It would be nice if the syntax was changed and generalized as follows:
def foo0(def x: Int) = ... x ... x ...
foo0: (=> Int) => R
def foo1(lazy x: Int) = ... x ... x ...
foo1: (=> Int) => R
// ^ semantically equivalent to:
def foo1(def x: Int) = { lazy val tmp = x; ... tmp ... tmp ... }
def foo2(var x: Int) = ... x ... x ...
foo2: (Int) => R
// ^ semantically equivalent to:
def foo2(x: Int) = { var tmp = x; ... tmp ... tmp ... }
I reckon the lazy x: T variant is actually the one that would be most often used, rather than def x: T, which is the only thing we have today (through the weird x: => T syntax).
I recall reading that a long, long, time ago, there were discussions about allowing syntaxes around these lines in Scala.