Kotlin协程的@RestrictsSuspension注解

Classes and interfaces marked with this annotation are restricted when used as receivers for >extension
suspend functions. These suspend extensions can only invoke other member or extension >suspend functions on this particular
receiver and are restricted from calling arbitrary suspension functions

简单来说，如果一个接口或者类被标注@RestrictsSuspension，那么当这个类作为suspend扩展函数接受者的时候，其suspend函数内部不能调用其它CoroutineScope的suspend函数。

suspend fun SequenceScope<String>.test(block: suspend GlobalScope.() -> Int){
    block()
}

报错：Restricted suspending functions can only invoke member or extension 
suspending functions on their restricted coroutine scope

上面的代码，在SequenceScope的test扩展函数中调用了GlobalScope的扩展lambda--block；而SequenceScope正是被@RestrictsSuspension注解修饰。所以编译器会报错！

@RestrictsSuspension
@SinceKotlin("1.3")
public abstract class SequenceScope<in T> internal constructor(){。。。}