Add explicit nulls attribute

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/tasty/AttributePickler.scala

@@ -14,10 +14,11 @@ object AttributePickler:
     pickler: TastyPickler,
     buf: TastyBuffer
   ): Unit =
-    if attributes.scala2StandardLibrary then // or any other attribute is set
+    if attributes.scala2StandardLibrary || attributes.explicitNulls then // or any other attribute is set
       pickler.newSection(AttributesSection, buf)
     // Pickle attributes
     if attributes.scala2StandardLibrary then buf.writeNat(TastyFormat.SCALA2STANDARDLIBRARYattr)
+    if attributes.explicitNulls then buf.writeNat(TastyFormat.EXPLICITNULLSattr)
 
   end pickleAttributes
 

compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala

@@ -17,13 +17,16 @@ class AttributeUnpickler(reader: TastyReader):
 
   lazy val attributes: Attributes = {
     var scala2StandardLibrary = false
+    var explicitNulls = false
     for attributeTag <- attributeTags do
       attributeTag match
         case TastyFormat.SCALA2STANDARDLIBRARYattr => scala2StandardLibrary = true
+        case TastyFormat.EXPLICITNULLSattr => explicitNulls = true
         case attribute =>
           assert(false, "Unexpected attribute value: " + attribute)
     Attributes(
       scala2StandardLibrary,
+      explicitNulls,
     )
   }
 

compiler/src/dotty/tools/dotc/core/tasty/Attributes.scala

@@ -2,4 +2,5 @@ package dotty.tools.dotc.core.tasty
 
 class Attributes(
   val scala2StandardLibrary: Boolean,
+  val explicitNulls: Boolean,
 )

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -102,6 +102,11 @@ class TreeUnpickler(reader: TastyReader,
   private val unpicklingScala2Library =
     attributeUnpicklerOpt.exists(_.attributes.scala2StandardLibrary)
 
+  /** This dependency was compiled with explicit nulls enabled */
+  // TODO Use this to tag the symbols of this dependency as compiled with explicit nulls (see use of unpicklingScala2Library).
+  private val explicitNulls =
+    attributeUnpicklerOpt.exists(_.attributes.explicitNulls)
+
   private def registerSym(addr: Addr, sym: Symbol) =
     symAtAddr(addr) = sym
 

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -110,6 +110,7 @@ class Pickler extends Phase {
 
           val attributes = Attributes(
             scala2StandardLibrary = ctx.settings.YcompileScala2Library.value,
+            explicitNulls = ctx.settings.YexplicitNulls.value,
           )
           AttributePickler.pickleAttributes(attributes, pickler, scratch.attributeBuffer)
 

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -271,6 +271,7 @@ Standard Section: "Comments" Comment*
 Standard Section: "Attributes" Attribute*
 ```none
   Attribute     = SCALA2STANDARDLIBRARYattr
+                  EXPLICITNULLSattr
 ```
 
 **************************************************************************************/
@@ -605,9 +606,10 @@ object TastyFormat {
   final val firstLengthTreeTag = PACKAGE
 
 
-  // Attribute tags
+  // Attributes tags
 
   final val SCALA2STANDARDLIBRARYattr = 1
+  final val EXPLICITNULLSattr = 2
 
   /** Useful for debugging */
   def isLegalTag(tag: Int): Boolean =
@@ -826,6 +828,7 @@ object TastyFormat {
 
   def attributeTagToString(tag: Int): String = tag match {
     case SCALA2STANDARDLIBRARYattr => "SCALA2STANDARDLIBRARYattr"
+    case EXPLICITNULLSattr => "EXPLICITNULLSattr"
   }
 
   /** @return If non-negative, the number of leading references (represented as nats) of a length/trees entry.