Trust case class unapplies to produce checkable type tests

Note that in the case where the unapply cannot match, exhaustivity
checker will issue a warning.

Author: abgruszecki

Diff for: compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -16,6 +16,7 @@ import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
 import config.Printers.patmatch
 import reporting.diagnostic.messages._
 import dotty.tools.dotc.ast._
+import util.Property.Key
 
 /** The pattern matching transform.
  *  After this phase, the only Match nodes remaining in the code are simple switches
@@ -52,6 +53,8 @@ object PatternMatcher {
   /** Minimal number of cases to emit a switch */
   final val MinSwitchCases = 4
 
+  object TrustedTypeTestKey extends Key[Unit]
+
   /** Was symbol generated by pattern matcher? */
   def isPatmatGenerated(sym: Symbol)(implicit ctx: Context): Boolean =
     sym.is(Synthetic) && sym.name.is(PatMatStdBinderName)
@@ -153,24 +156,24 @@ object PatternMatcher {
 
     /** The different kinds of tests */
     sealed abstract class Test
-    case class TypeTest(tpt: Tree) extends Test {                 // scrutinee.isInstanceOf[tpt]
+    case class TypeTest(tpt: Tree, trusted: Boolean) extends Test {  // scrutinee.isInstanceOf[tpt]
       override def equals(that: Any): Boolean = that match {
         case that: TypeTest => this.tpt.tpe =:= that.tpt.tpe
         case _ => false
       }
       override def hashCode: Int = tpt.tpe.hash
     }
-    case class EqualTest(tree: Tree) extends Test {               // scrutinee == tree
+    case class EqualTest(tree: Tree) extends Test {                  // scrutinee == tree
       override def equals(that: Any): Boolean = that match {
         case that: EqualTest => this.tree === that.tree
         case _ => false
       }
       override def hashCode: Int = tree.hash
     }
-    case class LengthTest(len: Int, exact: Boolean) extends Test  // scrutinee (== | >=) len
-    case object NonEmptyTest extends Test                         // !scrutinee.isEmpty
-    case object NonNullTest extends Test                          // scrutinee ne null
-    case object GuardTest extends Test                            // scrutinee
+    case class LengthTest(len: Int, exact: Boolean) extends Test     // scrutinee (== | >=) len
+    case object NonEmptyTest extends Test                            // !scrutinee.isEmpty
+    case object NonNullTest extends Test                             // scrutinee ne null
+    case object GuardTest extends Test                               // scrutinee
 
     // ------- Generating plans from trees ------------------------
 
@@ -352,7 +355,12 @@ object PatternMatcher {
       // begin patternPlan
       swapBind(tree) match {
         case Typed(pat, tpt) =>
-          TestPlan(TypeTest(tpt), scrutinee, tree.span,
+          val isTrusted = pat match {
+            case UnApply(extractor, _, _) =>
+              extractor.symbol.is(Synthetic) && extractor.symbol.owner.linkedClass.is(Case)
+            case _ => false
+          }
+          TestPlan(TypeTest(tpt, isTrusted), scrutinee, tree.span,
             letAbstract(ref(scrutinee).cast(tpt.tpe)) { casted =>
               nonNull += casted
               patternPlan(casted, pat, onSuccess)
@@ -685,7 +693,7 @@ object PatternMatcher {
               .select(defn.Seq_length.matchingMember(scrutinee.tpe))
               .select(if (exact) defn.Int_== else defn.Int_>=)
               .appliedTo(Literal(Constant(len)))
-        case TypeTest(tpt) =>
+        case TypeTest(tpt, trusted) =>
           val expectedTp = tpt.tpe
 
           // An outer test is needed in a situation like  `case x: y.Inner => ...`
@@ -716,6 +724,7 @@ object PatternMatcher {
               scrutinee.isInstance(expectedTp)  // will be translated to an equality test
             case _ =>
               val typeTest = scrutinee.select(defn.Any_typeTest).appliedToType(expectedTp)
+              if (trusted) typeTest.pushAttachment(TrustedTypeTestKey, ())
               if (outerTestNeeded) typeTest.and(outerTest) else typeTest
           }
       }
@@ -899,7 +908,7 @@ object PatternMatcher {
       val seen = mutable.Set[Int]()
       def showTest(test: Test) = test match {
         case EqualTest(tree) => i"EqualTest($tree)"
-        case TypeTest(tpt) => i"TypeTest($tpt)"
+        case TypeTest(tpt, trusted) => i"TypeTest($tpt, trusted=$trusted)"
         case _ => test.toString
       }
       def showPlan(plan: Plan): Unit =

Diff for: compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -298,7 +298,8 @@ object TypeTestsCasts {
 
         if (sym.isTypeTest) {
           val argType = tree.args.head.tpe
-          if (!checkable(expr.tpe, argType, tree.span))
+          val isTrusted = tree.getAttachment(PatternMatcher.TrustedTypeTestKey).nonEmpty
+          if (!isTrusted && !checkable(expr.tpe, argType, tree.span))
             ctx.warning(i"the type test for $argType cannot be checked at runtime", tree.sourcePos)
           transformTypeTest(expr, tree.args.head.tpe, flagUnrelated = true)
         }