This simple macro annotation outputs the annotated item as-is.

import scala.annotation.{compileTimeOnly, StaticAnnotation}
import scala.reflect.macros.whitebox.Context

@compileTimeOnly("enable macro paradise to expand macro annotations")
class noop extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro linkMacro.impl
}

object linkMacro {
  def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
    import c.universe._

    c.Expr[Any](q"{..$annottees}")
  }
}

The @compileTimeOnly annotation generates an error with a message indicating that the paradise compiler plugin must be included to use this macro. Instructions to include this via SBT are here.

You can use the above-defined macro like this:

@noop
case class Foo(a: String, b: Int)

@noop
object Bar {
  def f(): String = "hello"
}

@noop
def g(): Int = 10

When a method is defined to be a macro, the compiler takes the code that is passed as its argument and turns it into an AST. It then invokes the macro implementation with that AST, and it returns a new AST that is then spliced back to its call site.

import reflect.macros.blackbox.Context

object Macros {
  // This macro simply sees if the argument is the result of an addition expression.
  // E.g. isAddition(1+1) and isAddition("a"+1).
  // but !isAddition(1+1-1), as the addition is underneath a subtraction, and also
  // !isAddition(x.+), and !isAddition(x.+(a,b)) as there must be exactly one argument.
  def isAddition(x: Any): Boolean = macro isAddition_impl

  // The signature of the macro implementation is the same as the macro definition,
  // but with a new Context parameter, and everything else is wrapped in an Expr.
  def isAddition_impl(c: Context)(expr: c.Expr[Any]): c.Expr[Boolean] = {
    import c.universe._ // The universe contains all the useful methods and types
    val plusName = TermName("+").encodedName // Take the name + and encode it as $plus
    expr.tree match { // Turn expr into an AST representing the code in isAddition(...)
      case Apply(Select(_, `plusName`), List(_)) => reify(true)
      // Pattern match the AST to see whether we have an addition
      // Above we match this AST
      //             Apply (function application)
      //            /     \
      //         Select  List(_) (exactly one argument)
      // (selection ^ of entity, basically the . in x.y)
      //      /          \
      //    _              \
      //               `plusName` (method named +)
      case _                                     => reify(false)
      // reify is a macro you use when writing macros
      // It takes the code given as its argument and creates an Expr out of it
    }
  }
}

It is also possible to have macros that take Trees as arguments. Like how reify is used to create Exprs, the q (for quasiquote) string interpolator lets us create and deconstruct Trees. Note that we could have used q above (expr.tree is, surprise, a Tree itself) too, but didn't for demonstrative purposes.

// No Exprs, just Trees
def isAddition_impl(c: Context)(tree: c.Tree): c.Tree = {
  import c.universe._
  tree match {
    // q is a macro too, so it must be used with string literals.
    // It can destructure and create Trees.
    // Note how there was no need to encode + this time, as q is smart enough to do it itself.
    case q"${_} + ${_}" => q"true"
    case _              => q"false"
  }
}

Macros can trigger compiler warnings and errors through the use of their Context.

Say we're a particularly overzealous when it comes to bad code, and we want to mark every instance of technical debt with a compiler info message (let's not think about how bad this idea is). We can use a macro that does nothing except emit such a message.

import reflect.macros.blackbox.Context

def debtMark(message: String): Unit = macro debtMark_impl
def debtMarkImpl(c: Context)(message: c.Tree): c.Tree = {
  message match {
    case Literal(Constant(msg: String)) => c.info(c.enclosingPosition, msg, false)
    // false above means "do not force this message to be shown unless -verbose"
    case _                              => c.abort(c.enclosingPosition, "Message must be a string literal.")
    // Abort causes the compilation to completely fail. It's not even a compile error, where
    // multiple can stack up; this just kills everything.
  }
  q"()" // At runtime this method does nothing, so we return ()
}

Additionally, instead of using ??? to mark unimplemented code, we can create two macros, !!! and ?!?, that serve the same purpose, but emit compiler warnings. ?!? will cause a warning to be issued, and !!! will cause an outright error.

import reflect.macros.blackbox.Context

def ?!? : Nothing = macro impl_?!?
def !!! : Nothing = macro impl_!!!

def impl_?!?(c: Context): c.Tree = {
  import c.universe._
  c.warning(c.enclosingPosition, "Unimplemented!")
  q"${termNames.ROOTPKG}.scala.Predef.???"
  // If someone were to shadow the scala package, scala.Predef.??? would not work, as it
  // would end up referring to the scala that shadows and not the actual scala.
  // ROOTPKG is the very root of the tree, and acts like it is imported anew in every
  // expression. It is actually named _root_, but if someone were to shadow it, every
  // reference to it would be an error. It allows us to safely access ??? and know that
  // it is the one we want.
}

def impl_!!!(c: Context): c.Tree = {
  import c.universe._
  c.error(c.enclosingPosition, "Unimplemented!")
  q"${termNames.ROOTPKG}.scala.Predef.???"
}