Scala macro that returns an empty AST

Question

I've created a simple macro that generates compiler warnings (or errors) if the current date is passed the FIXME (or TODO) date specified.

The macro implementation (see here) for Scala 2.10.4 used c.Expr[Any] return type, in 2.11.0 it is a whitebox macro that returns c.Tree.

In either case returning c.Expr[Any](EmptyTree) or EmptyTree respectively returns a value. Supposing the following invocation of the macro,

def hi() {
  FIXME("2073/04/10: This will abort compilation if not fixed by 2073/04/10")
  println("hi")
}

Compilation generates the following,

def hi(): Unit = {
  (<empty>: scala.runtime.BoxedUnit);
  scala.this.Predef.println("hi")
};

at the macro call site. Is it possible to generate the following instead:

def hi(): Unit = {
  scala.this.Predef.println("hi")
};

Answer 1

You might consider making the fixable block an argument to the macro:

def hi = FIXME("...") {  println(...) }

Compare what the compiler does with elidable code: it elides to a "zero" value, not an empty tree.

https://github.com/scala/scala/blob/2.10.x/src/compiler/scala/tools/nsc/transform/UnCurry.scala#L515

You might also consider a macro annotation.