Storing a lambda expression in a variable removes inline optimization

Question

I am looking into optimizing the size of the call stack by using inline functions.

If you execute the following code:

inline fun function2( action: () -> Unit) {
 action()
}

fun function1() {

 function2{ printCurrentStack() }
}

fun printCurrentStack() {
 RuntimeException().printStackTrace()
}

fun main() {
 function1()
}

You will get the following output:

at com.kotlin.playground.functional.PrimeNumbersKt.printCurrentStack(PrimeNumbers.kt:17)
at com.kotlin.playground.functional.PrimeNumbersKt$function1$1.invoke(PrimeNumbers.kt:13)
at com.kotlin.playground.functional.PrimeNumbersKt$function1$1.invoke(PrimeNumbers.kt)
at com.kotlin.playground.functional.PrimeNumbersKt.function2(PrimeNumbers.kt:8)
at com.kotlin.playground.functional.PrimeNumbersKt.function1(PrimeNumbers.kt:13)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt:21)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt)

So, in this case, the stack has 7 elements.

• Calling main
• Calling function1
• Calling function2
• 2 calls for the lambda expression
• Calling printCurrentStack
• Calling printStackTrace

Now, if I mark function2 as inline and I execute the code, I get the following output:

at com.kotlin.playground.functional.PrimeNumbersKt.printCurrentStack(PrimeNumbers.kt:17)
at com.kotlin.playground.functional.PrimeNumbersKt.function1(PrimeNumbers.kt:13)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt:21)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt)

This seems to work great, the size of the call stack is reduced by 3.
However, if I update function1 to store the lambda in a variable before sending it to function2:

fun function1() {
 val lambda = { printCurrentStack() }
 function2(lambda)
}

The output changes to this:

at com.kotlin.playground.functional.PrimeNumbersKt.printCurrentStack(PrimeNumbers.kt:15)
at com.kotlin.playground.functional.PrimeNumbersKt$function1$lambda$1.invoke(PrimeNumbers.kt:10)
at com.kotlin.playground.functional.PrimeNumbersKt$function1$lambda$1.invoke(PrimeNumbers.kt)
at com.kotlin.playground.functional.PrimeNumbersKt.function1(PrimeNumbers.kt:21)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt:19)
at com.kotlin.playground.functional.PrimeNumbersKt.main(PrimeNumbers.kt)

The size of the call stack goes from 4 to 6 although function2 is still marked as an inline function.

It seems that the lambda calls cannot be inlined if the lambda is stored in a variable.
Why is this happening?