Add mode bit to track whether we are in safe nulls mode

The bit is set if

 - -Yexplicit-nulls is set
 - we are not in an `import.unsafeNulls` scope

Author: odersky

compiler/src/dotty/tools/dotc/Run.scala

@@ -71,11 +71,13 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       .setPeriod(Period(comp.nextRunId, FirstPhaseId))
       .setScope(rootScope)
     rootScope.enter(ctx.definitions.RootPackage)(using bootstrap)
-    val start = bootstrap.fresh
+    var start = bootstrap.fresh
       .setOwner(defn.RootClass)
       .setTyper(new Typer)
       .addMode(Mode.ImplicitsEnabled)
       .setTyperState(ctx.typerState.fresh(ctx.reporter))
+    if ctx.settings.YexplicitNulls.value then
+      start = start.addMode(Mode.SafeNulls)
     ctx.initialize()(using start) // re-initialize the base context with start
     start.setRun(this)
   }

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

@@ -27,6 +27,7 @@ import collection.mutable
 import printing._
 import config.{JavaPlatform, SJSPlatform, Platform, ScalaSettings}
 import classfile.ReusableDataReader
+import StdNames.nme
 
 import scala.annotation.internal.sharable
 
@@ -627,7 +628,14 @@ object Contexts {
     def setRun(run: Run): this.type = updateStore(runLoc, run)
     def setProfiler(profiler: Profiler): this.type = updateStore(profilerLoc, profiler)
     def setNotNullInfos(notNullInfos: List[NotNullInfo]): this.type = updateStore(notNullInfosLoc, notNullInfos)
-    def setImportInfo(importInfo: ImportInfo): this.type = updateStore(importInfoLoc, importInfo)
+    def setImportInfo(importInfo: ImportInfo): this.type =
+      importInfo.mentionsFeature(nme.unsafeNulls) match
+        case Some(true) =>
+          setMode(this.mode &~ Mode.SafeNulls)
+        case Some(false) if ctx.settings.YexplicitNulls.value =>
+          setMode(this.mode | Mode.SafeNulls)
+        case _ =>
+      updateStore(importInfoLoc, importInfo)
     def setTypeAssigner(typeAssigner: TypeAssigner): this.type = updateStore(typeAssignerLoc, typeAssigner)
 
     def setProperty[T](key: Key[T], value: T): this.type =

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

@@ -119,4 +119,7 @@ object Mode {
 
   /** Are we resolving a TypeTest node? */
   val InTypeTest: Mode = newMode(27, "InTypeTest")
+
+  /** Are we enforcing null safety */
+  val SafeNulls = newMode(28, "SafeNulls")
 }

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

@@ -613,6 +613,7 @@ object StdNames {
     val unapplySeq: N           = "unapplySeq"
     val unbox: N                = "unbox"
     val universe: N             = "universe"
+    val unsafeNulls: N          = "unsafeNulls"
     val update: N               = "update"
     val updateDynamic: N        = "updateDynamic"
     val using: N                = "using"

library/src/scala/runtime/stdLibPatches/language.scala

@@ -77,6 +77,8 @@ object language:
    */
   object adhocExtensions
 
+  object unsafeNulls
+
   /** Set source version to 3.0-migration.
     *
     * @see [[https://scalacenter.github.io/scala-3-migration-guide/docs/scala-3-migration-mode]]