add method, annotation and test cases

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1034,6 +1034,7 @@ class Definitions {
   @tu lazy val TransparentTraitAnnot: ClassSymbol = requiredClass("scala.annotation.transparentTrait")
   @tu lazy val NativeAnnot: ClassSymbol = requiredClass("scala.native")
   @tu lazy val RepeatedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Repeated")
+  @tu lazy val RuntimeCheckedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.RuntimeChecked")
   @tu lazy val SourceFileAnnot: ClassSymbol = requiredClass("scala.annotation.internal.SourceFile")
   @tu lazy val ScalaSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaSignature")
   @tu lazy val ScalaLongSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaLongSignature")

compiler/src/dotty/tools/dotc/semanticdb/ExtractSemanticDB.scala

@@ -458,14 +458,15 @@ object ExtractSemanticDB:
       def unapply(tree: ValDef)(using Context): Option[(Tree, Tree)] = tree.rhs match
 
         case Match(Typed(selected: Tree, tpt: TypeTree), CaseDef(pat: Tree, _, _) :: Nil)
-        if tpt.span.exists && !tpt.span.hasLength && tpt.tpe.isAnnotatedByUnchecked =>
+        if tpt.span.exists && !tpt.span.hasLength && tpt.tpe.isAnnotatedByUncheckedOrRuntimeChecked =>
           Some((pat, selected))
 
         case _ => None
 
       extension (tpe: Types.Type)
-        private inline def isAnnotatedByUnchecked(using Context) = tpe match
-          case Types.AnnotatedType(_, annot) => annot.symbol == defn.UncheckedAnnot
+        private inline def isAnnotatedByUncheckedOrRuntimeChecked(using Context) = tpe match
+          case Types.AnnotatedType(_, annot) =>
+            annot.symbol == defn.UncheckedAnnot || annot.symbol == defn.RuntimeCheckedAnnot
           case _                             => false
 
       def collectPats(pat: Tree): List[Tree] =

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -794,6 +794,7 @@ object SpaceEngine {
       }
 
     !sel.tpe.hasAnnotation(defn.UncheckedAnnot)
+    && !sel.tpe.hasAnnotation(defn.RuntimeCheckedAnnot)
     && {
       ctx.settings.YcheckAllPatmat.value
       || isCheckable(sel.tpe)
@@ -903,7 +904,7 @@ object SpaceEngine {
   def checkMatch(m: Match)(using Context): Unit =
     checkMatchExhaustivityOnly(m)
     if reachabilityCheckable(m.selector) then checkReachability(m)
-  
+
   def checkMatchExhaustivityOnly(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
 }

docs/_docs/reference/experimental/runtimeChecked.md

@@ -0,0 +1,125 @@
+---
+layout: doc-page
+title: "The runtimeChecked method"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/experimental/runtimeChecked.html
+---
+
+The `runtimeChecked` method is an extension method, defined in `scala.Predef`. It can be called on any expression. An expression marked as `runtimeChecked` is exempt from certain static checks in the compiler, for example pattern match exhaustivity. It is intended to replace `: @unchecked` type ascription in these cases.
+
+## Example
+
+A common use case for `runtimeChecked` is to assert that a pattern will always match, either for convenience, or because there is a known invariant that the types can not express.
+
+e.g. looking up an expected entry in a dynamically loaded dictionary-like structure
+```scala
+// example 1
+trait AppConfig:
+  def get(key: String): Option[String]
+
+val config: AppConfig = ???
+
+val Some(appVersion) = config.get("appVersion").runtimeChecked
+```
+
+or to assert that a value can only match some specific patterns:
+```scala
+// example 2
+enum Day:
+  case Mon, Tue, Wed, Thu, Fri, Sat, Sun
+
+val weekDay: Option[Day] = ???
+
+weekDay.runtimeChecked match
+  case Some(Mon | Tue | Wed | Thu | Fri) => println("got weekday")
+// case Some(Sat | Sun) => // weekend should not appear
+  case None =>
+```
+
+In both of these cases, without `runtimeChecked` then there would either be an error (example 1), or a warning (example 2), because statically, the compiler knows that there could be other cases at runtime - so is right to caution the programmer.
+
+```scala
+// warning in example 2 when we don't add `.runtimeChecked`.
+-- [E029] Pattern Match Exhaustivity Warning: ----------------------------------
+6 |weekDay match
+  |^^^^^^^
+  |match may not be exhaustive.
+  |
+  |It would fail on pattern case: Some(Sat), Some(Sun)
+```
+
+## Safety
+
+The `runtimeChecked` method only turns off static checks that can be soundly performed at runtime. This means that patterns with unchecked type-tests will still generate warnings. For example:
+```scala
+scala> val xs = List(1: Any)
+     | xs.runtimeChecked match {
+     |   case is: ::[Int] => is.head
+     | }
+1 warning found
+-- Unchecked Warning: ----------------------------------------------------------
+3 |  case is: ::[Int] => is.head
+  |       ^
+  |the type test for ::[Int] cannot be checked at runtime because its type arguments can't be determined from List[Any]
+val res0: Int = 1
+```
+As the warning hints, `::[Int]` can not be tested at runtime on a value of type `List[Any]`, so using `runtimeChecked` still protects the user against assertions that can not be validated.
+
+To fully avoid warnings, as with previous Scala versions, `@unchecked` should be put on the type argument:
+```scala
+scala> xs.runtimeChecked match {
+     |   case is: ::[Int @unchecked] => is.head
+     | }
+val res1: Int = 1
+```
+
+
+## Specification
+
+We add a new annotation `scala.internal.RuntimeChecked`, this is part of the standard Scala 3 library. A programmer is not expected to use this annotation directly.
+
+```scala
+package scala.annotation.internal
+
+@experimental
+final class RuntimeChecked extends Annotation
+```
+
+Any term that is the scrutinee of a pattern match, that has a type annotated with `RuntimeChecked`, is exempt from pattern match exhaustivity checking.
+
+
+The user facing API is provided by a new extension method `scala.Predef.runtimeChecked`, qualified for any value:
+```scala
+extension (x: Any)
+  @experimental
+  inline def runtimeChecked: x.type @RuntimeChecked = x: @RuntimeChecked
+```
+
+The `runtimeChecked` method returns its argument, refining its type with the `RuntimeChecked` annotation.
+
+## Motivation
+
+As described in [Pattern Bindings](../changed-features/pattern-bindings.md), under `-source:future` it is an error for a pattern definition to be refutable. For instance, consider:
+```scala
+def xs: List[Any] = ???
+val y :: ys = xs
+```
+
+This compiled without warning in 3.0, became a warning in 3.2, and we would like to make it an error by default in a future 3.x version.
+As an escape hatch in 3.2 we recommended to use a type ascription of `: @unchecked`:
+```
+-- Warning: ../../new/test.scala:6:16 ------------------------------------------
+6 |  val y :: ys = xs
+  |                ^^
+  |pattern's type ::[Any] is more specialized than the right hand side expression's type List[Any]
+  |
+  |If the narrowing is intentional, this can be communicated by adding `: @unchecked` after the expression,
+  |which may result in a MatchError at runtime.
+```
+
+We suggest that `: @unchecked` is syntactically awkward, and also a misnomer - in fact in this case the the pattern is fully checked, but the necessary checks occur at runtime. The `runtimeChecked` method is then a successor to `@unchecked` for this purpose.
+
+We propose that `@unchecked` will still be necessary for silencing warnings on unsound type tests.
+
+### Restoring Scala 2.13 semantics with runtimeChecked
+
+In Scala 3, the `: @unchecked` type ascription has the effect of turning off all pattern-match warnings on the match scrutinee - this differs from 2.13 in which it strictly turns off only pattern exhaustivity checking. `runtimeChecked` restores the semantics of Scala 2.13.

docs/sidebar.yml

@@ -157,6 +157,7 @@ subsection:
           - page: reference/experimental/named-tuples.md
           - page: reference/experimental/modularity.md
           - page: reference/experimental/typeclasses.md
+          - page: reference/experimental/runtimeChecked.md
       - page: reference/syntax.md
       - title: Language Versions
         index: reference/language-versions/language-versions.md

library/src/scala/annotation/internal/RuntimeChecked.scala

@@ -0,0 +1,11 @@
+package scala.annotation.internal
+
+import scala.annotation.Annotation
+import scala.annotation.experimental
+
+/**An annotation marking an intention that all checks on a value can be reliably performed at runtime.
+ *
+ * The compiler will remove certain static checks except those that can't be performed at runtime.
+ */
+@experimental
+final class RuntimeChecked() extends Annotation

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -1,6 +1,7 @@
 package scala.runtime.stdLibPatches
 
 import scala.annotation.experimental
+import scala.annotation.internal.RuntimeChecked
 
 object Predef:
   import compiletime.summonFrom
@@ -80,4 +81,16 @@ object Predef:
   @experimental
   infix type is[A <: AnyKind, B <: Any{type Self <: AnyKind}] = B { type Self = A }
 
+  extension (x: Any)
+    /**Asserts that a term should be exempt from static checks that can be reliably checked at runtime.
+     * @example {{{
+     * val xs: Option[Int] = Some(1)
+     * xs.runtimeChecked match {
+     *   case is: Some[Int] => is.get
+     * } // no warning about exhaustiveness, as all patterns can be checked at runtime.
+     * }}}
+     */
+    @experimental
+    inline def runtimeChecked: x.type @RuntimeChecked = x: @RuntimeChecked
+
 end Predef

tests/neg/runtimeChecked-2.check

@@ -0,0 +1,5 @@
+-- [E030] Match case Unreachable Warning: tests/neg/runtimeChecked-2.scala:13:11 ---------------------------------------
+13 |      case is: Some[t] => ??? // unreachable
+   |           ^^^^^^^^^^^
+   |           Unreachable case
+No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked-2.scala

@@ -0,0 +1,16 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+@experimental
+object Foo {
+
+  val xs: Option[Int] = Some(1)
+
+  def test: Int =
+    xs.runtimeChecked match { // this test asserts that reachability is not avoided by runtimeChecked
+      case is: Some[t] => is.get
+      case is: Some[t] => ??? // unreachable
+    }
+}
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked.check

@@ -0,0 +1,7 @@
+-- [E092] Pattern Match Unchecked Warning: tests/neg/runtimeChecked.scala:14:11 ----------------------------------------
+14 |      case is: ::[Int/* can not be checked so still err */] => is.head
+   |           ^
+   |the type test for ::[Int] cannot be checked at runtime because its type arguments can't be determined from List[Any]
+   |
+   | longer explanation available when compiling with `-explain`
+No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked.scala

@@ -0,0 +1,17 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+@experimental
+object Foo {
+
+  val xs: List[Any] = List(1: Any)
+
+  def test: Int =
+    xs.runtimeChecked match { // this test asserts that unsound type tests still require @unchecked
+      // tests/run/runtimeChecked.scala adds @unchecked to the
+      // unsound type test to avoid the warning.
+      case is: ::[Int/* can not be checked so still err */] => is.head
+    }
+}
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -85,6 +85,9 @@ val experimentalDefinitionInLibrary = Set(
   "scala.annotation.internal.WitnessNames",
   "scala.compiletime.package$package$.deferred",
   "scala.runtime.stdLibPatches.Predef$.is",
+
+  // New feature: SIP 57 - runtimeChecked replacement of @unchecked
+  "scala.Predef$.runtimeChecked", "scala.annotation.internal.RuntimeChecked"
 )
 
 

tests/run/runtimeChecked.scala

@@ -0,0 +1,16 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+
+val xs: List[Any] = List(1: Any)
+
+@experimental
+@main
+def Test: Unit =
+  val head = xs.runtimeChecked match {
+    // tests/neg/runtimeChecked.scala asserts that @unchecked is
+    // still needed for unsound type tests.
+    case is: ::[Int @unchecked] => is.head
+  }
+  assert(head == 1)