Fix #9100: Add error when experimental macros are not enabled

Author: nicolasstucki

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -60,6 +60,9 @@ object Feature:
   def dependentEnabled(using Context) =
     enabled(nme.dependent, defn.LanguageExperimentalModule.moduleClass)
 
+  def scala2ExperimentalMacroEnabled(using Context) =
+    enabled("macros".toTermName, defn.LanguageExperimentalModule.moduleClass)
+
   def sourceVersionSetting(using Context): SourceVersion =
     SourceVersion.valueOf(ctx.settings.source.value)
 

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3581,6 +3581,13 @@ class Typer extends Namer
       }
     }
     if ctx.phase.isTyper then
+      if !config.Feature.scala2ExperimentalMacroEnabled then
+        ctx.error(
+          """Scala 2 macro definition needs to be enabled
+            |by making the implicit value scala.language.experimental.macros visible.
+            |This can be achieved by adding the import clause 'import scala.language.experimental.macros'
+            |or by setting the compiler option -language:experimental.macros.
+          """.stripMargin, call.sourcePos)
       call match
         case call: untpd.Ident =>
           typedIdent(call, defn.AnyType)

tests/neg/i9100.scala

@@ -0,0 +1,3 @@
+object FooMacros {
+  def foo[T]: String = macro Foo.fooImpl[T] // error: Scala 2 macro definition needs to be enabled
+}

tests/pos/macro.scala

@@ -1,3 +1,4 @@
+import scala.language.experimental.macros
 class A {
   def foo: Int = macro ???
   inline def foo: Int = ???