improve error message for @tailrec

Now the compiler will give compile errors if the annotation is
used on other kinds then methods.

Author: ingarabr

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1825,10 +1825,15 @@ object messages {
       }
   }
 
-  case class TailrecNotApplicable(method: Symbol)(implicit ctx: Context)
+  case class TailrecNotApplicable(symbol: Symbol)(implicit ctx: Context)
     extends Message(TailrecNotApplicableID) {
     val kind = "Syntax"
-    val msg = hl"TailRec optimisation not applicable, $method is neither ${"private"} nor ${"final"}."
+    val symbolKind = symbol.showKind
+    val msg =
+      if (symbol.is(Method))
+        hl"TailRec optimisation not applicable, $symbol is neither ${"private"} nor ${"final"}."
+      else
+        hl"TailRec optimisation not applicable, ${symbolKind} isn't a method."
     val explanation =
       hl"A method annotated ${"@tailrec"} must be declared ${"private"} or ${"final"} so it can't be overridden."
   }

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -6,8 +6,6 @@ import ast.{TreeTypeMap, tpd}
 import core._
 import Contexts.Context
 import Decorators._
-import DenotTransformers.IdentityDenotTransformer
-import Denotations.SingleDenotation
 import Symbols._
 import Types._
 import NameKinds.TailLabelName
@@ -161,9 +159,6 @@ class TailRec extends MiniPhase with FullParameterization {
       case d: DefDef if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
         ctx.error(TailrecNotApplicable(sym), sym.pos)
         d
-      case d if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
-        ctx.error("TailRec optimisation not applicable, not a method", sym.pos)
-        d
       case _ => tree
     }
 

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

@@ -7,33 +7,25 @@ import ast._
 import Contexts._
 import Types._
 import Flags._
-import Denotations._
 import Names._
 import StdNames._
-import NameOps._
 import Symbols._
 import Trees._
 import TreeInfo._
 import ProtoTypes._
-import Constants._
-import Scopes._
 import CheckRealizable._
 import ErrorReporting.errorTree
 
-import annotation.unchecked
 import util.Positions._
-import util.Stats
-import util.common._
 import transform.SymUtils._
 import Decorators._
-import Uniques._
 import ErrorReporting.{err, errorType}
 import config.Printers.typr
 import NameKinds.DefaultGetterName
 
 import collection.mutable
 import SymDenotations.{NoCompleter, NoDenotation}
-import dotty.tools.dotc.reporting.diagnostic.{ErrorMessageID, Message}
+import dotty.tools.dotc.reporting.diagnostic.Message
 import dotty.tools.dotc.reporting.diagnostic.messages._
 import dotty.tools.dotc.transform.ValueClasses._
 
@@ -358,6 +350,9 @@ object Checking {
       fail(AbstractOverrideOnlyInTraits(sym))
     if (sym.is(Trait) && sym.is(Final))
       fail(TraitsMayNotBeFinal(sym))
+    // Skip ModuleVal since the annotation will also be on the ModuleClass
+    if (sym.hasAnnotation(defn.TailrecAnnot) && !sym.is(Method | ModuleVal))
+      fail(TailrecNotApplicable(sym))
     if (sym.hasAnnotation(defn.NativeAnnot)) {
       if (!sym.is(Deferred))
         fail(NativeMembersMayNotHaveImplementation(sym))

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

@@ -15,7 +15,7 @@ import ast.Trees._
 
 class FrontEnd extends Phase {
 
-  override def phaseName = "frontend"
+  override def phaseName = FrontEnd.name
   override def isTyper = true
   import ast.tpd
 
@@ -103,3 +103,7 @@ class FrontEnd extends Phase {
     typeCheck
   }
 }
+
+object FrontEnd {
+  val name = "frontend"
+}

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -7,6 +7,7 @@ import dotty.tools.dotc.core.Types.WildcardType
 import dotty.tools.dotc.parsing.Tokens
 import dotty.tools.dotc.reporting.diagnostic.messages._
 import dotty.tools.dotc.transform.PostTyper
+import dotty.tools.dotc.typer.FrontEnd
 import org.junit.Assert._
 import org.junit.Test
 
@@ -1361,4 +1362,26 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val (msg @ ImportRenamedTwice(ident)) :: Nil = messages
       assertEquals(ident.show, "Integer")
     }
+
+  @Test def tailRecOptimisation =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |import scala.annotation.tailrec
+        |@tailrec
+        |object Test {
+        |  @tailrec val a = ""
+        |  @tailrec var b = ""
+        |}
+        |@tailrec
+        |class Test {}
+        |
+      """.stripMargin
+    }.expect{ (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(4, messages)
+
+      val tailRegMessages = messages.map{ case m :TailrecNotApplicable => m.symbolKind}.toSet
+      assertEquals(tailRegMessages, Set("variable", "value", "object", "class"))
+    }
+
 }