Removing any2unit via a macro.

Macros.scala

import language.experimental.{ macros => scalaMacros }
import reflect.macros.Context

package object macros {
  def safe[A](expr: A) = macro safeImpl[A]

  def safeImpl[A](c: Context)(expr: c.Expr[A]): c.Expr[A] = {
    import c.universe._

    def isIgnoredStatement(tree: Tree) = tree match {
      // Scala creates synthetic blocks with <init> calls on classes.
      // The calls return Object so we need to ignore them.
      case Apply(Select(_, nme.CONSTRUCTOR), _) => true
      case _ => false
    }

    object NoUnit extends Traverser {
      override def traverse(tree: Tree) = {
        tree match {
          case Block(statements, _) =>
            statements.foreach { stat =>
              val unitLike = stat.tpe =:= typeOf[Unit] || stat.isDef || isIgnoredStatement(stat)
              if (!unitLike)
                c.error(stat.pos, "Statements must return Unit")
            }
          case _ =>
        }
        super.traverse(tree)
      }
    }
    NoUnit.traverse(expr.tree)

    expr
  }
}

NoAny2Unit.scala

// No any2unit!
Macros.safe {
  def x(a: Int) = {
    // Won't compile:
    // a
    println("Returning a")
    a
  }

  // Won't compile:
  // 1
  // x(1)
  // x _

  println("HELLO")
  println("HELLO")
}

• c.error(stat.pos, msg) is better form.
• you could use a Traverser, rather than a Transformer, the easiest way is via:

tree.foreach {
  case Block(stats, _) => ...
  case _ => 
}

• A bunch of standard names are in Universe#{nme, tpnme}

scala> reflect.runtime.universe.nme.CONSTRUCTOR
res1: reflect.runtime.universe.nme.NameType = <init>

scala> reflect.runtime.universe.tpnme.<TAB>
EMPTY           ERROR           NameType        PACKAGE         WILDCARD        WILDCARD_STAR   asInstanceOf    isInstanceOf    toString        

scala> reflect.runtime.universe.nme.<TAB>
CONSTRUCTOR           EMPTY                 ERROR                 LOCAL_SUFFIX_STRING   NameType              PACKAGE               
ROOTPKG               WILDCARD              asInstanceOf          isInstanceOf          toString      

@retronym that's much better. Thanks heaps!