Nullability Analysis without NotNull

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

@@ -5,11 +5,13 @@ import util.SourceFile
 import ast.{tpd, untpd}
 import tpd.{Tree, TreeTraverser}
 import typer.PrepareInlineable.InlineAccessors
+import typer.Nullables
 import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.SymDenotations.ClassDenotation
 import dotty.tools.dotc.core.Symbols._
 import dotty.tools.dotc.transform.SymUtils._
 import util.{NoSource, SourceFile}
+import util.Spans.Span
 import core.Decorators._
 
 class CompilationUnit protected (val source: SourceFile) {
@@ -42,6 +44,16 @@ class CompilationUnit protected (val source: SourceFile) {
       suspended = true
       ctx.run.suspendedUnits += this
     throw CompilationUnit.SuspendException()
+
+  private var myAssignmentSpans: Map[Int, List[Span]] = null
+
+  /** A map from (name-) offsets of all local variables in this compilation unit
+   *  that can be tracked for being not null to the list of spans of assignments
+   *  to these variables.
+   */
+  def assignmentSpans(given Context): Map[Int, List[Span]] =
+    if myAssignmentSpans == null then myAssignmentSpans = Nullables.assignmentSpans
+    myAssignmentSpans
 }
 
 object CompilationUnit {

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -88,6 +88,12 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
   /** If this is a block, its expression part */
   def stripBlock(tree: Tree): Tree = unsplice(tree) match {
     case Block(_, expr) => stripBlock(expr)
+    case Inlined(_, _, expr) => stripBlock(expr)
+    case _ => tree
+  }
+
+  def stripInlined(tree: Tree): Tree = unsplice(tree) match {
+    case Inlined(_, _, expr) => stripInlined(expr)
     case _ => tree
   }
 
@@ -391,7 +397,9 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       if (fn.symbol.is(Erased) || fn.symbol == defn.InternalQuoted_typeQuote) Pure else exprPurity(fn)
     case Apply(fn, args) =>
       def isKnownPureOp(sym: Symbol) =
-        sym.owner.isPrimitiveValueClass || sym.owner == defn.StringClass
+        sym.owner.isPrimitiveValueClass
+        || sym.owner == defn.StringClass
+        || defn.pureMethods.contains(sym)
       if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
           || (fn.symbol.isStableMember && !fn.symbol.is(Lazy))
           || fn.symbol.isPrimaryConstructor && fn.symbol.owner.isNoInitsClass) // TODO: include in isStable?

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -711,11 +711,19 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
   class TimeTravellingTreeCopier extends TypedTreeCopier {
     override def Apply(tree: Tree)(fun: Tree, args: List[Tree])(implicit ctx: Context): Apply =
-      ta.assignType(untpdCpy.Apply(tree)(fun, args), fun, args)
+      tree match
+        case tree: Apply
+        if (tree.fun eq fun) && (tree.args eq args)
+           && tree.tpe.isInstanceOf[ConstantType]
+           && isPureExpr(tree) => tree
+        case _ =>
+          ta.assignType(untpdCpy.Apply(tree)(fun, args), fun, args)
       // Note: Reassigning the original type if `fun` and `args` have the same types as before
-      // does not work here: The computed type depends on the widened function type, not
-      // the function type itself. A treetransform may keep the function type the
+      // does not work here in general: The computed type depends on the widened function type, not
+      // the function type itself. A tree transform may keep the function type the
       // same but its widened type might change.
+      // However, we keep constant types of pure expressions. This uses the underlying assumptions
+      // that pure functions yielding a constant will not change in later phases.
 
     override def TypeApply(tree: Tree)(fun: Tree, args: List[Tree])(implicit ctx: Context): TypeApply =
       ta.assignType(untpdCpy.TypeApply(tree)(fun, args), fun, args)

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -162,6 +162,7 @@ class ScalaSettings extends Settings.SettingGroup {
 
   // Extremely experimental language features
   val YnoKindPolymorphism: Setting[Boolean] = BooleanSetting("-Yno-kind-polymorphism", "Enable kind polymorphism (see https://dotty.epfl.ch/docs/reference/kind-polymorphism.html). Potentially unsound.")
+  val YexplicitNulls: Setting[Boolean] = BooleanSetting("-Yexplicit-nulls", "Make reference types non-nullable. Nullable types can be expressed with unions: e.g. String|Null.")
 
   /** Area-specific debug output */
   val YexplainLowlevel: Setting[Boolean] = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

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

@@ -15,7 +15,8 @@ import ast.Trees._
 import ast.untpd
 import Flags.GivenOrImplicit
 import util.{FreshNameCreator, NoSource, SimpleIdentityMap, SourceFile}
-import typer.{Implicits, ImportInfo, Inliner, NamerContextOps, SearchHistory, SearchRoot, TypeAssigner, Typer}
+import typer.{Implicits, ImportInfo, Inliner, NamerContextOps, SearchHistory, SearchRoot, TypeAssigner, Typer, Nullables}
+import Nullables.{NotNullInfo, given}
 import Implicits.ContextualImplicits
 import config.Settings._
 import config.Config
@@ -47,7 +48,11 @@ object Contexts {
   private val (compilationUnitLoc,  store6) = store5.newLocation[CompilationUnit]()
   private val (runLoc,              store7) = store6.newLocation[Run]()
   private val (profilerLoc,         store8) = store7.newLocation[Profiler]()
-  private val initialStore = store8
+  private val (notNullInfosLoc,     store9) = store8.newLocation[List[NotNullInfo]]()
+  private val initialStore = store9
+
+  /** The current context */
+  def curCtx(given ctx: Context): Context = ctx
 
   /** A context is passed basically everywhere in dotc.
    *  This is convenient but carries the risk of captured contexts in
@@ -207,6 +212,9 @@ object Contexts {
     /**  The current compiler-run profiler */
     def profiler: Profiler = store(profilerLoc)
 
+    /** The paths currently known to be not null */
+    def notNullInfos = store(notNullInfosLoc)
+
     /** The new implicit references that are introduced by this scope */
     protected var implicitsCache: ContextualImplicits = null
     def implicits: ContextualImplicits = {
@@ -556,6 +564,7 @@ object Contexts {
     def setRun(run: Run): this.type = updateStore(runLoc, run)
     def setProfiler(profiler: Profiler): this.type = updateStore(profilerLoc, profiler)
     def setFreshNames(freshNames: FreshNameCreator): this.type = updateStore(freshNamesLoc, freshNames)
+    def setNotNullInfos(notNullInfos: List[NotNullInfo]): this.type = updateStore(notNullInfosLoc, notNullInfos)
 
     def setProperty[T](key: Key[T], value: T): this.type =
       setMoreProperties(moreProperties.updated(key, value))
@@ -587,6 +596,17 @@ object Contexts {
     def setDebug: this.type = setSetting(base.settings.Ydebug, true)
   }
 
+  given (c: Context)
+    def addNotNullInfo(info: NotNullInfo) =
+      c.withNotNullInfos(c.notNullInfos.extendWith(info))
+
+    def addNotNullRefs(refs: Set[TermRef]) =
+      c.addNotNullInfo(NotNullInfo(refs, Set()))
+
+    def withNotNullInfos(infos: List[NotNullInfo]): Context =
+      if c.notNullInfos eq infos then c else c.fresh.setNotNullInfos(infos)
+
+  // TODO: Fix issue when converting ModeChanges and FreshModeChanges to extension givens
   implicit class ModeChanges(val c: Context) extends AnyVal {
     final def withModeBits(mode: Mode): Context =
       if (mode != c.mode) c.fresh.setMode(mode) else c
@@ -615,7 +635,9 @@ object Contexts {
     typeAssigner = TypeAssigner
     moreProperties = Map.empty
     source = NoSource
-    store = initialStore.updated(settingsStateLoc, settingsGroup.defaultState)
+    store = initialStore
+      .updated(settingsStateLoc, settingsGroup.defaultState)
+      .updated(notNullInfosLoc, Nil)
     typeComparer = new TypeComparer(this)
     searchHistory = new SearchRoot
     gadt = EmptyGadtConstraint

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

@@ -310,6 +310,10 @@ class Definitions {
     def ObjectMethods: List[TermSymbol] = List(Object_eq, Object_ne, Object_synchronized, Object_clone,
         Object_finalize, Object_notify, Object_notifyAll, Object_wait, Object_waitL, Object_waitLI)
 
+  /** Methods in Object and Any that do not have a side effect */
+  @tu lazy val pureMethods: List[TermSymbol] = List(Any_==, Any_!=, Any_equals, Any_hashCode,
+    Any_toString, Any_##, Any_getClass, Any_isInstanceOf, Any_typeTest, Object_eq, Object_ne)
+
   @tu lazy val AnyKindClass: ClassSymbol = {
     val cls = ctx.newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil)
     if (!ctx.settings.YnoKindPolymorphism.value)

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

@@ -195,6 +195,7 @@ object StdNames {
     final val ExprApi: N             = "ExprApi"
     final val Mirror: N              = "Mirror"
     final val Nothing: N             = "Nothing"
+    final val NotNull: N             = "NotNull"
     final val Null: N                = "Null"
     final val Object: N              = "Object"
     final val Product: N             = "Product"
@@ -261,6 +262,7 @@ object StdNames {
     val MIRROR_PREFIX: N            = "$m."
     val MIRROR_SHORT: N             = "$m"
     val MIRROR_UNTYPED: N           = "$m$untyped"
+    val NOT_NULL: N                 = "$nn"
     val REIFY_FREE_PREFIX: N        = "free$"
     val REIFY_FREE_THIS_SUFFIX: N   = "$this"
     val REIFY_FREE_VALUE_SUFFIX: N  = "$value"

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

@@ -1838,7 +1838,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
     else if (!tp2.exists) tp1
     else tp.derivedAndType(tp1, tp2)
 
-  /** If some (&-operand of) this type is a supertype of `sub` replace it with `NoType`.
+  /** If some (&-operand of) `tp` is a supertype of `sub` replace it with `NoType`.
    */
   private def dropIfSuper(tp: Type, sub: Type): Type =
     if (isSubTypeWhenFrozen(sub, tp)) NoType

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

@@ -81,7 +81,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
                   // called which we override to set the `approximated` flag.
                   range(defn.NothingType, pre)
               else pre
-            else if ((pre.termSymbol is Package) && !(thiscls is Package))
+            else if (pre.termSymbol.is(Package) && !thiscls.is(Package))
               toPrefix(pre.select(nme.PACKAGE), cls, thiscls)
             else
               toPrefix(pre.baseType(cls).normalizedPrefix, cls.owner, thiscls)

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

@@ -1068,8 +1068,9 @@ object Types {
      *  instead of `ArrayBuffer[? >: Int | A <: Int & A]`
      */
     def widenUnion(implicit ctx: Context): Type = widen match {
-      case OrType(tp1, tp2) =>
-        ctx.typeComparer.lub(tp1.widenUnion, tp2.widenUnion, canConstrain = true) match {
+      case tp @ OrType(tp1, tp2) =>
+        if tp1.isNull || tp2.isNull then tp
+        else ctx.typeComparer.lub(tp1.widenUnion, tp2.widenUnion, canConstrain = true) match {
           case union: OrType => union.join
           case res => res
         }
@@ -1399,6 +1400,11 @@ object Types {
       case _ => true
     }
 
+    /** Is this (an alias of) the `scala.Null` type? */
+    final def isNull(given Context) =
+      isRef(defn.NullClass)
+      || classSymbol.name == tpnme.Null // !!! temporary kludge for being able to test without the explicit nulls PR
+
     /** The resultType of a LambdaType, or ExprType, the type itself for others */
     def resultType(implicit ctx: Context): Type = this
 
@@ -2293,7 +2299,7 @@ object Types {
   }
 
   /** The singleton type for path prefix#myDesignator.
-    */
+   */
   abstract case class TermRef(override val prefix: Type,
                               private var myDesignator: Designator)
     extends NamedType with SingletonType with ImplicitRef {
@@ -2886,6 +2892,24 @@ object Types {
       else apply(tp1, tp2)
   }
 
+  /** An extractor for `T | Null` or `Null | T`, returning the `T` */
+  object OrNull with
+    private def stripNull(tp: Type)(given Context): Type = tp match
+      case tp @ OrType(tp1, tp2) =>
+        if tp1.isNull then tp2
+        else if tp2.isNull then tp1
+        else tp.derivedOrType(stripNull(tp1), stripNull(tp2))
+      case tp @ AndType(tp1, tp2) =>
+        tp.derivedAndType(stripNull(tp1), stripNull(tp2))
+      case _ =>
+        tp
+    def apply(tp: Type)(given Context) =
+      OrType(tp, defn.NullType)
+    def unapply(tp: Type)(given Context): Option[Type] =
+      val tp1 = stripNull(tp)
+      if tp1 ne tp then Some(tp1) else None
+  end OrNull
+
   // ----- ExprType and LambdaTypes -----------------------------------
 
   // Note: method types are cached whereas poly types are not. The reason

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -207,7 +207,7 @@ object Completion {
     def addMemberCompletions(qual: Tree)(implicit ctx: Context): Unit =
       if (!qual.tpe.widenDealias.isBottomType) {
         addAccessibleMembers(qual.tpe)
-        if (!mode.is(Mode.Import) && !qual.tpe.isRef(defn.NullClass))
+        if (!mode.is(Mode.Import) && !qual.tpe.isNull)
           // Implicit conversions do not kick in when importing
           // and for `NullClass` they produce unapplicable completions (for unclear reasons)
           implicitConversionTargets(qual)(ctx.fresh.setExploreTyperState())

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

@@ -740,11 +740,12 @@ object Erasure {
     override def typedAnnotated(tree: untpd.Annotated, pt: Type)(implicit ctx: Context): Tree =
       typed(tree.arg, pt)
 
-    override def typedStats(stats: List[untpd.Tree], exprOwner: Symbol)(implicit ctx: Context): List[Tree] = {
+    override def typedStats(stats: List[untpd.Tree], exprOwner: Symbol)(implicit ctx: Context): (List[Tree], Context) = {
       val stats1 =
         if (takesBridges(ctx.owner)) new Bridges(ctx.owner.asClass, erasurePhase).add(stats)
         else stats
-      super.typedStats(stats1, exprOwner).filter(!_.isEmpty)
+      val (stats2, finalCtx) = super.typedStats(stats1, exprOwner)
+      (stats2.filter(!_.isEmpty), finalCtx)
     }
 
     override def adapt(tree: Tree, pt: Type, locked: TypeVars)(implicit ctx: Context): Tree =

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

@@ -160,8 +160,9 @@ class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
     constToLiteral(tree)
 
   override def transformIf(tree: If)(implicit ctx: Context): Tree =
-    tree.cond match {
-      case Literal(Constant(c: Boolean)) => if (c) tree.thenp else tree.elsep
+    tree.cond.tpe match {
+      case ConstantType(Constant(c: Boolean)) if isPureExpr(tree.cond) =>
+        if (c) tree.thenp else tree.elsep
       case _ => tree
     }
 

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

@@ -434,9 +434,9 @@ class TreeChecker extends Phase with SymTransformer {
         }
       }
 
-    override def typedCase(tree: untpd.CaseDef, selType: Type, pt: Type)(implicit ctx: Context): CaseDef =
+    override def typedCase(tree: untpd.CaseDef, sel: Tree, selType: Type, pt: Type)(implicit ctx: Context): CaseDef =
       withPatSyms(tpd.patVars(tree.pat.asInstanceOf[tpd.Tree])) {
-        super.typedCase(tree, selType, pt)
+        super.typedCase(tree, sel, selType, pt)
       }
 
     override def typedClosure(tree: untpd.Closure, pt: Type)(implicit ctx: Context): Tree = {
@@ -466,7 +466,7 @@ class TreeChecker extends Phase with SymTransformer {
      *  is that we should be able to pull out an expression as an initializer
      *  of a helper value without having to do a change owner traversal of the expression.
      */
-    override def typedStats(trees: List[untpd.Tree], exprOwner: Symbol)(implicit ctx: Context): List[Tree] = {
+    override def typedStats(trees: List[untpd.Tree], exprOwner: Symbol)(implicit ctx: Context): (List[Tree], Context) = {
       for (tree <- trees) tree match {
         case tree: untpd.DefTree => checkOwner(tree)
         case _: untpd.Thicket => assert(false, i"unexpanded thicket $tree in statement sequence $trees%\n%")

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

@@ -22,6 +22,7 @@ import NameKinds.DefaultGetterName
 import ProtoTypes._
 import Inferencing._
 import transform.TypeUtils._
+import Nullables.given
 
 import collection.mutable
 import config.Printers.{overload, typr, unapp}
@@ -864,8 +865,9 @@ trait Applications extends Compatibility {
               if (proto.allArgTypesAreCurrent())
                 new ApplyToTyped(tree, fun1, funRef, proto.unforcedTypedArgs, pt)
               else
-                new ApplyToUntyped(tree, fun1, funRef, proto, pt)(argCtx(tree))
-            convertNewGenericArray(app.result)
+                new ApplyToUntyped(tree, fun1, funRef, proto, pt)(
+                  given fun1.nullableInArgContext(given argCtx(tree)))
+            convertNewGenericArray(app.result).computeNullable()
           case _ =>
             handleUnexpectedFunType(tree, fun1)
         }

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

@@ -11,6 +11,7 @@ import Constants._
 import Names._
 import StdNames._
 import Contexts._
+import Nullables.{CompareNull, TrackedRef}
 
 object ConstFold {
 
@@ -19,15 +20,17 @@ object ConstFold {
   /** If tree is a constant operation, replace with result. */
   def apply[T <: Tree](tree: T)(implicit ctx: Context): T = finish(tree) {
     tree match {
+      case CompareNull(TrackedRef(ref), testEqual)
+      if ctx.settings.YexplicitNulls.value && ctx.notNullInfos.impliesNotNull(ref) =>
+        // TODO maybe drop once we have general Nullability?
+        Constant(!testEqual)
       case Apply(Select(xt, op), yt :: Nil) =>
-        xt.tpe.widenTermRefExpr.normalized match {
+        xt.tpe.widenTermRefExpr.normalized match
           case ConstantType(x) =>
-            yt.tpe.widenTermRefExpr match {
+            yt.tpe.widenTermRefExpr match
               case ConstantType(y) => foldBinop(op, x, y)
               case _ => null
-            }
           case _ => null
-        }
       case Select(xt, op) =>
         xt.tpe.widenTermRefExpr match {
           case ConstantType(x) => foldUnop(op, x)

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

@@ -33,7 +33,7 @@ object Docstrings {
         expandComment(sym).map { expanded =>
           val typedUsecases = expanded.usecases.map { usecase =>
             ctx.typer.enterSymbol(ctx.typer.createSymbol(usecase.untpdCode))
-            ctx.typer.typedStats(usecase.untpdCode :: Nil, owner) match {
+            ctx.typer.typedStats(usecase.untpdCode :: Nil, owner)._1 match {
               case List(df: tpd.DefDef) =>
                 usecase.typed(df)
               case _ =>

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

@@ -24,6 +24,7 @@ import ErrorReporting.errorTree
 import dotty.tools.dotc.tastyreflect.ReflectionImpl
 import dotty.tools.dotc.util.{SimpleIdentityMap, SimpleIdentitySet, SourceFile, SourcePosition}
 import dotty.tools.dotc.parsing.Parsers.Parser
+import Nullables.given
 
 import collection.mutable
 import reporting.trace
@@ -1064,10 +1065,10 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
             errorTree(tree, em"""cannot reduce inline if
                                 | its condition   ${tree.cond}
                                 | is not a constant value""")
-          else {
+          else
+            cond1.computeNullableDeeply()
             val if1 = untpd.cpy.If(tree)(cond = untpd.TypedSplice(cond1))
             super.typedIf(if1, pt)
-          }
       }
 
     override def typedApply(tree: untpd.Apply, pt: Type)(implicit ctx: Context): Tree =