What is the difference between crossinline and noinline in Kotlin?

Question

1. This code compiles with a warning (insignificant performance impact):

   inline fun test(noinline f: () -> Unit) {
       thread(         
   
   
           

Answer 1

Q1: How come (2) does not compile but (4) does?

From their doc:

Inlinable lambdas can only be called inside the inline functions or passed as inlinable arguments...

Answer:

The method thread(...) is not an inline method so you won't be able to pass f as an argument.

Q2: What exactly is the difference between noinline and crossinline?

Answer:

noinline will prevent the inlining of lambdas. This becomes useful when you have multiple lambda arguments and you want only some of the lambdas passed to an inline function to be inlined.

crossinline is used to mark lambdas that mustn't allow non-local returns, especially when such lambda is passed to another execution context. In other words, you won't be able to do a use a return in such lambdas. Using your example:

inline fun test(crossinline f: () -> Unit) {
    thread { f() }
}

//another method in the class
fun foo() {

    test{ 

       //Error! return is not allowed here.
       return
    }

}

Q3: If (3) does not generates a no performance improvements, why would (4) do?

Answer:

That is because the only lambda you have in (3) has been marked with noinline which means you'll have the overhead cost of creating the Function object to house the body of your lamda. For (4) the lambda is still inlined (performance improvement) only that it won't allow non-local returns.