How are coroutines implemented in JVM langs without JVM support?

Question

This question came up after reading the Loom proposal, which describes an approach of implementing coroutines in the Java programming language.

Particularly this pro

Answer 1

Coroutines do not rely on features of the operating system or the JVM. Instead, coroutines and suspend functions are transformed by the compiler producing a state machine capable of handling suspensions in general and passing around suspending coroutines keeping their state. This is enabled by Continuations, which are added as a parameter to each and every suspending function by the compiler; this technique is called “Continuation-passing style”(CPS).

One example can be observed in the transformation of suspend functions:

suspend fun  CompletableFuture.await(): T

The following shows its signature after CPS transformation:

fun  CompletableFuture.await(continuation: Continuation): Any?

If you want to know the hard details, you need to read this explanation.