Add nullTrackable annotation to force tracking mutale fields

Author: noti0na1

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

@@ -1101,6 +1101,7 @@ class Definitions {
   @tu lazy val RetainsByNameAnnot: ClassSymbol = requiredClass("scala.annotation.retainsByName")
   @tu lazy val PublicInBinaryAnnot: ClassSymbol = requiredClass("scala.annotation.publicInBinary")
   @tu lazy val WitnessNamesAnnot: ClassSymbol = requiredClass("scala.annotation.internal.WitnessNames")
+  @tu lazy val NullTrackableAnnot: ClassSymbol = requiredClass("scala.annotation.nullTrackable")
 
   @tu lazy val JavaRepeatableAnnot: ClassSymbol = requiredClass("java.lang.annotation.Repeatable")
 

compiler/src/dotty/tools/dotc/typer/Nullables.scala

@@ -186,15 +186,22 @@ object Nullables:
    *  Check `usedOutOfOrder` to see the explaination and example of "out of order".
    *  See more examples in `tests/explicit-nulls/neg/var-ref-in-closure.scala`.
    */
-  def isTracked(ref: TermRef)(using Context) =
+  def isTracked(ref: TermRef)(using Context) = // true
+    val sym = ref.symbol
+
+    def isNullTrackableField: Boolean =
+      ref.prefix.isStable
+      && sym.isField
+      && sym.hasAnnotation(defn.NullTrackableAnnot)
+
+    // println(s"isTracked: $ref, usedOutOfOrder = ${ref.usedOutOfOrder}, isStable = ${ref.isStable}, span = ${ref.symbol.span}, assignmentSpans = ${ctx.compilationUnit.assignmentSpans.get(ref.symbol.span.start)}")
     ref.isStable
-    || { val sym = ref.symbol
-         val unit = ctx.compilationUnit
+    || isNullTrackableField
+    || { val unit = ctx.compilationUnit
          !ref.usedOutOfOrder
          && sym.span.exists
          && (unit ne NoCompilationUnit) // could be null under -Ytest-pickler
-         && unit.assignmentSpans.contains(sym.span.start)
-      }
+         && unit.assignmentSpans.contains(sym.span.start) }
 
   /** The nullability context to be used after a case that matches pattern `pat`.
    *  If `pat` is `null`, this will assert that the selector `sel` is not null afterwards.

library/src/scala/annotation/nullTrackable.scala

@@ -0,0 +1,3 @@
+package scala.annotation
+
+final class nullTrackable extends StaticAnnotation

tests/explicit-nulls/pos/force-track-var-fields.scala

@@ -0,0 +1,14 @@
+import scala.annotation.nullTrackable
+
+class A:
+  @nullTrackable var s: String | Null = null
+  def getS: String =
+    if s == null then s = ""
+    s
+
+def test(a: A): String =
+  if a.s == null then
+    a.s = ""
+    a.s
+  else
+    a.s