diff --git a/compiler/src/dotty/tools/dotc/CompilationUnit.scala b/compiler/src/dotty/tools/dotc/CompilationUnit.scala
index 25151126eb20..caca90bad586 100644
--- a/compiler/src/dotty/tools/dotc/CompilationUnit.scala
+++ b/compiler/src/dotty/tools/dotc/CompilationUnit.scala
@@ -16,6 +16,7 @@ import typer.Nullables
 import transform.SymUtils._
 import core.Decorators._
 import config.SourceVersion
+import scala.annotation.internal.sharable
 
 class CompilationUnit protected (val source: SourceFile) {
 
@@ -89,15 +90,24 @@ class CompilationUnit protected (val source: SourceFile) {
     myAssignmentSpans.nn
 }
 
+@sharable object NoCompilationUnit extends CompilationUnit(NoSource) {
+
+  override def isJava: Boolean = false
+
+  override def suspend()(using Context): Nothing =
+    throw CompilationUnit.SuspendException()
+
+  override def assignmentSpans(using Context): Map[Int, List[Span]] = Map.empty
+}
+
 object CompilationUnit {
 
   class SuspendException extends Exception
 
   /** Make a compilation unit for top class `clsd` with the contents of the `unpickled` tree */
   def apply(clsd: ClassDenotation, unpickled: Tree, forceTrees: Boolean)(using Context): CompilationUnit =
-    val file = clsd.symbol.associatedFile
-    // TODO: could file be null?
-    apply(new SourceFile(file.nn, Array.empty[Char]), unpickled, forceTrees)
+    val file = clsd.symbol.associatedFile.nn
+    apply(new SourceFile(file, Array.empty[Char]), unpickled, forceTrees)
 
   /** Make a compilation unit, given picked bytes and unpickled tree */
   def apply(source: SourceFile, unpickled: Tree, forceTrees: Boolean)(using Context): CompilationUnit = {
diff --git a/compiler/src/dotty/tools/dotc/Driver.scala b/compiler/src/dotty/tools/dotc/Driver.scala
index 311060f49aa3..f6d9df7d3822 100644
--- a/compiler/src/dotty/tools/dotc/Driver.scala
+++ b/compiler/src/dotty/tools/dotc/Driver.scala
@@ -42,10 +42,7 @@ class Driver {
         finish(compiler, run)
       catch
         case ex: FatalError =>
-          val msg = ex.getMessage
-          if msg != null then
-            report.error(msg) // signals that we should fail compilation.
-          else report.error("null")
+          report.error(ex.getMessage.nn) // signals that we should fail compilation.
         case ex: TypeError =>
           println(s"${ex.toMessage} while compiling ${files.map(_.path).mkString(", ")}")
           throw ex
diff --git a/compiler/src/dotty/tools/dotc/Resident.scala b/compiler/src/dotty/tools/dotc/Resident.scala
index 6025dd93078d..f1370d915250 100644
--- a/compiler/src/dotty/tools/dotc/Resident.scala
+++ b/compiler/src/dotty/tools/dotc/Resident.scala
@@ -55,8 +55,8 @@ class Resident extends Driver {
           }
           if (line.startsWith(quit)) ctx.reporter
           else
-            // assuming split returns non-nullable values
-            loop((line split "\\s+").asInstanceOf, nextCtx)
+            // split returns non-nullable values
+            loop((line split "\\s+").asInstanceOf[Array[String]], nextCtx)
         case None =>
           prevCtx.reporter
     }
diff --git a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
index 7fa21372a6ff..cf5c1fdb9b70 100644
--- a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -825,8 +825,8 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *  tree must be reachable from come tree stored in an enclosing context.
    */
   def definingStats(sym: Symbol)(using Context): List[Tree] =
-    val unit: CompilationUnit | Null = ctx.compilationUnit
-    if (!sym.span.exists || (ctx eq NoContext) || unit == null) Nil
+    val unit = ctx.compilationUnit
+    if (!sym.span.exists || (ctx eq NoContext) || (unit eq NoCompilationUnit)) Nil
     else defPath(sym, unit.tpdTree) match {
       case defn :: encl :: _ =>
         def verify(stats: List[Tree]) =
diff --git a/compiler/src/dotty/tools/dotc/config/Feature.scala b/compiler/src/dotty/tools/dotc/config/Feature.scala
index dccbc9eb21ee..a037a0d4fcbc 100644
--- a/compiler/src/dotty/tools/dotc/config/Feature.scala
+++ b/compiler/src/dotty/tools/dotc/config/Feature.scala
@@ -78,9 +78,7 @@ object Feature:
     SourceVersion.valueOf(ctx.settings.source.value)
 
   def sourceVersion(using Context): SourceVersion =
-    val unit: CompilationUnit | Null = ctx.compilationUnit
-    if unit == null then sourceVersionSetting
-    else unit.sourceVersion match
+    ctx.compilationUnit.sourceVersion match
       case Some(v) => v
       case none => sourceVersionSetting
 
diff --git a/compiler/src/dotty/tools/dotc/core/Contexts.scala b/compiler/src/dotty/tools/dotc/core/Contexts.scala
index 9f0400ffefde..64311f75c657 100644
--- a/compiler/src/dotty/tools/dotc/core/Contexts.scala
+++ b/compiler/src/dotty/tools/dotc/core/Contexts.scala
@@ -349,8 +349,7 @@ object Contexts {
         if ctx1 == null then
           util.Stats.record("Context.withSource.new")
           val ctx2 = fresh.setSource(source)
-          val ctx2unit: CompilationUnit | Null = ctx2.compilationUnit
-          if ctx2unit == null then
+          if ctx2.compilationUnit eq NoCompilationUnit then
             // `source` might correspond to a file not necessarily
             // in the current project (e.g. when inlining library code),
             // so set `mustExist` to false.
@@ -394,12 +393,7 @@ object Contexts {
     final def erasedTypes = phase.erasedTypes
 
     /** Are we in a Java compilation unit? */
-    final def isJava: Boolean =
-      // FIXME: It would be much nicer if compilationUnit was non-nullable,
-      // perhaps we need to introduce a `NoCompilationUnit` compilation unit
-      // to be used as a default value.
-      val unit: CompilationUnit | Null = compilationUnit
-      unit != null && unit.isJava
+    final def isJava: Boolean = compilationUnit.isJava
 
     /** Is current phase after TyperPhase? */
     final def isAfterTyper = base.isAfterTyper(phase)
@@ -837,6 +831,7 @@ object Contexts {
     store = initialStore
       .updated(settingsStateLoc, settingsGroup.defaultState)
       .updated(notNullInfosLoc, Nil)
+      .updated(compilationUnitLoc, NoCompilationUnit)
     searchHistory = new SearchRoot
     gadt = EmptyGadtConstraint
   }
diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index 59f108fe0330..933d63667273 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -1431,8 +1431,7 @@ class Definitions {
 
   /** Are we compiling a java source file? */
   private def isJavaContext(using Context): Boolean =
-    val unit: CompilationUnit | Null = ctx.compilationUnit
-    unit != null && unit.isJava
+    ctx.compilationUnit.isJava
 
   private def unqualifiedTypes(refs: List[TermRef]) =
     val types = refs.toSet[NamedType]
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 6bbd487374dc..3a95a212437e 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1230,10 +1230,10 @@ object SymDenotations {
      *     lookup its companion in the same scope.
      */
     private def companionNamed(name: TypeName)(using Context): Symbol =
-      val unit: CompilationUnit | Null = ctx.compilationUnit
+      val unit = ctx.compilationUnit
       if (owner.isClass)
         owner.unforcedDecls.lookup(name).suchThat(_.isCoDefinedWith(symbol)).symbol
-      else if (!owner.exists || unit == null)
+      else if (!owner.exists || (unit eq NoCompilationUnit))
         NoSymbol
       else if (!unit.tpdTree.isEmpty)
         tpd.definingStats(symbol).iterator
diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index 63a67ce1f4c0..d08265566834 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -167,10 +167,10 @@ object Nullables:
   def isTracked(ref: TermRef)(using Context) =
     ref.isStable
     || { val sym = ref.symbol
-         val unit: CompilationUnit | Null = ctx.compilationUnit
+         val unit = ctx.compilationUnit
          !ref.usedOutOfOrder
          && sym.span.exists
-         && unit != null // could be null under -Ytest-pickler
+         && (unit ne NoCompilationUnit) // could be null under -Ytest-pickler
          && unit.assignmentSpans.contains(sym.span.start)
       }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 3397a5da8fdf..a0ad8ec9aae3 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -313,7 +313,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
        */
       def isDefinedInCurrentUnit(denot: Denotation)(using Context): Boolean = denot match {
         case MultiDenotation(d1, d2) => isDefinedInCurrentUnit(d1) || isDefinedInCurrentUnit(d2)
-        case denot: SingleDenotation => (ctx.compilationUnit: CompilationUnit | Null) != null && denot.symbol.source == ctx.compilationUnit.source
+        case denot: SingleDenotation => (ctx.compilationUnit ne NoCompilationUnit) && denot.symbol.source == ctx.compilationUnit.source
       }
 
       /** Is `denot` the denotation of a self symbol? */
diff --git a/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala b/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
index 926b3fd04003..0ab435991354 100644
--- a/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
+++ b/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
@@ -14,6 +14,7 @@ import dotty.tools.dotc.quoted.reflect._
 import dotty.tools.dotc.quoted.QuoteUtils._
 import dotty.tools.dotc.core.Decorators._
 import dotty.tools.dotc.CompilationUnit
+import dotty.tools.dotc.NoCompilationUnit
 
 import dotty.tools.dotc.quoted.{MacroExpansion, PickledQuotes, QuoteUtils}
 
@@ -2797,8 +2798,8 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
 
     object SourceFile extends SourceFileModule {
       def current: SourceFile =
-        val unit: CompilationUnit | Null = ctx.compilationUnit
-        if unit == null then
+        val unit = ctx.compilationUnit
+        if unit == NoCompilationUnit then
           throw new java.lang.UnsupportedOperationException(
             "`reflect.SourceFile.current` cannot be called within the TASTy ispector")
         else unit.source