Scala StringContext, how can I intercept expressions before evaluation?

I'm trying to use the Scala extensions to extend the String context to time the execution of a costly method And I came with this:

extension(sc: StringContext)
  def timeit(args: Any*): String = {
    val startTime = System.currentTimeMillis();
    // sc.parts.evaluate??
    val result = sc.s(args*) + s.parts().mkstring("");
    result + " took " + (System.currentTimeMillis() - startTime) + " millisecs"
  }

Unfortunately I don't see a way to intercept before the sc.parts are evaluated, so there is not much sense in measuring time. Is there a way to record the startTime before the sc.parts are evaluated? Do a call by name somehow? Or some other ways?

You need to use inline def, but you also need to make sure that all arguments are inline as well (so that their code would be inlined, not their result):

// code where all arguments are inlined
extension(inline sc: StringContext)
  inline def timeit(inline args: Any*): String = {
    val startTime = System.currentTimeMillis()
    val result = sc.s(args*)
    val endTime = System.currentTimeMillis()
    s"$result took ${endTime - startTime} millisecs"
  }

println(timeit"This string ${ { Thread.sleep(100); "is" } } taking ${ { Thread.sleep(250); "a while" } }")

// code where arguments are NOT inlined
extension(sc: StringContext)
  inline def timeit2(args: Any*): String = {
    val startTime = System.currentTimeMillis()
    val result = sc.s(args*)
    val endTime = System.currentTimeMillis()
    s"$result took ${endTime - startTime} millisecs"
  }

println(timeit2"This string ${ { Thread.sleep(100); "is" } } taking ${ { Thread.sleep(250); "a while" } }")

This string is taking a while took 354 millisecs
This string is taking a while took 1 millisecs