From 38b0168cc2cbba99e39bab89e459bfea51bda783 Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Thu, 2 Nov 2023 09:44:43 +0100
Subject: [PATCH] Use `compiletime.uninitialized` in compiler

---
 .../tools/backend/jvm/BCodeHelpers.scala      |  3 +-
 .../dotty/tools/backend/jvm/GenBCode.scala    | 13 ++---
 .../backend/jvm/GeneratedClassHandler.scala   |  3 +-
 .../tools/backend/sjs/JSDefinitions.scala     |  3 +-
 compiler/src/dotty/tools/dotc/Bench.scala     |  3 +-
 compiler/src/dotty/tools/dotc/Run.scala       |  4 +-
 .../src/dotty/tools/dotc/ast/Positioned.scala |  4 +-
 compiler/src/dotty/tools/dotc/ast/tpd.scala   |  3 +-
 .../tools/dotc/core/ConstraintRunInfo.scala   |  3 +-
 .../src/dotty/tools/dotc/core/Contexts.scala  |  2 +-
 .../dotty/tools/dotc/core/Definitions.scala   |  5 +-
 .../dotty/tools/dotc/core/Denotations.scala   |  6 ++-
 .../tools/dotc/core/OrderingConstraint.scala  |  4 +-
 .../src/dotty/tools/dotc/core/Phases.scala    | 51 ++++++++++---------
 .../src/dotty/tools/dotc/core/Scopes.scala    |  1 +
 .../tools/dotc/core/SymDenotations.scala      |  3 +-
 .../src/dotty/tools/dotc/core/Symbols.scala   |  4 +-
 .../dotty/tools/dotc/core/TyperState.scala    | 19 +++----
 .../src/dotty/tools/dotc/core/Types.scala     | 28 +++++-----
 .../dotc/core/classfile/ClassfileParser.scala | 10 ++--
 .../classfile/ClassfileTastyUUIDParser.scala  |  3 +-
 .../dotc/core/tasty/PositionUnpickler.scala   |  8 +--
 .../tools/dotc/core/tasty/TreeUnpickler.scala |  3 +-
 .../tools/dotc/parsing/CharArrayReader.scala  |  4 +-
 .../dotc/parsing/xml/MarkupParsers.scala      |  3 +-
 .../dotc/parsing/xml/SymbolicXMLBuilder.scala |  3 +-
 .../dotty/tools/dotc/plugins/Plugins.scala    |  6 ++-
 .../tools/dotc/sbt/ExtractDependencies.scala  | 10 ++--
 .../tools/dotc/transform/CapturedVars.scala   |  4 +-
 .../dotty/tools/dotc/transform/CtxLazy.scala  |  4 +-
 .../tools/dotc/transform/Dependencies.scala   |  5 +-
 .../dotty/tools/dotc/transform/Flatten.scala  |  4 +-
 .../dotc/transform/ForwardDepChecks.scala     |  8 +--
 .../dotc/transform/Instrumentation.scala      | 12 +++--
 .../tools/dotc/transform/LambdaLift.scala     |  4 +-
 .../dotty/tools/dotc/transform/LazyVals.scala |  3 +-
 .../tools/dotc/transform/MegaPhase.scala      |  8 +--
 .../dotty/tools/dotc/transform/Memoize.scala  |  3 +-
 .../dotc/transform/OverridingPairs.scala      |  6 ++-
 .../dotty/tools/dotc/transform/TailRec.scala  |  4 +-
 .../transform/sjs/ExplicitJSClasses.scala     |  3 +-
 .../dotty/tools/dotc/typer/Implicits.scala    |  3 +-
 .../dotty/tools/dotc/typer/ImportInfo.scala   |  6 ++-
 .../src/dotty/tools/dotc/typer/Namer.scala    |  3 +-
 .../tools/dotc/typer/VarianceChecker.scala    |  4 +-
 .../tools/dotc/util/GenericHashMap.scala      |  8 +--
 .../tools/dotc/util/GenericHashSet.scala      |  8 +--
 .../src/dotty/tools/dotc/util/IntMap.scala    |  4 +-
 .../tools/dotc/util/PerfectHashing.scala      |  8 +--
 .../dotty/tools/dotc/util/SourceFile.scala    |  3 +-
 .../tools/repl/CollectTopLevelImports.scala   |  4 +-
 compiler/src/dotty/tools/repl/Rendering.scala |  5 +-
 .../src/dotty/tools/repl/ReplDriver.scala     |  9 ++--
 .../tools/dotc/reporting/TestReporter.scala   |  7 +--
 .../tools/vulpix/RunnerOrchestration.scala    |  5 +-
 55 files changed, 216 insertions(+), 141 deletions(-)

diff --git a/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala b/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
index c36c8c546635..3085c9411222 100644
--- a/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
+++ b/compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
@@ -9,6 +9,7 @@ import scala.tools.asm
 import scala.tools.asm.AnnotationVisitor
 import scala.tools.asm.ClassWriter
 import scala.collection.mutable
+import scala.compiletime.uninitialized
 
 import dotty.tools.dotc.CompilationUnit
 import dotty.tools.dotc.ast.tpd
@@ -576,7 +577,7 @@ trait BCodeHelpers extends BCodeIdiomatic {
   /* builder of mirror classes */
   class JMirrorBuilder extends JCommonBuilder {
 
-    private var cunit: CompilationUnit = _
+    private var cunit: CompilationUnit = uninitialized
     def getCurrentCUnit(): CompilationUnit = cunit;
 
     /* Generate a mirror class for a top-level module. A mirror class is a class
diff --git a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
index 8c1691e88b80..6dcaaa23c61d 100644
--- a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
+++ b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
@@ -9,6 +9,7 @@ import Contexts._
 import Symbols._
 import dotty.tools.io._
 import scala.collection.mutable
+import scala.compiletime.uninitialized
 
 class GenBCode extends Phase { self =>
 
@@ -25,7 +26,7 @@ class GenBCode extends Phase { self =>
   private val entryPoints = new mutable.HashSet[String]()
   def registerEntryPoint(s: String): Unit = entryPoints += s
 
-  private var _backendInterface: DottyBackendInterface = _
+  private var _backendInterface: DottyBackendInterface = uninitialized
   def backendInterface(using ctx: Context): DottyBackendInterface = {
     if _backendInterface eq null then
       // Enforce usage of FreshContext so we would be able to modify compilation unit between runs
@@ -36,7 +37,7 @@ class GenBCode extends Phase { self =>
     _backendInterface
   }
 
-  private var _codeGen: CodeGen = _
+  private var _codeGen: CodeGen = uninitialized
   def codeGen(using Context): CodeGen = {
     if _codeGen eq null then
       val int = backendInterface
@@ -45,28 +46,28 @@ class GenBCode extends Phase { self =>
     _codeGen
   }
 
-  private var _bTypes: BTypesFromSymbols[DottyBackendInterface] = _
+  private var _bTypes: BTypesFromSymbols[DottyBackendInterface] = uninitialized
   def bTypes(using Context): BTypesFromSymbols[DottyBackendInterface] = {
     if _bTypes eq null then
       _bTypes = BTypesFromSymbols(backendInterface, frontendAccess)
     _bTypes
   }
 
-  private var _frontendAccess: PostProcessorFrontendAccess | Null = _
+  private var _frontendAccess: PostProcessorFrontendAccess | Null = uninitialized
   def frontendAccess(using Context): PostProcessorFrontendAccess = {
     if _frontendAccess eq null then
       _frontendAccess = PostProcessorFrontendAccess.Impl(backendInterface, entryPoints)
     _frontendAccess.nn
   }
 
-  private var _postProcessor: PostProcessor | Null = _
+  private var _postProcessor: PostProcessor | Null = uninitialized
   def postProcessor(using Context): PostProcessor = {
     if _postProcessor eq null then
       _postProcessor = new PostProcessor(frontendAccess, bTypes)
     _postProcessor.nn
   }
 
-  private var _generatedClassHandler: GeneratedClassHandler | Null = _
+  private var _generatedClassHandler: GeneratedClassHandler | Null = uninitialized
   def generatedClassHandler(using Context): GeneratedClassHandler = {
     if _generatedClassHandler eq null then
       _generatedClassHandler = GeneratedClassHandler(postProcessor)
diff --git a/compiler/src/dotty/tools/backend/jvm/GeneratedClassHandler.scala b/compiler/src/dotty/tools/backend/jvm/GeneratedClassHandler.scala
index fc02d9597efe..f17f9d620d90 100644
--- a/compiler/src/dotty/tools/backend/jvm/GeneratedClassHandler.scala
+++ b/compiler/src/dotty/tools/backend/jvm/GeneratedClassHandler.scala
@@ -15,6 +15,7 @@ import dotty.tools.dotc.core.Phases
 import dotty.tools.dotc.core.Decorators.em
 
 import scala.language.unsafeNulls
+import scala.compiletime.uninitialized
 
 /**
  * Interface to handle post-processing and classfile writing (see [[PostProcessor]]) of generated
@@ -185,7 +186,7 @@ final private class CompilationUnitInPostProcess(private var classes: List[Gener
   }
 
   /** the main async task submitted onto the scheduler */
-  var task: Future[Unit] = _
+  var task: Future[Unit] = uninitialized
 
   val bufferedReporting = new PostProcessorFrontendAccess.BufferingBackendReporting()
 }
\ No newline at end of file
diff --git a/compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala b/compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala
index 5336d60129ac..49755a8ee83e 100644
--- a/compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala
+++ b/compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala
@@ -12,6 +12,7 @@ import Symbols._
 import StdNames._
 
 import dotty.tools.dotc.config.SJSPlatform
+import scala.compiletime.uninitialized
 
 object JSDefinitions {
   /** The Scala.js-specific definitions for the current context. */
@@ -249,7 +250,7 @@ final class JSDefinitions()(using Context) {
     @threadUnsafe lazy val Selectable_reflectiveSelectableFromLangReflectiveCallsR = SelectableModule.requiredMethodRef("reflectiveSelectableFromLangReflectiveCalls")
     def Selectable_reflectiveSelectableFromLangReflectiveCalls(using Context) = Selectable_reflectiveSelectableFromLangReflectiveCallsR.symbol
 
-  private var allRefClassesCache: Set[Symbol] = _
+  private var allRefClassesCache: Set[Symbol] = uninitialized
   def allRefClasses(using Context): Set[Symbol] = {
     if (allRefClassesCache == null) {
       val baseNames = List("Object", "Boolean", "Character", "Byte", "Short",
diff --git a/compiler/src/dotty/tools/dotc/Bench.scala b/compiler/src/dotty/tools/dotc/Bench.scala
index 5f5e9fc799b5..40ad66a0a9e6 100644
--- a/compiler/src/dotty/tools/dotc/Bench.scala
+++ b/compiler/src/dotty/tools/dotc/Bench.scala
@@ -6,6 +6,7 @@ import reporting.Reporter
 import io.AbstractFile
 
 import scala.annotation.internal.sharable
+import scala.compiletime.uninitialized
 
 /** A main class for running compiler benchmarks. Can instantiate a given
  *  number of compilers and run each (sequentially) a given number of times
@@ -17,7 +18,7 @@ object Bench extends Driver:
   @sharable private var numCompilers = 1
   @sharable private var waitAfter = -1
   @sharable private var curCompiler = 0
-  @sharable private var times: Array[Int] = _
+  @sharable private var times: Array[Int] = uninitialized
 
   override def doCompile(compiler: Compiler, files: List[AbstractFile])(using Context): Reporter =
     var reporter: Reporter = emptyReporter
diff --git a/compiler/src/dotty/tools/dotc/Run.scala b/compiler/src/dotty/tools/dotc/Run.scala
index d85e03e92d60..7b09e7ef833a 100644
--- a/compiler/src/dotty/tools/dotc/Run.scala
+++ b/compiler/src/dotty/tools/dotc/Run.scala
@@ -65,7 +65,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
 
   private var myUnits: List[CompilationUnit] = Nil
   private var myUnitsCached: List[CompilationUnit] = Nil
-  private var myFiles: Set[AbstractFile] = _
+  private var myFiles: Set[AbstractFile] = uninitialized
 
   // `@nowarn` annotations by source file, populated during typer
   private val mySuppressions: mutable.LinkedHashMap[SourceFile, mutable.ListBuffer[Suppression]] = mutable.LinkedHashMap.empty
@@ -511,7 +511,7 @@ object Run {
     var currentCompletedSubtraversalCount: Int = 0 // completed subphases in the current phase
     var seenPhaseCount: Int = 0 // how many phases we've seen so far
 
-    private var currPhase: Phase = uninitialized  // initialized by enterPhase
+    private var currPhase: Phase = uninitialized      // initialized by enterPhase
     private var subPhases: SubPhases = uninitialized  // initialized by enterPhase
     private var currPhaseName: String = uninitialized // initialized by enterPhase
     private var nextPhaseName: String = uninitialized // initialized by enterPhase
diff --git a/compiler/src/dotty/tools/dotc/ast/Positioned.scala b/compiler/src/dotty/tools/dotc/ast/Positioned.scala
index dd783be7a9e1..980042f8292e 100644
--- a/compiler/src/dotty/tools/dotc/ast/Positioned.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Positioned.scala
@@ -13,12 +13,14 @@ import ast.Trees.mods
 import annotation.constructorOnly
 import annotation.internal.sharable
 
+import scala.compiletime.uninitialized
+
 /** A base class for things that have positions (currently: modifiers and trees)
  */
 abstract class Positioned(implicit @constructorOnly src: SourceFile) extends SrcPos, Product, Cloneable {
   import Positioned.{ids, nextId, debugId}
 
-  private var mySpan: Span = _
+  private var mySpan: Span = uninitialized
 
   private var mySource: SourceFile = src
 
diff --git a/compiler/src/dotty/tools/dotc/ast/tpd.scala b/compiler/src/dotty/tools/dotc/ast/tpd.scala
index e0b282e2d8db..7625a61d48aa 100644
--- a/compiler/src/dotty/tools/dotc/ast/tpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/tpd.scala
@@ -18,6 +18,7 @@ import typer.ConstFold
 
 import scala.annotation.tailrec
 import scala.collection.mutable.ListBuffer
+import scala.compiletime.uninitialized
 
 /** Some creators for typed trees */
 object tpd extends Trees.Instance[Type] with TypedTreeInfo {
@@ -1309,7 +1310,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   trait TreeProvider {
     protected def computeRootTrees(using Context): List[Tree]
 
-    private var myTrees: List[Tree] | Null = _
+    private var myTrees: List[Tree] | Null = uninitialized
 
     /** Get trees defined by this provider. Cache them if -Yretain-trees is set. */
     def rootTrees(using Context): List[Tree] =
diff --git a/compiler/src/dotty/tools/dotc/core/ConstraintRunInfo.scala b/compiler/src/dotty/tools/dotc/core/ConstraintRunInfo.scala
index d2b1246a8149..412b4228677d 100644
--- a/compiler/src/dotty/tools/dotc/core/ConstraintRunInfo.scala
+++ b/compiler/src/dotty/tools/dotc/core/ConstraintRunInfo.scala
@@ -3,10 +3,11 @@ package core
 
 import Contexts._
 import config.Printers.{default, typr}
+import scala.compiletime.uninitialized
 
 trait ConstraintRunInfo { self: Run =>
   private var maxSize = 0
-  private var maxConstraint: Constraint | Null = _
+  private var maxConstraint: Constraint | Null = uninitialized
   def recordConstraintSize(c: Constraint, size: Int): Unit =
     if (size > maxSize) {
       maxSize = size
diff --git a/compiler/src/dotty/tools/dotc/core/Contexts.scala b/compiler/src/dotty/tools/dotc/core/Contexts.scala
index a2adc0058938..b6474ca7973c 100644
--- a/compiler/src/dotty/tools/dotc/core/Contexts.scala
+++ b/compiler/src/dotty/tools/dotc/core/Contexts.scala
@@ -558,7 +558,7 @@ object Contexts {
     private var _owner: Symbol = uninitialized
     final def owner: Symbol = _owner
 
-    private var _tree: Tree[?]= _
+    private var _tree: Tree[?] = uninitialized
     final def tree: Tree[?] = _tree
 
     private var _scope: Scope = uninitialized
diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index 97b1b7db1b21..205d43cd07ca 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -20,6 +20,7 @@ import cc.{CaptureSet, RetainingType}
 import ast.tpd.ref
 
 import scala.annotation.tailrec
+import scala.compiletime.uninitialized
 
 object Definitions {
 
@@ -44,7 +45,7 @@ object Definitions {
 class Definitions {
   import Definitions._
 
-  private var initCtx: Context = _
+  private var initCtx: Context = uninitialized
   private given currentContext[Dummy_so_its_a_def]: Context = initCtx
 
   private def newPermanentSymbol[N <: Name](owner: Symbol, name: N, flags: FlagSet, info: Type) =
@@ -2001,7 +2002,7 @@ class Definitions {
 
   class PerRun[T](generate: Context ?=> T) {
     private var current: RunId = NoRunId
-    private var cached: T = _
+    private var cached: T = uninitialized
     def apply()(using Context): T = {
       if (current != ctx.runId) {
         cached = generate
diff --git a/compiler/src/dotty/tools/dotc/core/Denotations.scala b/compiler/src/dotty/tools/dotc/core/Denotations.scala
index 640ba8015be7..1969c87d7a86 100644
--- a/compiler/src/dotty/tools/dotc/core/Denotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/Denotations.scala
@@ -24,6 +24,8 @@ import util.common._
 import typer.ProtoTypes.NoViewsAllowed
 import collection.mutable.ListBuffer
 
+import scala.compiletime.uninitialized
+
 /** Denotations represent the meaning of symbols and named types.
  *  The following diagram shows how the principal types of denotations
  *  and their denoting entities relate to each other. Lines ending in
@@ -121,8 +123,8 @@ object Denotations {
     /** Map `f` over all single denotations and aggregate the results with `g`. */
     def aggregate[T](f: SingleDenotation => T, g: (T, T) => T): T
 
-    private var cachedPrefix: Type = _
-    private var cachedAsSeenFrom: AsSeenFromResult = _
+    private var cachedPrefix: Type = uninitialized
+    private var cachedAsSeenFrom: AsSeenFromResult = uninitialized
     private var validAsSeenFrom: Period = Nowhere
 
     type AsSeenFromResult <: PreDenotation
diff --git a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
index 0328cea9b3ca..0f05778266fd 100644
--- a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
+++ b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
@@ -14,6 +14,8 @@ import annotation.tailrec
 import annotation.internal.sharable
 import cc.{CapturingType, derivedCapturingType}
 
+import scala.compiletime.uninitialized
+
 object OrderingConstraint {
 
   /** If true, use reverse dependencies in `replace` to avoid checking the bounds
@@ -881,7 +883,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
         i += 1
     }
 
-  private var myUninstVars: mutable.ArrayBuffer[TypeVar] | Null = _
+  private var myUninstVars: mutable.ArrayBuffer[TypeVar] | Null = uninitialized
 
   /** The uninstantiated typevars of this constraint */
   def uninstVars: collection.Seq[TypeVar] = {
diff --git a/compiler/src/dotty/tools/dotc/core/Phases.scala b/compiler/src/dotty/tools/dotc/core/Phases.scala
index 9ba8332a8634..dd354b66cb4a 100644
--- a/compiler/src/dotty/tools/dotc/core/Phases.scala
+++ b/compiler/src/dotty/tools/dotc/core/Phases.scala
@@ -21,6 +21,7 @@ import typer.ImportInfo.withRootImports
 import ast.{tpd, untpd}
 import scala.annotation.internal.sharable
 import scala.util.control.NonFatal
+import scala.compiletime.uninitialized
 
 object Phases {
 
@@ -205,30 +206,30 @@ object Phases {
         if nextDenotTransformerId(i) == phase.id then
           nextDenotTransformerId(i) = nextDenotTransformerId(phase.id + 1)
 
-    private var myParserPhase: Phase = _
-    private var myTyperPhase: Phase = _
-    private var myPostTyperPhase: Phase = _
-    private var mySbtExtractDependenciesPhase: Phase = _
-    private var myPicklerPhase: Phase = _
-    private var myInliningPhase: Phase = _
-    private var myStagingPhase: Phase = _
-    private var mySplicingPhase: Phase = _
-    private var myFirstTransformPhase: Phase = _
-    private var myCollectNullableFieldsPhase: Phase = _
-    private var myRefChecksPhase: Phase = _
-    private var myPatmatPhase: Phase = _
-    private var myElimRepeatedPhase: Phase = _
-    private var myElimByNamePhase: Phase = _
-    private var myExtensionMethodsPhase: Phase = _
-    private var myExplicitOuterPhase: Phase = _
-    private var myGettersPhase: Phase = _
-    private var myErasurePhase: Phase = _
-    private var myElimErasedValueTypePhase: Phase = _
-    private var myLambdaLiftPhase: Phase = _
-    private var myCountOuterAccessesPhase: Phase = _
-    private var myFlattenPhase: Phase = _
-    private var myGenBCodePhase: Phase = _
-    private var myCheckCapturesPhase: Phase = _
+    private var myParserPhase: Phase = uninitialized
+    private var myTyperPhase: Phase = uninitialized
+    private var myPostTyperPhase: Phase = uninitialized
+    private var mySbtExtractDependenciesPhase: Phase = uninitialized
+    private var myPicklerPhase: Phase = uninitialized
+    private var myInliningPhase: Phase = uninitialized
+    private var myStagingPhase: Phase = uninitialized
+    private var mySplicingPhase: Phase = uninitialized
+    private var myFirstTransformPhase: Phase = uninitialized
+    private var myCollectNullableFieldsPhase: Phase = uninitialized
+    private var myRefChecksPhase: Phase = uninitialized
+    private var myPatmatPhase: Phase = uninitialized
+    private var myElimRepeatedPhase: Phase = uninitialized
+    private var myElimByNamePhase: Phase = uninitialized
+    private var myExtensionMethodsPhase: Phase = uninitialized
+    private var myExplicitOuterPhase: Phase = uninitialized
+    private var myGettersPhase: Phase = uninitialized
+    private var myErasurePhase: Phase = uninitialized
+    private var myElimErasedValueTypePhase: Phase = uninitialized
+    private var myLambdaLiftPhase: Phase = uninitialized
+    private var myCountOuterAccessesPhase: Phase = uninitialized
+    private var myFlattenPhase: Phase = uninitialized
+    private var myGenBCodePhase: Phase = uninitialized
+    private var myCheckCapturesPhase: Phase = uninitialized
 
     final def parserPhase: Phase = myParserPhase
     final def typerPhase: Phase = myTyperPhase
@@ -389,7 +390,7 @@ object Phases {
     def printingContext(ctx: Context): Context = ctx
 
     private var myPeriod: Period = Periods.InvalidPeriod
-    private var myBase: ContextBase = _
+    private var myBase: ContextBase = uninitialized
     private var myErasedTypes = false
     private var myFlatClasses = false
     private var myRefChecked = false
diff --git a/compiler/src/dotty/tools/dotc/core/Scopes.scala b/compiler/src/dotty/tools/dotc/core/Scopes.scala
index 99076b422358..7762ce8da339 100644
--- a/compiler/src/dotty/tools/dotc/core/Scopes.scala
+++ b/compiler/src/dotty/tools/dotc/core/Scopes.scala
@@ -19,6 +19,7 @@ import printing.Printer
 import SymDenotations.NoDenotation
 
 import collection.mutable
+import scala.compiletime.uninitialized
 
 object Scopes {
 
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 395164b1dd48..c2e0e6f717dd 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -27,6 +27,7 @@ import transform.TypeUtils._
 import cc.{CapturingType, derivedCapturingType}
 
 import scala.annotation.internal.sharable
+import scala.compiletime.uninitialized
 
 object SymDenotations {
 
@@ -2431,7 +2432,7 @@ object SymDenotations {
     initPrivateWithin: Symbol)
     extends ClassDenotation(symbol, ownerIfExists, name, initFlags, initInfo, initPrivateWithin) {
 
-    private var packageObjsCache: List[ClassDenotation] = _
+    private var packageObjsCache: List[ClassDenotation] = uninitialized
     private var packageObjsRunId: RunId = NoRunId
     private var ambiguityWarningIssued: Boolean = false
 
diff --git a/compiler/src/dotty/tools/dotc/core/Symbols.scala b/compiler/src/dotty/tools/dotc/core/Symbols.scala
index 1e21c1a743f4..9c7c6a9a18bf 100644
--- a/compiler/src/dotty/tools/dotc/core/Symbols.scala
+++ b/compiler/src/dotty/tools/dotc/core/Symbols.scala
@@ -33,6 +33,8 @@ import scala.annotation.internal.sharable
 import config.Printers.typr
 import dotty.tools.dotc.classpath.FileUtils.isScalaBinary
 
+import scala.compiletime.uninitialized
+
 object Symbols {
 
   implicit def eqSymbol: CanEqual[Symbol, Symbol] = CanEqual.derived
@@ -88,7 +90,7 @@ object Symbols {
       ctx.settings.YcheckInitGlobal.value
 
     /** The last denotation of this symbol */
-    private var lastDenot: SymDenotation = _
+    private var lastDenot: SymDenotation = uninitialized
     private var checkedPeriod: Period = Nowhere
 
     private[core] def invalidateDenotCache(): Unit = { checkedPeriod = Nowhere }
diff --git a/compiler/src/dotty/tools/dotc/core/TyperState.scala b/compiler/src/dotty/tools/dotc/core/TyperState.scala
index d2df2a2aebef..54ddd4c0eb6b 100644
--- a/compiler/src/dotty/tools/dotc/core/TyperState.scala
+++ b/compiler/src/dotty/tools/dotc/core/TyperState.scala
@@ -14,6 +14,7 @@ import util.{Stats, SimpleIdentityMap}
 import Decorators._
 
 import scala.annotation.internal.sharable
+import scala.compiletime.uninitialized
 
 object TyperState {
   @sharable private var nextId: Int = 0
@@ -44,19 +45,19 @@ object TyperState {
 class TyperState() {
   import TyperState.LevelMap
 
-  private var myId: Int = _
+  private var myId: Int = uninitialized
   def id: Int = myId
 
-  private var previous: TyperState | Null = _
+  private var previous: TyperState | Null = uninitialized
 
-  private var myReporter: Reporter = _
+  private var myReporter: Reporter = uninitialized
 
   def reporter: Reporter = myReporter
 
   /** A fresh type state with the same constraint as this one and the given reporter */
   def setReporter(reporter: Reporter): this.type = { myReporter = reporter; this }
 
-  private var myConstraint: Constraint = _
+  private var myConstraint: Constraint = uninitialized
 
   def constraint: Constraint = myConstraint
   def constraint_=(c: Constraint)(using Context): Unit = {
@@ -66,9 +67,9 @@ class TyperState() {
       c.checkConsistentVars()
   }
 
-  private var previousConstraint: Constraint = _
+  private var previousConstraint: Constraint = uninitialized
 
-  private var myIsCommittable: Boolean = _
+  private var myIsCommittable: Boolean = uninitialized
 
   def isCommittable: Boolean = myIsCommittable
 
@@ -79,7 +80,7 @@ class TyperState() {
   def isGlobalCommittable: Boolean =
     isCommittable && (previous == null || previous.uncheckedNN.isGlobalCommittable)
 
-  private var isCommitted: Boolean = _
+  private var isCommitted: Boolean = uninitialized
 
   /** The set of uninstantiated type variables which have this state as their owning state.
    *
@@ -87,11 +88,11 @@ class TyperState() {
    *   if `tstate.isCommittable` then
    *     `tstate.ownedVars.contains(tvar)` iff `tvar.owningState.get eq tstate`
    */
-  private var myOwnedVars: TypeVars = _
+  private var myOwnedVars: TypeVars = uninitialized
   def ownedVars: TypeVars = myOwnedVars
   def ownedVars_=(vs: TypeVars): Unit = myOwnedVars = vs
 
-  private var upLevels: LevelMap = _
+  private var upLevels: LevelMap = uninitialized
 
   /** Initializes all fields except reporter, isCommittable, which need to be
    *  set separately.
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index eb878b430183..f26cf432277f 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -2164,7 +2164,7 @@ object Types {
 
   /** A trait for references in CaptureSets. These can be NamedTypes, ThisTypes or ParamRefs */
   trait CaptureRef extends SingletonType:
-    private var myCaptureSet: CaptureSet | Null = _
+    private var myCaptureSet: CaptureSet | Null = uninitialized
     private var myCaptureSetRunId: Int = NoRunId
     private var mySingletonCaptureSet: CaptureSet.Const | Null = null
 
@@ -2285,7 +2285,7 @@ object Types {
     private var lastSymbol: Symbol | Null = null
     private var checkedPeriod: Period = Nowhere
     private var myStableHash: Byte = 0
-    private var mySignature: Signature = _
+    private var mySignature: Signature = uninitialized
     private var mySignatureRunId: Int = NoRunId
 
     // Invariants:
@@ -2941,7 +2941,7 @@ object Types {
     type ThisName = TypeName
 
     private var myCanDropAliasPeriod: Period = Nowhere
-    private var myCanDropAlias: Boolean = _
+    private var myCanDropAlias: Boolean = uninitialized
 
     /** Given an alias type `type A = B` where a recursive comparison with `B` yields
      *  `false`, can we conclude that the comparison is definitely false?
@@ -3405,7 +3405,7 @@ object Types {
   abstract case class AndType(tp1: Type, tp2: Type) extends AndOrType {
     def isAnd: Boolean = true
     private var myBaseClassesPeriod: Period = Nowhere
-    private var myBaseClasses: List[ClassSymbol] = _
+    private var myBaseClasses: List[ClassSymbol] = uninitialized
     /** Base classes are the merge of the operand base classes. */
     override final def baseClasses(using Context): List[ClassSymbol] = {
       if (myBaseClassesPeriod != ctx.period) {
@@ -3498,7 +3498,7 @@ object Types {
     def isAnd: Boolean = false
     def isSoft: Boolean
     private var myBaseClassesPeriod: Period = Nowhere
-    private var myBaseClasses: List[ClassSymbol] = _
+    private var myBaseClasses: List[ClassSymbol] = uninitialized
     /** Base classes are the intersection of the operand base classes. */
     override final def baseClasses(using Context): List[ClassSymbol] = {
       if (myBaseClassesPeriod != ctx.period) {
@@ -3527,7 +3527,7 @@ object Types {
         myFactorCount
       else 1
 
-    private var myJoin: Type = _
+    private var myJoin: Type = uninitialized
     private var myJoinPeriod: Period = Nowhere
 
     /** Replace or type by the closest non-or type above it */
@@ -3541,7 +3541,7 @@ object Types {
       myJoin
     }
 
-    private var myUnion: Type = _
+    private var myUnion: Type = uninitialized
     private var myUnionPeriod: Period = Nowhere
 
     override def widenUnionWithoutNull(using Context): Type =
@@ -3556,8 +3556,8 @@ object Types {
       myUnion
 
     private var atomsRunId: RunId = NoRunId
-    private var myAtoms: Atoms = _
-    private var myWidened: Type = _
+    private var myAtoms: Atoms = uninitialized
+    private var myWidened: Type = uninitialized
 
     private def computeAtoms()(using Context): Atoms =
       val tp1n = tp1.normalized
@@ -3797,11 +3797,11 @@ object Types {
     // (1) mySignatureRunId != NoRunId      =>  mySignature != null
     // (2) myJavaSignatureRunId != NoRunId  =>  myJavaSignature != null
 
-    private var mySignature: Signature = _
+    private var mySignature: Signature = uninitialized
     private var mySignatureRunId: Int = NoRunId
-    private var myJavaSignature: Signature = _
+    private var myJavaSignature: Signature = uninitialized
     private var myJavaSignatureRunId: Int = NoRunId
-    private var myScala2Signature: Signature = _
+    private var myScala2Signature: Signature = uninitialized
     private var myScala2SignatureRunId: Int = NoRunId
 
     /** If `isJava` is false, the Scala signature of this method. Otherwise, its Java signature.
@@ -4993,7 +4993,7 @@ object Types {
     def underlying(using Context): Type = bound
 
     private var myReduced: Type | Null = null
-    private var reductionContext: util.MutableMap[Type, Type] = _
+    private var reductionContext: util.MutableMap[Type, Type] = uninitialized
 
     override def tryNormalize(using Context): Type =
       try
@@ -5420,7 +5420,7 @@ object Types {
     override def stripped(using Context): Type = parent.stripped
 
     private var isRefiningKnown = false
-    private var isRefiningCache: Boolean = _
+    private var isRefiningCache: Boolean = uninitialized
 
     def isRefining(using Context): Boolean = {
       if (!isRefiningKnown) {
diff --git a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
index 5e816502f359..349dbc445971 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
@@ -25,6 +25,8 @@ import io.{AbstractFile, ZipArchive}
 import scala.util.control.NonFatal
 import dotty.tools.dotc.classpath.FileUtils.classToTasty
 
+import scala.compiletime.uninitialized
+
 object ClassfileParser {
 
   import ClassfileConstants._
@@ -237,11 +239,11 @@ class ClassfileParser(
 
   protected val staticModule: Symbol = moduleRoot.sourceModule(using ictx)
 
-  protected val instanceScope: MutableScope = newScope(0)     // the scope of all instance definitions
-  protected val staticScope: MutableScope = newScope(0)       // the scope of all static definitions
-  protected var pool: ConstantPool = _              // the classfile's constant pool
+  protected val instanceScope: MutableScope = newScope(0) // the scope of all instance definitions
+  protected val staticScope: MutableScope = newScope(0)   // the scope of all static definitions
+  protected var pool: ConstantPool = uninitialized        // the classfile's constant pool
 
-  protected var currentClassName: SimpleName = _      // JVM name of the current class
+  protected var currentClassName: SimpleName = uninitialized // JVM name of the current class
   protected var classTParams: Map[Name, Symbol] = Map()
 
   private var Scala2UnpicklingMode = Mode.Scala2Unpickling
diff --git a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileTastyUUIDParser.scala b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileTastyUUIDParser.scala
index a9c91a68bb60..4c4885fd5313 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileTastyUUIDParser.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileTastyUUIDParser.scala
@@ -2,6 +2,7 @@ package dotty.tools.dotc
 package core.classfile
 
 import scala.language.unsafeNulls
+import scala.compiletime.uninitialized
 
 import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.core.Decorators._
@@ -21,7 +22,7 @@ class ClassfileTastyUUIDParser(classfile: AbstractFile)(ictx: Context) {
 
   import ClassfileConstants._
 
-  private var pool: ConstantPool = _              // the classfile's constant pool
+  private var pool: ConstantPool = uninitialized // the classfile's constant pool
 
   def checkTastyUUID(tastyUUID: UUID)(using Context): Unit = try ctx.base.reusableDataReader.withInstance { reader =>
     implicit val reader2 = reader.reset(classfile)
diff --git a/compiler/src/dotty/tools/dotc/core/tasty/PositionUnpickler.scala b/compiler/src/dotty/tools/dotc/core/tasty/PositionUnpickler.scala
index 9c66e43eae80..bc58138f2db6 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/PositionUnpickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/PositionUnpickler.scala
@@ -3,6 +3,8 @@ package dotc
 package core
 package tasty
 
+import scala.compiletime.uninitialized
+
 import dotty.tools.tasty.{TastyFormat, TastyBuffer, TastyReader}
 import TastyFormat.SOURCE
 import TastyBuffer.{Addr, NameRef}
@@ -14,9 +16,9 @@ import Names.TermName
 class PositionUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName) {
   import reader._
 
-  private var myLineSizes: Array[Int] = _
-  private var mySpans: util.HashMap[Addr, Span] = _
-  private var mySourcePaths: util.HashMap[Addr, String] = _
+  private var myLineSizes: Array[Int] = uninitialized
+  private var mySpans: util.HashMap[Addr, Span] = uninitialized
+  private var mySourcePaths: util.HashMap[Addr, String] = uninitialized
   private var isDefined = false
 
   def ensureDefined(): Unit = {
diff --git a/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala b/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
index 1748b1edd08e..840bb3bd94fb 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
@@ -46,6 +46,7 @@ import dotty.tools.tasty.TastyFormat._
 
 import scala.annotation.constructorOnly
 import scala.annotation.internal.sharable
+import scala.compiletime.uninitialized
 
 /** Unpickler for typed trees
  *  @param reader              the reader from which to unpickle
@@ -88,7 +89,7 @@ class TreeUnpickler(reader: TastyReader,
   private var seenRoots: Set[Symbol] = Set()
 
   /** The root owner tree. See `OwnerTree` class definition. Set by `enterTopLevel`. */
-  private var ownerTree: OwnerTree = _
+  private var ownerTree: OwnerTree = uninitialized
 
   /** Was unpickled class compiled with pureFunctions? */
   private var withPureFuns: Boolean = false
diff --git a/compiler/src/dotty/tools/dotc/parsing/CharArrayReader.scala b/compiler/src/dotty/tools/dotc/parsing/CharArrayReader.scala
index c63409d0d52b..a3769f4f813d 100644
--- a/compiler/src/dotty/tools/dotc/parsing/CharArrayReader.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/CharArrayReader.scala
@@ -4,6 +4,8 @@ package parsing
 
 import util.Chars._
 
+import scala.compiletime.uninitialized
+
 abstract class CharArrayReader { self =>
 
   val buf: Array[Char]
@@ -16,7 +18,7 @@ abstract class CharArrayReader { self =>
   protected def error(msg: String, offset: Int): Unit
 
   /** the last read character */
-  var ch: Char = _
+  var ch: Char = uninitialized
 
   /** The offset one past the last read character */
   var charOffset: Int = startFrom
diff --git a/compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala b/compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala
index b3f41fab9eaa..0f0a2a3ee61f 100644
--- a/compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala
@@ -4,6 +4,7 @@ package parsing
 package xml
 
 import scala.language.unsafeNulls
+import scala.compiletime.uninitialized
 
 import scala.collection.mutable
 import scala.collection.BufferedIterator
@@ -71,7 +72,7 @@ object MarkupParsers {
       if (ch == SU) throw TruncatedXMLControl
       else reportSyntaxError(msg)
 
-    var input : CharArrayReader = _
+    var input : CharArrayReader = uninitialized
     def lookahead(): BufferedIterator[Char] =
       (input.buf drop input.charOffset).iterator.buffered
 
diff --git a/compiler/src/dotty/tools/dotc/parsing/xml/SymbolicXMLBuilder.scala b/compiler/src/dotty/tools/dotc/parsing/xml/SymbolicXMLBuilder.scala
index 0e70cc077fa4..6d49360feb85 100644
--- a/compiler/src/dotty/tools/dotc/parsing/xml/SymbolicXMLBuilder.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/xml/SymbolicXMLBuilder.scala
@@ -4,6 +4,7 @@ package parsing
 package xml
 
 import scala.language.unsafeNulls
+import scala.compiletime.uninitialized
 
 import scala.collection.mutable
 import core._
@@ -32,7 +33,7 @@ class SymbolicXMLBuilder(parser: Parser, preserveWS: Boolean)(using Context) {
 
   import parser.atSpan
 
-  private[parsing] var isPattern: Boolean = _
+  private[parsing] var isPattern: Boolean = uninitialized
 
   private object xmltypes extends ScalaTypeNames {
     val _Comment: TypeName             = "Comment"
diff --git a/compiler/src/dotty/tools/dotc/plugins/Plugins.scala b/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
index c44fe4cf59b4..12acc3d7b4c3 100644
--- a/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
+++ b/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
@@ -11,6 +11,8 @@ import dotty.tools.io._
 import Phases._
 import config.Printers.plugins.{ println => debug }
 
+import scala.compiletime.uninitialized
+
 /** Support for run-time loading of compiler plugins.
  *
  *  @author Lex Spoon
@@ -44,7 +46,7 @@ trait Plugins {
     goods map (_.get)
   }
 
-  private var _roughPluginsList: List[Plugin] = _
+  private var _roughPluginsList: List[Plugin] = uninitialized
   protected def roughPluginsList(using Context): List[Plugin] =
     if (_roughPluginsList == null) {
       _roughPluginsList = loadRoughPluginsList
@@ -96,7 +98,7 @@ trait Plugins {
     plugs
   }
 
-  private var _plugins: List[Plugin] = _
+  private var _plugins: List[Plugin] = uninitialized
   def plugins(using Context): List[Plugin] =
     if (_plugins == null) {
       _plugins = loadPlugins
diff --git a/compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala b/compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala
index d0330a955148..60cfeeced209 100644
--- a/compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala
+++ b/compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala
@@ -29,7 +29,7 @@ import xsbti.api.DependencyContext._
 import scala.jdk.CollectionConverters.*
 
 import scala.collection.{Set, mutable}
-
+import scala.compiletime.uninitialized
 
 /** This phase sends information on classes' dependencies to sbt via callbacks.
  *
@@ -519,9 +519,9 @@ class DependencyRecorder {
     }
   }
 
-  private var lastOwner: Symbol = _
-  private var lastDepSource: Symbol = _
-  private var lastFoundCache: FoundDepsInClass | Null = _
+  private var lastOwner: Symbol = uninitialized
+  private var lastDepSource: Symbol = uninitialized
+  private var lastFoundCache: FoundDepsInClass | Null = uninitialized
 
   /** The source of the dependency according to `nonLocalEnclosingClass`
    *  if it exists, otherwise fall back to `responsibleForImports`.
@@ -558,7 +558,7 @@ class DependencyRecorder {
     clazz
   }
 
-  private var _responsibleForImports: Symbol = _
+  private var _responsibleForImports: Symbol = uninitialized
 
   /** Top level import dependencies are registered as coming from a first top level
    *  class/trait/object declared in the compilation unit. If none exists, issue a warning and return NoSymbol.
diff --git a/compiler/src/dotty/tools/dotc/transform/CapturedVars.scala b/compiler/src/dotty/tools/dotc/transform/CapturedVars.scala
index 8964beb26051..a018bbd1a3ac 100644
--- a/compiler/src/dotty/tools/dotc/transform/CapturedVars.scala
+++ b/compiler/src/dotty/tools/dotc/transform/CapturedVars.scala
@@ -14,6 +14,8 @@ import core.Constants._
 import util.Store
 import dotty.tools.uncheckedNN
 
+import scala.compiletime.uninitialized
+
 /** This phase translates variables that are captured in closures to
  *  heap-allocated refs.
  */
@@ -25,7 +27,7 @@ class CapturedVars extends MiniPhase with IdentityDenotTransformer:
 
   override def description: String = CapturedVars.description
 
-  private[this] var Captured: Store.Location[util.ReadOnlySet[Symbol]] = _
+  private[this] var Captured: Store.Location[util.ReadOnlySet[Symbol]] = uninitialized
   private def captured(using Context) = ctx.store(Captured)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/transform/CtxLazy.scala b/compiler/src/dotty/tools/dotc/transform/CtxLazy.scala
index 808cf928ecc2..4f8f1b195bb3 100644
--- a/compiler/src/dotty/tools/dotc/transform/CtxLazy.scala
+++ b/compiler/src/dotty/tools/dotc/transform/CtxLazy.scala
@@ -3,6 +3,8 @@ package transform
 
 import core.Contexts._
 
+import scala.compiletime.uninitialized
+
 /** Utility class for lazy values whose evaluation depends on a context.
  *  This should be used whenever the evaluation of a lazy expression
  *  depends on some context, but the value can be re-used afterwards
@@ -12,7 +14,7 @@ import core.Contexts._
  *  the expression intiializing the lazy val depends only on the root context, but not any changes afterwards.
  */
 class CtxLazy[T](expr: Context ?=> T) {
-  private var myValue: T = _
+  private var myValue: T = uninitialized
   private var forced = false
   def apply()(using Context): T = {
     if (!forced) {
diff --git a/compiler/src/dotty/tools/dotc/transform/Dependencies.scala b/compiler/src/dotty/tools/dotc/transform/Dependencies.scala
index 08189e107d94..46285c324307 100644
--- a/compiler/src/dotty/tools/dotc/transform/Dependencies.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Dependencies.scala
@@ -6,6 +6,7 @@ import Symbols.*, Contexts.*, Types.*, Flags.*, Decorators.*
 import SymUtils.*
 import collection.mutable.{LinkedHashMap, LinkedHashSet}
 import annotation.constructorOnly
+import scala.compiletime.uninitialized
 
 import dotty.tools.backend.sjs.JSDefinitions.jsdefn
 
@@ -51,10 +52,10 @@ abstract class Dependencies(root: ast.tpd.Tree, @constructorOnly rootContext: Co
   private val logicOwner = new LinkedHashMap[Symbol, Symbol]
 
   /** A flag to indicate whether new free variables have been found */
-  private var changedFreeVars: Boolean = _
+  private var changedFreeVars: Boolean = uninitialized
 
   /** A flag to indicate whether lifted owners have changed */
-  private var changedLogicOwner: Boolean = _
+  private var changedLogicOwner: Boolean = uninitialized
 
   private def newSymSet: LinkedHashSet[Symbol] = new LinkedHashSet[Symbol]
 
diff --git a/compiler/src/dotty/tools/dotc/transform/Flatten.scala b/compiler/src/dotty/tools/dotc/transform/Flatten.scala
index 25df51d0916d..84f3d35c80b7 100644
--- a/compiler/src/dotty/tools/dotc/transform/Flatten.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Flatten.scala
@@ -10,6 +10,8 @@ import collection.mutable
 import MegaPhase.MiniPhase
 import util.Store
 
+import scala.compiletime.uninitialized
+
 /** Lift nested classes to toplevel */
 class Flatten extends MiniPhase with SymTransformer {
   import ast.tpd._
@@ -24,7 +26,7 @@ class Flatten extends MiniPhase with SymTransformer {
 
   override def changesMembers: Boolean = true // the phase removes inner classes
 
-  private var LiftedDefs: Store.Location[mutable.ListBuffer[Tree] | Null] = _
+  private var LiftedDefs: Store.Location[mutable.ListBuffer[Tree] | Null] = uninitialized
   private def liftedDefs(using Context) = ctx.store(LiftedDefs)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/transform/ForwardDepChecks.scala b/compiler/src/dotty/tools/dotc/transform/ForwardDepChecks.scala
index bf8a6fa6c7bf..afe78ce1296d 100644
--- a/compiler/src/dotty/tools/dotc/transform/ForwardDepChecks.scala
+++ b/compiler/src/dotty/tools/dotc/transform/ForwardDepChecks.scala
@@ -10,6 +10,8 @@ import collection.immutable
 import ast.tpd
 import MegaPhase.MiniPhase
 
+import scala.compiletime.uninitialized
+
 object ForwardDepChecks:
 
   import tpd.*
@@ -37,8 +39,8 @@ object ForwardDepChecks:
         (m1, idx + 1)
       }._1
     var maxIndex: Int = Int.MinValue
-    var refSpan: Span = _
-    var refSym: Symbol = _
+    var refSpan: Span = uninitialized
+    var refSym: Symbol = uninitialized
 
     override def enterReference(sym: Symbol, span: Span): Unit =
       if (sym.exists && sym.owner.isTerm)
@@ -63,7 +65,7 @@ class ForwardDepChecks extends MiniPhase:
 
   override def runsAfter: Set[String] = Set(ElimByName.name)
 
-  private var LevelInfo: Store.Location[OptLevelInfo] = _
+  private var LevelInfo: Store.Location[OptLevelInfo] = uninitialized
   private def currentLevel(using Context): OptLevelInfo = ctx.store(LevelInfo)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/transform/Instrumentation.scala b/compiler/src/dotty/tools/dotc/transform/Instrumentation.scala
index 046147f20d82..9802de34a6e0 100644
--- a/compiler/src/dotty/tools/dotc/transform/Instrumentation.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Instrumentation.scala
@@ -12,6 +12,8 @@ import MegaPhase._
 import Names._
 import Constants.Constant
 
+import scala.compiletime.uninitialized
+
 
 /** The phase is enabled if the -Yinstrument option is set.
  *  If enabled, it counts the number of closures or allocations for each source position.
@@ -40,11 +42,11 @@ class Instrumentation extends MiniPhase { thisPhase =>
     "::", "+=", "toString", "newArray", "box", "toCharArray", "termName", "typeName",
     "slice", "staticRef", "requiredClass")
 
-  private var namesToRecord: Set[Name] = _
-  private var collectionNamesToRecord: Set[Name] = _
-  private var Stats_doRecord: Symbol = _
-  private var Stats_doRecordSize: Symbol = _
-  private var CollectionIterableClass: ClassSymbol = _
+  private var namesToRecord: Set[Name] = uninitialized
+  private var collectionNamesToRecord: Set[Name] = uninitialized
+  private var Stats_doRecord: Symbol = uninitialized
+  private var Stats_doRecordSize: Symbol = uninitialized
+  private var CollectionIterableClass: ClassSymbol = uninitialized
 
   override def prepareForUnit(tree: Tree)(using Context): Context =
     namesToRecord = namesOfInterest.map(_.toTermName).toSet
diff --git a/compiler/src/dotty/tools/dotc/transform/LambdaLift.scala b/compiler/src/dotty/tools/dotc/transform/LambdaLift.scala
index 6ec0f330efff..5be43fd56952 100644
--- a/compiler/src/dotty/tools/dotc/transform/LambdaLift.scala
+++ b/compiler/src/dotty/tools/dotc/transform/LambdaLift.scala
@@ -18,6 +18,8 @@ import ExplicitOuter.outer
 import util.Store
 import collection.mutable.{HashMap, LinkedHashMap, ListBuffer}
 
+import scala.compiletime.uninitialized
+
 object LambdaLift:
   import ast.tpd._
 
@@ -266,7 +268,7 @@ class LambdaLift extends MiniPhase with IdentityDenotTransformer { thisPhase =>
     // lambda lift for super calls right. Witness the implementation restrictions to
     // this effect in scalac.
 
-  private var Lifter: Store.Location[Lifter] = _
+  private var Lifter: Store.Location[Lifter] = uninitialized
   private def lifter(using Context) = ctx.store(Lifter)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/transform/LazyVals.scala b/compiler/src/dotty/tools/dotc/transform/LazyVals.scala
index b433e37e39c0..4fabaf99f843 100644
--- a/compiler/src/dotty/tools/dotc/transform/LazyVals.scala
+++ b/compiler/src/dotty/tools/dotc/transform/LazyVals.scala
@@ -19,6 +19,7 @@ import transform.MegaPhase.MiniPhase
 import transform.SymUtils.*
 
 import scala.collection.mutable
+import scala.compiletime.uninitialized
 
 class LazyVals extends MiniPhase with IdentityDenotTransformer {
   import LazyVals._
@@ -47,7 +48,7 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
   val containerFlagsMask: FlagSet = Method | Lazy | Accessor | Module
 
   /** A map of lazy values to the fields they should null after initialization. */
-  private var lazyValNullables: IdentityHashMap[Symbol, mutable.ListBuffer[Symbol]] | Null = _
+  private var lazyValNullables: IdentityHashMap[Symbol, mutable.ListBuffer[Symbol]] | Null = uninitialized
   private def nullableFor(sym: Symbol)(using Context) = {
     // optimisation: value only used once, we can remove the value from the map
     val nullables = lazyValNullables.nn.remove(sym)
diff --git a/compiler/src/dotty/tools/dotc/transform/MegaPhase.scala b/compiler/src/dotty/tools/dotc/transform/MegaPhase.scala
index fe70a1659036..eff634b485b8 100644
--- a/compiler/src/dotty/tools/dotc/transform/MegaPhase.scala
+++ b/compiler/src/dotty/tools/dotc/transform/MegaPhase.scala
@@ -2,6 +2,8 @@ package dotty.tools
 package dotc
 package transform
 
+import scala.compiletime.uninitialized
+
 import core._
 import Contexts._, Phases._, Symbols._, Decorators._
 import Flags.PackageVal
@@ -31,8 +33,8 @@ object MegaPhase {
    */
   abstract class MiniPhase extends Phase {
 
-    private[MegaPhase] var superPhase: MegaPhase = _
-    private[MegaPhase] var idxInGroup: Int = _
+    private[MegaPhase] var superPhase: MegaPhase = uninitialized
+    private[MegaPhase] var idxInGroup: Int = uninitialized
 
     /** List of names of phases that should have finished their processing of all compilation units
      *  before this phase starts
@@ -151,7 +153,7 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
     else
       s"MegaPhase{${miniPhases.head.phaseName},...,${miniPhases.last.phaseName}}"
 
-  private var relaxedTypingCache: Boolean = _
+  private var relaxedTypingCache: Boolean = uninitialized
   private var relaxedTypingKnown = false
 
   override final def relaxedTyping: Boolean = {
diff --git a/compiler/src/dotty/tools/dotc/transform/Memoize.scala b/compiler/src/dotty/tools/dotc/transform/Memoize.scala
index af6533cfc17f..91f394866035 100644
--- a/compiler/src/dotty/tools/dotc/transform/Memoize.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Memoize.scala
@@ -19,6 +19,7 @@ import StdNames.nme
 import sjs.JSSymUtils._
 
 import util.Store
+import scala.compiletime.uninitialized
 
 object Memoize {
   val name: String = "memoize"
@@ -52,7 +53,7 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
 
   override def description: String = Memoize.description
 
-  private var MyState: Store.Location[MyState] = _
+  private var MyState: Store.Location[MyState] = uninitialized
   private def myState(using Context): MyState = ctx.store(MyState)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala b/compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala
index 0e38e9c074cd..92039a8b9af8 100644
--- a/compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala
+++ b/compiler/src/dotty/tools/dotc/transform/OverridingPairs.scala
@@ -10,6 +10,8 @@ import collection.immutable.BitSet
 import scala.annotation.tailrec
 import cc.isCaptureChecking
 
+import scala.compiletime.uninitialized
+
 /** A module that can produce a kind of iterator (`Cursor`),
  *  which yields all pairs of overriding/overridden symbols
  *  that are visible in some baseclass, unless there's a parent class
@@ -118,10 +120,10 @@ object OverridingPairs:
     private var nextEntry = curEntry
 
     /** The current candidate symbol for overriding */
-    var overriding: Symbol = _
+    var overriding: Symbol = uninitialized
 
     /** If not null: The symbol overridden by overriding */
-    var overridden: Symbol = _
+    var overridden: Symbol = uninitialized
 
     //@M: note that next is called once during object initialization
     final def hasNext: Boolean = nextEntry != null
diff --git a/compiler/src/dotty/tools/dotc/transform/TailRec.scala b/compiler/src/dotty/tools/dotc/transform/TailRec.scala
index 741b9d1627fe..45330b6e9f5d 100644
--- a/compiler/src/dotty/tools/dotc/transform/TailRec.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TailRec.scala
@@ -13,6 +13,8 @@ import transform.MegaPhase.MiniPhase
 import util.LinearSet
 import dotty.tools.uncheckedNN
 
+import scala.compiletime.uninitialized
+
 /** A Tail Rec Transformer.
  *
  *  What it does:
@@ -232,7 +234,7 @@ class TailRec extends MiniPhase {
     var failureReported: Boolean = false
 
     /** The `tailLabelN` label symbol, used to encode a `continue` from the infinite `while` loop. */
-    private var myContinueLabel: Symbol | Null = _
+    private var myContinueLabel: Symbol | Null = uninitialized
     def continueLabel(using Context): Symbol = {
       if (myContinueLabel == null)
         myContinueLabel = newSymbol(method, TailLabelName.fresh(), Label, defn.UnitType)
diff --git a/compiler/src/dotty/tools/dotc/transform/sjs/ExplicitJSClasses.scala b/compiler/src/dotty/tools/dotc/transform/sjs/ExplicitJSClasses.scala
index 705b3cc404a8..71783c509596 100644
--- a/compiler/src/dotty/tools/dotc/transform/sjs/ExplicitJSClasses.scala
+++ b/compiler/src/dotty/tools/dotc/transform/sjs/ExplicitJSClasses.scala
@@ -3,6 +3,7 @@ package dotc
 package transform
 package sjs
 
+import scala.compiletime.uninitialized
 
 import MegaPhase._
 import core.Annotations._
@@ -236,7 +237,7 @@ class ExplicitJSClasses extends MiniPhase with InfoTransformer { thisPhase =>
 
   override def description: String = ExplicitJSClasses.description
 
-  private var MyState: Store.Location[MyState] = _
+  private var MyState: Store.Location[MyState] = uninitialized
   private def myState(using Context) = ctx.store(MyState)
 
   override def initContext(ctx: FreshContext): Unit =
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 5cba406a302e..58323f708144 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -37,6 +37,7 @@ import annotation.tailrec
 
 import scala.annotation.internal.sharable
 import scala.annotation.threadUnsafe
+import scala.compiletime.uninitialized
 
 /** Implicit resolution */
 object Implicits:
@@ -620,7 +621,7 @@ trait ImplicitRunInfo:
 
     object collectParts extends TypeTraverser:
 
-      private var parts: mutable.LinkedHashSet[Type] = _
+      private var parts: mutable.LinkedHashSet[Type] = uninitialized
       private val partSeen = util.HashSet[Type]()
 
       def traverse(t: Type) = try
diff --git a/compiler/src/dotty/tools/dotc/typer/ImportInfo.scala b/compiler/src/dotty/tools/dotc/typer/ImportInfo.scala
index ba05cba229ae..4850e7bdffa9 100644
--- a/compiler/src/dotty/tools/dotc/typer/ImportInfo.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ImportInfo.scala
@@ -12,6 +12,8 @@ import StdNames.nme
 import printing.Texts.Text
 import NameKinds.QualifiedName
 
+import scala.compiletime.uninitialized
+
 object ImportInfo {
 
   case class RootRef(refFn: () => TermRef, isPredef: Boolean = false)
@@ -66,7 +68,7 @@ class ImportInfo(symf: Context ?=> Symbol,
     }
     mySym.uncheckedNN
   }
-  private var mySym: Symbol | Null = _
+  private var mySym: Symbol | Null = uninitialized
 
   /** The (TermRef) type of the qualifier of the import clause */
   def site(using Context): Type = importSym.info match {
@@ -180,7 +182,7 @@ class ImportInfo(symf: Context ?=> Symbol,
 
   private val isLanguageImport: Boolean = untpd.languageImport(qualifier).isDefined
 
-  private var myUnimported: Symbol | Null = _
+  private var myUnimported: Symbol | Null = uninitialized
 
   private var featureCache: SimpleIdentityMap[TermName, java.lang.Boolean] = SimpleIdentityMap.empty
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 44d8077e44db..6d6580e740db 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -30,6 +30,7 @@ import reporting._
 import config.Feature.sourceVersion
 import config.SourceVersion._
 
+import scala.compiletime.uninitialized
 
 /** This class creates symbols from definitions and imports and gives them
  *  lazy types.
@@ -1078,7 +1079,7 @@ class Namer { typer: Typer =>
 
     protected implicit val completerCtx: Context = localContext(cls)
 
-    private var localCtx: Context = _
+    private var localCtx: Context = uninitialized
 
     /** info to be used temporarily while completing the class, to avoid cyclic references. */
     private var tempInfo: TempClassInfo | Null = null
diff --git a/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala b/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
index 21fa9eed0df4..7d129e128518 100644
--- a/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
+++ b/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
@@ -13,6 +13,8 @@ import config.Feature.migrateTo3
 import reporting.trace
 import printing.Formatting.hl
 
+import scala.compiletime.uninitialized
+
 /** Provides `check` method to check that all top-level definitions
  *  in tree are variance correct. Does not recurse inside methods.
  *  The method should be invoked once for each Template.
@@ -67,7 +69,7 @@ class VarianceChecker(using Context) {
   import tpd._
 
   private object Validator extends TypeAccumulator[Option[VarianceError]] {
-    private var base: Symbol = _
+    private var base: Symbol = uninitialized
 
     /** The variance of a symbol occurrence of `tvar` seen at the level of the definition of `base`.
      *  The search proceeds from `base` to the owner of `tvar`.
diff --git a/compiler/src/dotty/tools/dotc/util/GenericHashMap.scala b/compiler/src/dotty/tools/dotc/util/GenericHashMap.scala
index 6d013717ec52..8c0506573109 100644
--- a/compiler/src/dotty/tools/dotc/util/GenericHashMap.scala
+++ b/compiler/src/dotty/tools/dotc/util/GenericHashMap.scala
@@ -1,6 +1,8 @@
 package dotty.tools
 package dotc.util
 
+import scala.compiletime.uninitialized
+
 object GenericHashMap:
 
   /** The number of elements up to which dense packing is used.
@@ -27,9 +29,9 @@ abstract class GenericHashMap[Key, Value]
     (initialCapacity: Int, capacityMultiple: Int) extends MutableMap[Key, Value]:
   import GenericHashMap.DenseLimit
 
-  protected var used: Int = _
-  protected var limit: Int = _
-  protected var table: Array[AnyRef | Null] = _
+  protected var used: Int = uninitialized
+  protected var limit: Int = uninitialized
+  protected var table: Array[AnyRef | Null] = uninitialized
   clear()
 
   private def allocate(capacity: Int) =
diff --git a/compiler/src/dotty/tools/dotc/util/GenericHashSet.scala b/compiler/src/dotty/tools/dotc/util/GenericHashSet.scala
index 7abe40a8e13d..3c30e8e73300 100644
--- a/compiler/src/dotty/tools/dotc/util/GenericHashSet.scala
+++ b/compiler/src/dotty/tools/dotc/util/GenericHashSet.scala
@@ -2,6 +2,8 @@ package dotty.tools.dotc.util
 
 import dotty.tools.uncheckedNN
 
+import scala.compiletime.uninitialized
+
 object GenericHashSet:
 
   /** The number of elements up to which dense packing is used.
@@ -24,9 +26,9 @@ object GenericHashSet:
 abstract class GenericHashSet[T](initialCapacity: Int = 8, capacityMultiple: Int = 2) extends MutableSet[T] {
   import GenericHashSet.DenseLimit
 
-  protected var used: Int = _
-  protected var limit: Int = _
-  protected var table: Array[AnyRef | Null] = _
+  protected var used: Int = uninitialized
+  protected var limit: Int = uninitialized
+  protected var table: Array[AnyRef | Null] = uninitialized
 
   clear()
 
diff --git a/compiler/src/dotty/tools/dotc/util/IntMap.scala b/compiler/src/dotty/tools/dotc/util/IntMap.scala
index 008ea866f70e..1d04567e99c7 100644
--- a/compiler/src/dotty/tools/dotc/util/IntMap.scala
+++ b/compiler/src/dotty/tools/dotc/util/IntMap.scala
@@ -1,5 +1,7 @@
 package dotty.tools.dotc.util
 
+import scala.compiletime.uninitialized
+
 /** A dense map from some `Key` type to `Int. Dense means: All keys and values
  *  are stored in arrays from 0 up to the size of the map. Keys and values
  *  can be obtained by index using `key(index)` and `value(index)`. Values
@@ -19,7 +21,7 @@ package dotty.tools.dotc.util
  */
 final class IntMap[Key](initialCapacity: Int = 8, capacityMultiple: Int = 2)
 extends PerfectHashing[Key](initialCapacity, capacityMultiple):
-  private var values: Array[Int] = _
+  private var values: Array[Int] = uninitialized
 
   def default: Int = -1
 
diff --git a/compiler/src/dotty/tools/dotc/util/PerfectHashing.scala b/compiler/src/dotty/tools/dotc/util/PerfectHashing.scala
index fca790837959..1351afb68f3d 100644
--- a/compiler/src/dotty/tools/dotc/util/PerfectHashing.scala
+++ b/compiler/src/dotty/tools/dotc/util/PerfectHashing.scala
@@ -1,5 +1,7 @@
 package dotty.tools.dotc.util
 
+import scala.compiletime.uninitialized
+
 object PerfectHashing:
 
   /** The number of elements up to which dense packing is used.
@@ -22,9 +24,9 @@ object PerfectHashing:
 class PerfectHashing[Key](initialCapacity: Int = 8, capacityMultiple: Int = 2):
   import PerfectHashing.DenseLimit
 
-  private var used: Int = _
-  private var table: Array[Int] = _
-  private var keys: Array[AnyRef] = _
+  private var used: Int = uninitialized
+  private var table: Array[Int] = uninitialized
+  private var keys: Array[AnyRef] = uninitialized
 
   clear()
 
diff --git a/compiler/src/dotty/tools/dotc/util/SourceFile.scala b/compiler/src/dotty/tools/dotc/util/SourceFile.scala
index 3462036d7ba6..d35509f591a3 100644
--- a/compiler/src/dotty/tools/dotc/util/SourceFile.scala
+++ b/compiler/src/dotty/tools/dotc/util/SourceFile.scala
@@ -13,6 +13,7 @@ import Chars._
 import scala.annotation.internal.sharable
 import scala.collection.mutable
 import scala.collection.mutable.ArrayBuffer
+import scala.compiletime.uninitialized
 import scala.util.chaining.given
 
 import java.io.File.separator
@@ -137,7 +138,7 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
     buf.toArray
   }
 
-  private var lineIndicesCache: Array[Int] = _
+  private var lineIndicesCache: Array[Int] = uninitialized
   private def lineIndices: Array[Int] =
     if lineIndicesCache eq null then
       lineIndicesCache = calculateLineIndicesFromContents()
diff --git a/compiler/src/dotty/tools/repl/CollectTopLevelImports.scala b/compiler/src/dotty/tools/repl/CollectTopLevelImports.scala
index d539c1986f8d..8a12ae22be37 100644
--- a/compiler/src/dotty/tools/repl/CollectTopLevelImports.scala
+++ b/compiler/src/dotty/tools/repl/CollectTopLevelImports.scala
@@ -4,6 +4,8 @@ import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.core.Phases.Phase
 
+import scala.compiletime.uninitialized
+
 /** A phase that collects user defined top level imports.
  *
  *  These imports must be collected as typed trees and therefore
@@ -14,7 +16,7 @@ class CollectTopLevelImports extends Phase {
 
   def phaseName: String = "collectTopLevelImports"
 
-  private var myImports: List[Import] = _
+  private var myImports: List[Import] = uninitialized
   def imports: List[Import] = myImports
 
   def run(using Context): Unit = {
diff --git a/compiler/src/dotty/tools/repl/Rendering.scala b/compiler/src/dotty/tools/repl/Rendering.scala
index c647ef302bb9..972baa39877b 100644
--- a/compiler/src/dotty/tools/repl/Rendering.scala
+++ b/compiler/src/dotty/tools/repl/Rendering.scala
@@ -10,6 +10,7 @@ import reporting.Diagnostic
 import transform.ValueClasses
 import util.StackTraceOps.*
 
+import scala.compiletime.uninitialized
 import scala.util.control.NonFatal
 
 /** This rendering object uses `ClassLoader`s to accomplish crossing the 4th
@@ -24,10 +25,10 @@ private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None):
 
   import Rendering._
 
-  var myClassLoader: AbstractFileClassLoader = _
+  var myClassLoader: AbstractFileClassLoader = uninitialized
 
   /** (value, maxElements, maxCharacters) => String */
-  var myReplStringOf: (Object, Int, Int) => String = _
+  var myReplStringOf: (Object, Int, Int) => String = uninitialized
 
   /** Class loader used to load compiled code */
   private[repl] def classLoader()(using Context) =
diff --git a/compiler/src/dotty/tools/repl/ReplDriver.scala b/compiler/src/dotty/tools/repl/ReplDriver.scala
index 2471f6bece42..6bb3d21dd45f 100644
--- a/compiler/src/dotty/tools/repl/ReplDriver.scala
+++ b/compiler/src/dotty/tools/repl/ReplDriver.scala
@@ -36,6 +36,7 @@ import org.jline.reader._
 
 import scala.annotation.tailrec
 import scala.collection.mutable
+import scala.compiletime.uninitialized
 import scala.jdk.CollectionConverters._
 import scala.util.control.NonFatal
 import scala.util.Using
@@ -116,10 +117,10 @@ class ReplDriver(settings: Array[String],
     rendering = new Rendering(classLoader)
   }
 
-  private var rootCtx: Context = _
-  private var shouldStart: Boolean = _
-  private var compiler: ReplCompiler = _
-  protected var rendering: Rendering = _
+  private var rootCtx: Context = uninitialized
+  private var shouldStart: Boolean = uninitialized
+  private var compiler: ReplCompiler = uninitialized
+  protected var rendering: Rendering = uninitialized
 
   // initialize the REPL session as part of the constructor so that once `run`
   // is called, we're in business
diff --git a/compiler/test/dotty/tools/dotc/reporting/TestReporter.scala b/compiler/test/dotty/tools/dotc/reporting/TestReporter.scala
index 03b61c393d35..a96a2765d56a 100644
--- a/compiler/test/dotty/tools/dotc/reporting/TestReporter.scala
+++ b/compiler/test/dotty/tools/dotc/reporting/TestReporter.scala
@@ -17,6 +17,7 @@ import dotty.Properties
 import interfaces.Diagnostic.{ERROR, WARNING}
 
 import scala.io.Codec
+import scala.compiletime.uninitialized
 
 class TestReporter protected (outWriter: PrintWriter, logLevel: Int)
 extends Reporter with UniqueMessagePositions with HideNonSensicalMessages with MessageRendering {
@@ -81,9 +82,9 @@ object TestReporter {
   private val failedTestsFileName: String = "last-failed.log"
   private val failedTestsFile: JFile = new JFile(s"$testLogsDirName/$failedTestsFileName")
 
-  private var outFile: JFile = _
-  private var logWriter: PrintWriter = _
-  private var failedTestsWriter: PrintWriter = _
+  private var outFile: JFile = uninitialized
+  private var logWriter: PrintWriter = uninitialized
+  private var failedTestsWriter: PrintWriter = uninitialized
 
   private def initLog() = if (logWriter eq null) {
     val date = new Date
diff --git a/compiler/test/dotty/tools/vulpix/RunnerOrchestration.scala b/compiler/test/dotty/tools/vulpix/RunnerOrchestration.scala
index 8e9a27e766b4..9047bb6737dc 100644
--- a/compiler/test/dotty/tools/vulpix/RunnerOrchestration.scala
+++ b/compiler/test/dotty/tools/vulpix/RunnerOrchestration.scala
@@ -14,6 +14,7 @@ import scala.concurrent.duration.Duration
 import scala.concurrent.{ Await, Future }
 import scala.concurrent.ExecutionContext.Implicits.global
 import scala.collection.mutable
+import scala.compiletime.uninitialized
 
 /** Vulpix spawns JVM subprocesses (`numberOfSlaves`) in order to run tests
  *  without compromising the main JVM
@@ -70,8 +71,8 @@ trait RunnerOrchestration {
       withRunner(_.runMain(classPath))
 
     private class Runner(private var process: Process) {
-      private var childStdout: BufferedReader = _
-      private var childStdin: PrintStream = _
+      private var childStdout: BufferedReader = uninitialized
+      private var childStdin: PrintStream = uninitialized
 
       /** Checks if `process` is still alive
        *