Exactly! Most code doesn't care whether it's sync or async, the compiler should decide which specialization to use when the program is built. The only places where you usually care is either 1: the very top of the program (some event loop handler called near main) or 2: the very bottom, when you're implementing an IO library. The specialization is decided by what you do at the very top; that decision propagates down the call chain, specializing everything in the middle that doesn't care; until it gets to a function in your IO library where it chooses between the pre-defined sync or pre-defined async implementations.

So you have a synchronous http server and you decide you want to make it async? Ok, no problem, switch to an async-enabled request handler in main, and boom everything that you wrote is recompiled into a state machine a la async, and at the very bottom where it actually calls the library to write out to the socket it, the library knows what the context is and can choose the async-enabled implementation.

I'm glossing over some important details and restrictions that might make this more complicated in practice, but I think it should at least be possible for functions to opt-in to 'async-specializability' to avoid having to rewrite the world for async.