Add support for `magnolia.debug` annotation

2 files changed, 19 insertions, 1 deletions

diff --git a/core/shared/src/main/scala/interface.scala b/core/shared/src/main/scala/interface.scala
index f0f104c..93aa045 100644
--- a/core/shared/src/main/scala/interface.scala
+++ b/core/shared/src/main/scala/interface.scala
@@ -188,4 +188,14 @@ final class SealedTrait[Typeclass[_], Type](val typeName: TypeName,
   */
 final case class TypeName(ownerName: String, short: String) {
   def full: String = s"$ownerName.$short"
-}
\ No newline at end of file
+}
+
+/**
+  * This annotation can be attached to the implicit `gen` method of a type class companion,
+  * which is implemented by the `Magnolia.gen` macro.
+  * It causes magnolia to dump the macro-generated code to the console during compilation.
+  *
+  * @param typeNamePart If non-empty restricts the output generation to types
+  *                     whose full name contains the given [[String]]
+  */
+final class debug(typeNamePart: String = "") extends scala.annotation.StaticAnnotation
\ No newline at end of file
diff --git a/core/shared/src/main/scala/magnolia.scala b/core/shared/src/main/scala/magnolia.scala
index b4179f9..67e948e 100644
--- a/core/shared/src/main/scala/magnolia.scala
+++ b/core/shared/src/main/scala/magnolia.scala
@@ -69,6 +69,10 @@ object Magnolia {
     import c.universe._
     import internal._
 
+    val debug = c.macroApplication.symbol.annotations
+      .find(_.tree.tpe <:< typeOf[debug])
+      .flatMap(_.tree.children.tail.collectFirst { case Literal(Constant(s: String)) => s })
+
     val magnoliaPkg = q"_root_.magnolia"
     val scalaPkg = q"_root_.scala"
 
@@ -448,6 +452,10 @@ object Magnolia {
     if (currentStack.frames.isEmpty) recursionStack = ListMap()
 
     val dereferencedResult = result.map { tree =>
+      if (debug.isDefined && genericType.toString.contains(debug.get)) {
+        c.echo(c.enclosingPosition, s"Magnolia macro expansion for $genericType")
+        c.echo(NoPosition, s"... = ${showCode(tree)}\n\n")
+      }
       if (currentStack.frames.isEmpty) c.untypecheck(removeDeferred.transform(tree)) else tree
     }