[cogman10]

It's basically how Java does it (circa 17) as well.

It's something you really can't do without a pretty significant language runtime. You also really need people working within your runtime to prefer being in your runtime. Environments that do a lot of FFI don't work well with a colorblind runtime. That's because if the little C library you call does IO then you've got an incongruous interaction that you need to worry about.

[gpderetta]

Neither Java nor Go make all functions async. What they provide is stackful coroutines (or equivalently one shot continuations) that allow composing async and sync functions transparently.

[FlyingSnake]

Golang functions are synchronous but all goroutine code is essentially async or “coloured”.