zig doesn't switch. It's statically typed, and I think (not 100% sure) it detects if you have called a function with the async keyword and when that happens it

zig doesn't switch. It's statically typed, and I think (not 100% sure) it detects if you have called a function with the async keyword and when that happens it triggers compiling a second version of the function with async support, so there are actually two versions of the function floating around in your binary. Since zig is itself a parsimonious compiler, if that's never in a codepath (say it's provided by a library), it doesn't get compiled. The reverse is true, too, if you ONLY call a function with the async keyword, it doesn't compile the non-async version. Again, with the caveat that I think that's what's going on.