#1589: Add error message for not emitting switch.

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -118,6 +118,7 @@ public enum ErrorMessageID {
     StaticFieldsOnlyAllowedInObjectsID,
     CyclicInheritanceID,
     UnableToExtendSealedClassID,
+    UnableToEmitSwitchID,
     ;
 
     public int errorNumber() {

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

@@ -1972,4 +1972,33 @@ object messages {
     val msg = hl"Cannot extend ${"sealed"} $pclazz in a different source file"
     val explanation = "A sealed class or trait can only be extended in the same file as its declaration"
   }
+
+  case class UnableToEmitSwitch()(implicit ctx: Context)
+  extends Message(UnableToEmitSwitchID) {
+    val kind = "Syntax"
+    val msg = hl"Could not emit switch for ${"@switch"} annotated match"
+    val explanation = {
+      val codeExample =
+        """val ConstantB = 'B'
+          |final val ConstantC = 'C'
+          |def tokenMe(ch: Char) = (ch: @switch) match {
+          |  case '\t' | '\n' => 1
+          |  case 'A'         => 2
+          |  case ConstantB   => 3  // a non-literal may prevent switch generation: this would not compile
+          |  case ConstantC   => 4  // a constant value is allowed
+          |  case _           => 5
+          |}""".stripMargin
+
+      hl"""If annotated with ${"@switch"}, the compiler will verify that the match has been compiled to a
+          |tableswitch or lookupswitch and issue an error if it instead compiles into a series of conditional
+          |expressions. Example usage:
+          |
+          |$codeExample
+          |
+          |The compiler will not apply the optimisation if:
+          |- the matched value is not of type ${"Int"}, ${"Byte"}, ${"Short"} or ${"Char"}
+          |- the matched value is not a constant literal
+          |- there are less than three cases"""
+    }
+  }
 }

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

@@ -14,6 +14,7 @@ import Decorators._
 import patmat.Space
 import NameKinds.{UniqueNameKind, PatMatStdBinderName, PatMatCaseName}
 import config.Printers.patmatch
+import reporting.diagnostic.messages._
 
 /** The pattern matching transform.
  *  After this phase, the only Match nodes remaining in the code are simple switches
@@ -908,7 +909,7 @@ object PatternMatcher {
           tpes.toSet.size: Int // without the type ascription, testPickling fails because of #2840.
         }
         if (numConsts(resultCases) < numConsts(original.cases))
-          ctx.warning(i"could not emit switch for @switch annotated match", original.pos)
+          ctx.warning(UnableToEmitSwitch(), original.pos)
       case _ =>
     }