Unfortunately, there’s no built-in support for formatting in string templates yet, as a workaround, you can use something like:
"pi = ${pi.format(2)}"
the .format(n) function you’d need to define yourself as
fun Double.format(digits: Int) = "%.${digits}f".format(this)
This will work only in Kotlin/JVM.
There’s clearly a piece of functionality here that is missing from Kotlin at the moment, we’ll fix it.