective-C APIs which exist in OSes from only one manufacturer. You can't separate Swift from that. The fact that it's an amalgam of OS X specific technology makes it unsuitable for other OSes. It doesn't really have that much in common with Rust either.