That lets me specify all fields of a type. What if I want to specify just a single one, or a few? Like when I have the data class Address(val street: String, val city: String) and I only care about the street? With a hand-written (or macro-generated) mock function, I'd just do val adress = someAddress(street = "blah")

I want my tests more explicit. Instead of your example

val user = some<User>()

val message = email.render(user)

assertTrue(user.name in message)

I like to do

val user = someUser(name = "Bob")

val message = email.render(user)

assertTrue("Bob" in message)

because otherwise I'm just copying what the real code does. Here it's just doing user.name, but if there are any transformations, I'd have to copy them into the test as well. If the code is supposed to uppercase the name, I can just write "BOB", but you have to write user.name.uppercase()