Don't want to chase NPEs around for the rest of...

Don't want to chase NPEs around for the rest of my life. Created
"NoCompilationUnit" and "NoSourceFile" objects to represent not-present
versions of these items. Seems a lot better than null. References
SI-4859, got past NPE only to uncover the actual problem. No review.

8 files changed, 40 insertions, 19 deletions

diff --git a/src/compiler/scala/tools/nsc/CompilationUnits.scala b/src/compiler/scala/tools/nsc/CompilationUnits.scala
index aba17ca290..f5e32fbb09 100644
--- a/src/compiler/scala/tools/nsc/CompilationUnits.scala
+++ b/src/compiler/scala/tools/nsc/CompilationUnits.scala
@@ -5,12 +5,20 @@
 
 package scala.tools.nsc
 
-import util.{ FreshNameCreator,Position,NoPosition,SourceFile }
+import util.{ FreshNameCreator, Position, NoPosition, SourceFile, NoSourceFile }
 import scala.collection.mutable
 import scala.collection.mutable.{ LinkedHashSet, ListBuffer }
 
 trait CompilationUnits { self: Global =>
 
+  /** An object representing a missing compilation unit.
+   */
+  object NoCompilationUnit extends CompilationUnit(NoSourceFile) {
+    override lazy val isJava = false
+    override def exists = false
+    override def toString() = "NoCompilationUnit"
+  }
+
   /** One unit of compilation that has been submitted to the compiler.
     * It typically corresponds to a single file of source code.  It includes
     * error-reporting hooks.  */
@@ -25,6 +33,8 @@ trait CompilationUnits { self: Global =>
     /** the content of the compilation unit in tree form */
     var body: Tree = EmptyTree
 
+    def exists = source != NoSourceFile && source != null
+
 //    def parseSettings() = {
 //      val argsmarker = "SCALAC_ARGS"
 //      if(comments nonEmpty) {
diff --git a/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala b/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
index 76af2a7a03..75019f7624 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/GenICode.scala
@@ -48,7 +48,7 @@ abstract class GenICode extends SubComponent  {
 
     override def description = "Generate ICode from the AST"
 
-    var unit: CompilationUnit = _
+    var unit: CompilationUnit = NoCompilationUnit
 
     override def run() {
       scalaPrimitives.init
@@ -61,7 +61,7 @@ abstract class GenICode extends SubComponent  {
       unit.icode.clear
       informProgress("Generating icode for " + unit)
       gen(unit.body)
-      this.unit = null
+      this.unit = NoCompilationUnit
     }
 
     def gen(tree: Tree): Context = gen(tree, new Context())
diff --git a/src/compiler/scala/tools/nsc/interactive/Global.scala b/src/compiler/scala/tools/nsc/interactive/Global.scala
index fc53a34c45..3d7d14627c 100644
--- a/src/compiler/scala/tools/nsc/interactive/Global.scala
+++ b/src/compiler/scala/tools/nsc/interactive/Global.scala
@@ -205,7 +205,7 @@ class Global(settings: Settings, reporter: Reporter, projectName: String = "")
    */
   override def signalDone(context: Context, old: Tree, result: Tree) {
     if (interruptsEnabled && analyzer.lockedCount == 0) {
-      if (context.unit != null &&
+      if (context.unit.exists &&
           result.pos.isOpaqueRange &&
           (result.pos includes context.unit.targetPos)) {
         var located = new TypedLocator(context.unit.targetPos) locateIn result
diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index 96b27b58ae..44c28cf2f5 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -21,6 +21,7 @@ trait Contexts { self: Analyzer =>
   val NoContext = new Context {
     override def implicitss: List[List[ImplicitInfo]] = List()
     outer = this
+    override def toString = "NoContext"
   }
   NoContext.enclClass = NoContext
   NoContext.enclMethod = NoContext
@@ -89,7 +90,7 @@ trait Contexts { self: Analyzer =>
   }
 
   class Context private[typechecker] {
-    var unit: CompilationUnit = _
+    var unit: CompilationUnit = NoCompilationUnit
     var tree: Tree = _                      // Tree associated with this context
     var owner: Symbol = NoSymbol            // The current owner
     var scope: Scope = _                    // The current scope
@@ -164,12 +165,11 @@ trait Contexts { self: Analyzer =>
      */
     def make(unit: CompilationUnit, tree: Tree, owner: Symbol,
              scope: Scope, imports: List[ImportInfo]): Context = {
-      val c = new Context
-      c.unit = unit
-      c.tree = tree
+      val c   = new Context
+      c.unit  = unit
+      c.tree  = tree
       c.owner = owner
       c.scope = scope
-
       c.outer = this
 
       tree match {
@@ -203,6 +203,7 @@ trait Contexts { self: Analyzer =>
       c.retyping = this.retyping
       c.openImplicits = this.openImplicits
       registerContext(c.asInstanceOf[analyzer.Context])
+      debuglog("Created context: " + this + " ==> " + c)
       c
     }
 
@@ -364,11 +365,9 @@ trait Contexts { self: Analyzer =>
     def nextEnclosing(p: Context => Boolean): Context =
       if (this == NoContext || p(this)) this else outer.nextEnclosing(p)
 
-    override def toString = (
-      if (this == NoContext) "NoContext"
-      else "Context(%s@%s scope=%s)".format(owner.fullName, tree.getClass.getName split "[.$]" last, scope.##)
+    override def toString = "Context(%s@%s unit=%s scope=%s)".format(
+      owner.fullName, tree.shortClass, unit, scope.##
     )
-
     /** Is `sub` a subclass of `base` or a companion object of such a subclass?
      */
     def isSubClassOrCompanion(sub: Symbol, base: Symbol) =
diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index 2a6fc122e2..7305cef34f 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -249,7 +249,7 @@ trait Infer {
         tree setSymbol sym setType ErrorType
       } else {
         val topClass = context.owner.toplevelClass
-        if (context.unit != null)
+        if (context.unit.exists)
           context.unit.depends += sym.toplevelClass
 
         var sym1 = sym filter (alt => context.isAccessible(alt, pre, site.isInstanceOf[Super]))
diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index 513522e017..2c424d17d7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
@@ -414,7 +414,7 @@ trait TypeDiagnostics {
         // Error suppression will squash some of these warnings unless we circumvent it.
         // It is presumed if you are using a -Y option you would really like to hear
         // the warnings you've requested.
-        if (settings.warnDeadCode.value && context.unit != null && treeOK(tree) && exprOK) {
+        if (settings.warnDeadCode.value && context.unit.exists && treeOK(tree) && exprOK) {
           val saved = context.reportGeneralErrors
           try {
             context.reportGeneralErrors = true
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index d8c09cc685..f305644837 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -2101,7 +2101,7 @@ trait Typers extends Modes with Adaptations {
     def typedStats(stats: List[Tree], exprOwner: Symbol): List[Tree] = {
       val inBlock = exprOwner == context.owner
       def includesTargetPos(tree: Tree) =
-        tree.pos.isRange && context.unit != null && (tree.pos includes context.unit.targetPos)
+        tree.pos.isRange && context.unit.exists && (tree.pos includes context.unit.targetPos)
       val localTarget = stats exists includesTargetPos
       def typedStat(stat: Tree): Tree = {
         if (context.owner.isRefinementClass && !treeInfo.isDeclarationOrTypeDef(stat))
@@ -3779,7 +3779,7 @@ trait Typers extends Modes with Adaptations {
             // compilation units. Defined symbols take precedence over erroneous imports.
             if (defSym.definedInPackage &&
                 (!currentRun.compiles(defSym) ||
-                 (context.unit ne null) && defSym.sourceFile != context.unit.source.file))
+                 context.unit.exists && defSym.sourceFile != context.unit.source.file))
               defSym = NoSymbol
             else if (impSym.isError || impSym.name == nme.CONSTRUCTOR)
               impSym = NoSymbol
@@ -4328,8 +4328,7 @@ trait Typers extends Modes with Adaptations {
         case ex: Exception =>
           if (settings.debug.value) // @M causes cyclic reference error
             Console.println("exception when typing "+tree+", pt = "+pt)
-          if ((context ne null) && (context.unit ne null) &&
-              (context.unit.source ne null) && (tree ne null))
+          if (context != null && context.unit.exists && tree != null)
             logError("AT: " + (tree.pos).dbgString, ex)
           throw ex
       }
diff --git a/src/compiler/scala/tools/nsc/util/SourceFile.scala b/src/compiler/scala/tools/nsc/util/SourceFile.scala
index 44e06441ce..42489c5aaf 100644
--- a/src/compiler/scala/tools/nsc/util/SourceFile.scala
+++ b/src/compiler/scala/tools/nsc/util/SourceFile.scala
@@ -51,6 +51,19 @@ abstract class SourceFile {
   def identifier(pos: Position): Option[String] = None
 }
 
+/** An object representing a missing source file.
+ */
+object NoSourceFile extends SourceFile {
+  def content                   = Array()
+  def file                      = null    // TODO: push NPE-defense out another level or two
+  def isLineBreak(idx: Int)     = false
+  def isSelfContained           = true
+  def length                    = -1
+  def offsetToLine(offset: Int) = -1
+  def lineToOffset(index : Int) = -1
+  override def toString = "NoSourceFile"
+}
+
 object ScriptSourceFile {
   /** Length of the script header from the given content, if there is one.
    *  The header begins with "#!" or "::#!" and ends with a line starting