Throw a more informative error when scala_macro_library isn't used

src/java/io/bazel/rulesscala/scalac/scala_2/ScalacInvoker.java

@@ -11,14 +11,14 @@
 
 //Invokes Scala 2 compiler
 class ScalacInvoker{
-   
+
   public static ScalacInvokerResults invokeCompiler(CompileOptions ops, String[] compilerArgs)
     throws IOException, Exception{
 
     ReportableMainClass comp = new ReportableMainClass(ops);
 
     ScalacInvokerResults results = new ScalacInvokerResults();
-    
+
     results.startTime = System.currentTimeMillis();
     try {
       comp.process(compilerArgs);
@@ -28,7 +28,15 @@ public static ScalacInvokerResults invokeCompiler(CompileOptions ops, String[] c
       } else if (ex.toString().contains("java.lang.StackOverflowError")) {
         throw new ScalacWorker.CompilationFailed("with StackOverflowError", ex);
       } else if (isMacroException(ex)) {
-        throw new ScalacWorker.CompilationFailed("during macro expansion", ex);
+        String reason;
+
+        if (ex instanceof ClassFormatError) {
+          reason = "during macro expansion. You may have declared a target containing a macro as a `scala_library` target instead of a `scala_macro_library` target";
+        } else {
+          reason = "during macro expansion";
+        }
+
+        throw new ScalacWorker.CompilationFailed(reason, ex);
       } else {
         throw ex;
       }
@@ -37,7 +45,7 @@ public static ScalacInvokerResults invokeCompiler(CompileOptions ops, String[] c
     }
 
     results.stopTime = System.currentTimeMillis();
-    
+
     ConsoleReporter reporter = (ConsoleReporter) comp.getReporter();
     if (reporter == null) {
       // Can happen only when `ReportableMainClass::newCompiler` was not invoked,

test/macros/BUILD

@@ -0,0 +1,24 @@
+load("//scala:scala.bzl", "scala_library", "scala_macro_library")
+
+scala_library(
+    name = "incorrect-macro",
+    srcs = ["IdentityMacro.scala"],
+)
+
+scala_macro_library(
+    name = "correct-macro",
+    srcs = ["IdentityMacro.scala"],
+)
+
+scala_library(
+    name = "incorrect-macro-user",
+    srcs = ["MacroUser.scala"],
+    tags = ["manual"],
+    deps = [":incorrect-macro"],
+)
+
+scala_library(
+    name = "correct-macro-user",
+    srcs = ["MacroUser.scala"],
+    deps = [":correct-macro"],
+)

test/macros/IdentityMacro.scala

@@ -0,0 +1,9 @@
+package macros
+
+import scala.language.experimental.macros
+import scala.reflect.macros.blackbox
+
+object IdentityMacro {
+  def identityMacro[A](value: A): A = macro identityMacroImpl[A]
+  def identityMacroImpl[A](context: blackbox.Context)(value: context.Expr[A]): context.Expr[A] = value
+}

test/macros/MacroUser.scala

@@ -0,0 +1,5 @@
+package macros
+
+object Main {
+  def main(arguments: Array[String]): Unit = println(IdentityMacro.identityMacro("Hello, world!"))
+}

test/shell/test_macros.sh

@@ -0,0 +1,17 @@
+# shellcheck source=./test_runner.sh
+dir=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
+. "${dir}"/test_runner.sh
+. "${dir}"/test_helper.sh
+runner=$(get_test_runner "${1:-local}")
+
+incorrect_macro_user_does_not_build() {
+  (! bazel build //test/macros:incorrect-macro-user) |&
+    grep --fixed-strings 'Build failure during macro expansion. You may have declared a target containing a macro as a `scala_library` target instead of a `scala_macro_library` target'
+}
+
+correct_macro_user_builds() {
+  bazel build //test/macros:correct-macro-user
+}
+
+$runner incorrect_macro_user_does_not_build
+$runner correct_macro_user_builds