Add fromNullable to Predef for explicit nulls (#20222)

The attempt to enable explicit nulls for community projects has shown
the usefulness of `fromNullable`,
so we decide to include it in the std library.

It is a helper function which can turn a nullable value into an option.

Author: sjrd

library/src-bootstrapped/scala/annotation/experimental.scala

@@ -5,6 +5,5 @@ package scala.annotation
  *  @see [[https://dotty.epfl.ch/docs/reference/other-new-features/experimental-defs]]
  *  @syntax markdown
  */
-@deprecatedInheritance("Scheduled for being final in the future", "3.4.0")
-class experimental(message: String) extends StaticAnnotation:
+final class experimental(message: String) extends StaticAnnotation:
   def this() = this("")

library/src-non-bootstrapped/scala/annotation/experimental.scala

@@ -1,4 +1,3 @@
 package scala.annotation
 
-@deprecatedInheritance("Scheduled for being final in the future", "3.4.0")
-class experimental extends StaticAnnotation
+final class experimental extends StaticAnnotation

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

@@ -62,4 +62,8 @@ object Predef:
      *  `eq` or `ne` methods, only `==` and `!=` inherited from `Any`. */
     inline def ne(inline y: AnyRef | Null): Boolean =
       !(x eq y)
+
+  extension (opt: Option.type)
+    @experimental
+    inline def fromNullable[T](t: T | Null): Option[T] = Option(t).asInstanceOf[Option[T]]
 end Predef

project/MiMaFilters.scala

@@ -9,6 +9,7 @@ object MiMaFilters {
       // Additions that require a new minor version of the library
       Build.mimaPreviousDottyVersion -> Seq(
         ProblemFilters.exclude[DirectMissingMethodProblem]("scala.annotation.experimental.this"),
+        ProblemFilters.exclude[FinalClassProblem]("scala.annotation.experimental"),
       ),
 
       // Additions since last LTS
@@ -56,6 +57,8 @@ object MiMaFilters {
           ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#MethodTypeModule.apply"),
           ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#MethodTypeMethods.methodTypeKind"),
           ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#MethodTypeMethods.isContextual"),
+          // Change `experimental` annotation to a final class
+          ProblemFilters.exclude[FinalClassProblem]("scala.annotation.experimental"),
         ),
 
       // Breaking changes since last LTS

tests/explicit-nulls/neg/from-nullable.scala

@@ -0,0 +1,6 @@
+import scala.annotation.experimental
+
+@experimental def testFromNullable =
+  val s: String | Null = "abc"
+  val sopt1: Option[String] = Option(s) // error
+  val sopt2: Option[String] = Option.fromNullable(s) // ok

tests/explicit-nulls/run/from-nullable.check

@@ -0,0 +1,2 @@
+hello
+None

tests/explicit-nulls/run/from-nullable.scala

@@ -0,0 +1,17 @@
+object Test:
+  import scala.annotation.experimental
+
+  @experimental def main(args: Array[String]): Unit =
+    val s1: String | Null = "hello"
+    val s2: String | Null = null
+
+    val opts1: Option[String] = Option.fromNullable(s1)
+    val opts2: Option[String] = Option.fromNullable(s2)
+
+    opts1 match
+      case Some(s) => println(s)
+      case None => println("None")
+
+    opts2 match
+      case Some(s) => println(s)
+      case None => println("None")

tests/neg/experimentalExperimental.scala

@@ -0,0 +1 @@
+class MyExperimentalAnnot extends scala.annotation.experimental // error

tests/pos/experimentalExperimental.scala

@@ -76,6 +76,9 @@ val experimentalDefinitionInLibrary = Set(
   "scala.Tuple$.Reverse", // can be stabilized in 3.5
   "scala.Tuple$.ReverseOnto", // can be stabilized in 3.5
   "scala.runtime.Tuples$.reverse", // can be stabilized in 3.5
+
+  // New feature: fromNullable for explicit nulls
+  "scala.Predef$.fromNullable",
 )