Allow to specify per-callsite @tailrec annotation.

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

@@ -73,6 +73,20 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
   final val labelPrefix = "tailLabel"
   final val labelFlags = Flags.Synthetic | Flags.Label
 
+  /** Symbols of methods that have @tailrec annotatios inside */
+  private val methodsWithInnerAnnots = new collection.mutable.HashSet[Symbol]()
+
+  override def transformUnit(tree: Tree)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    methodsWithInnerAnnots.clear()
+    tree
+  }
+
+  override def transformTyped(tree: Typed)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    if (tree.tpt.tpe.hasAnnotation(defn.TailrecAnnot))
+      methodsWithInnerAnnots += ctx.owner.enclosingMethod
+    tree
+  }
+
   private def mkLabel(method: Symbol, abstractOverClass: Boolean)(implicit c: Context): TermSymbol = {
     val name = c.freshName(labelPrefix)
 
@@ -137,10 +151,10 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
             }
           })
         }
-      case d: DefDef if d.symbol.hasAnnotation(defn.TailrecAnnot) =>
+      case d: DefDef if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
         ctx.error("TailRec optimisation not applicable, method is neither private nor final so can be overridden", d.pos)
         d
-      case d if d.symbol.hasAnnotation(defn.TailrecAnnot) =>
+      case d if d.symbol.hasAnnotation(defn.TailrecAnnot) || methodsWithInnerAnnots.contains(d.symbol) =>
         ctx.error("TailRec optimisation not applicable, not a method", d.pos)
         d
       case _ => tree
@@ -180,7 +194,7 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
 
     override def transform(tree: Tree)(implicit c: Context): Tree = {
       /* A possibly polymorphic apply to be considered for tail call transformation. */
-      def rewriteApply(tree: Tree, sym: Symbol): Tree = {
+      def rewriteApply(tree: Tree, sym: Symbol, required: Boolean = false): Tree = {
         def receiverArgumentsAndSymbol(t: Tree, accArgs: List[List[Tree]] = Nil, accT: List[Tree] = Nil):
             (Tree, Tree, List[List[Tree]], List[Tree], Symbol) = t match {
           case TypeApply(fun, targs) if fun.symbol eq t.symbol => receiverArgumentsAndSymbol(fun, accArgs, targs)
@@ -216,7 +230,7 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
           }
         }
         def fail(reason: String) = {
-          if (isMandatory) c.error(s"Cannot rewrite recursive call: $reason", tree.pos)
+          if (isMandatory || required) c.error(s"Cannot rewrite recursive call: $reason", tree.pos)
           else c.debuglog("Cannot rewrite recursive call at: " + tree.pos + " because: " + reason)
           continue
         }
@@ -299,7 +313,8 @@ class TailRec extends MiniPhaseTransform with DenotTransformer with FullParamete
             noTailTransforms(stats),
             transform(expr)
           )
-
+        case tree @ Typed(t: Apply, tpt) if tpt.tpe.hasAnnotation(defn.TailrecAnnot) =>
+          tpd.Typed(rewriteApply(t, t.fun.symbol, required = true), tpt)
         case tree@If(cond, thenp, elsep) =>
           tpd.cpy.If(tree)(
             noTailTransform(cond),

tests/neg/tailcall/i1221.scala

@@ -0,0 +1,10 @@
+import annotation.tailrec
+
+object I1221{
+  final def foo(a: Int): Int = {
+  if ((foo(a - 1): @tailrec) > 0) // error: not in tail position
+    foo(a - 1): @tailrec
+  else
+    foo(a - 2): @tailrec
+  }
+}

tests/neg/tailcall/i1221b.scala

@@ -0,0 +1,10 @@
+import annotation.tailrec
+
+class Test {
+  def foo(a: Int): Int = { // error: method is not final
+    if ((foo(a - 1): @tailrec) > 0)
+      foo(a - 1): @tailrec
+    else
+      foo(a - 2): @tailrec
+    }
+}

tests/pos/tailcall/i1221.scala

@@ -0,0 +1,10 @@
+import annotation.tailrec
+
+object i1221{
+  final def foo(a: Int): Int = {
+  if (foo(a - 1) > 0) 
+    foo(a - 1): @tailrec 
+  else 
+    foo(a - 2): @tailrec
+  }
+}