diff --git a/compiler/src/dotty/tools/MainGenericCompiler.scala b/compiler/src/dotty/tools/MainGenericCompiler.scala
index aa924a237f73..2c3f6f97e79e 100644
--- a/compiler/src/dotty/tools/MainGenericCompiler.scala
+++ b/compiler/src/dotty/tools/MainGenericCompiler.scala
@@ -1,7 +1,5 @@
 package dotty.tools
 
-import scala.language.unsafeNulls
-
 import scala.annotation.tailrec
 import scala.io.Source
 import scala.util.Try
diff --git a/compiler/src/dotty/tools/backend/jvm/AsmUtils.scala b/compiler/src/dotty/tools/backend/jvm/AsmUtils.scala
index e6393ce82054..8a71a09aa7ab 100644
--- a/compiler/src/dotty/tools/backend/jvm/AsmUtils.scala
+++ b/compiler/src/dotty/tools/backend/jvm/AsmUtils.scala
@@ -2,8 +2,6 @@ package dotty.tools
 package backend
 package jvm
 
-import scala.language.unsafeNulls
-
 import scala.tools.asm.tree.{AbstractInsnNode}
 import java.io.PrintWriter
 import scala.tools.asm.util.{TraceClassVisitor, TraceMethodVisitor, Textifier}
diff --git a/compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala b/compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala
index e2730c1e84ab..e2ce73f79028 100644
--- a/compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala
+++ b/compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala
@@ -18,8 +18,6 @@ import BTypes.InternalName
 import scala.util.chaining.*
 import dotty.tools.io.JarArchive
 
-import scala.language.unsafeNulls
-
 /** !!! This file is now copied in `dotty.tools.io.FileWriters` in a more general way that does not rely upon
  * `PostProcessorFrontendAccess`, this should probably be changed to wrap that class instead.
  *
@@ -54,11 +52,11 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
     def close(): Unit
 
     protected def classRelativePath(className: InternalName, suffix: String = ".class"): String =
-      className.replace('.', '/').nn + suffix
+      className.replace('.', '/') + suffix
   }
 
   object ClassfileWriter {
-    private def getDirectory(dir: String): Path = Paths.get(dir).nn
+    private def getDirectory(dir: String): Path = Paths.get(dir)
 
     def apply(): ClassfileWriter = {
       val jarManifestMainClass: Option[String] = compilerSettings.mainClass.orElse {
@@ -137,7 +135,7 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
         new JarEntryWriter(jarFile, jarManifestMainClass, jarCompressionLevel)
       }
       else if (file.isVirtual) new VirtualFileWriter(file)
-      else if (file.isDirectory) new DirEntryWriter(file.file.toPath.nn)
+      else if (file.isDirectory) new DirEntryWriter(file.file.toPath)
       else throw new IllegalStateException(s"don't know how to handle an output of $file [${file.getClass}]")
   }
 
@@ -151,7 +149,7 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
     val jarWriter: JarOutputStream = {
       import scala.util.Properties.*
       val manifest = new Manifest
-      val attrs = manifest.getMainAttributes.nn
+      val attrs = manifest.getMainAttributes
       attrs.put(MANIFEST_VERSION, "1.0")
       attrs.put(ScalaCompilerVersion, versionNumberString)
       mainClass.foreach(c => attrs.put(MAIN_CLASS, c))
@@ -184,7 +182,7 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
       // important detail here, even on Windows, Zinc expects the separator within the jar
       // to be the system default, (even if in the actual jar file the entry always uses '/').
       // see https://github.com/sbt/zinc/blob/dcddc1f9cfe542d738582c43f4840e17c053ce81/internal/compiler-bridge/src/main/scala/xsbt/JarUtils.scala#L47
-      val pathInJar = 
+      val pathInJar =
         if File.separatorChar == '/' then relativePath
         else relativePath.replace('/', File.separatorChar)
       PlainFile.toPlainFile(Paths.get(s"${file.absolutePath}!$pathInJar"))
@@ -293,5 +291,5 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
   }
 
   /** Can't output a file due to the state of the file system. */
-  class FileConflictException(msg: String, cause: Throwable = null) extends IOException(msg, cause)
+  class FileConflictException(msg: String, cause: Throwable | Null = null) extends IOException(msg, cause)
 }
diff --git a/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala b/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
index 06c3c7f1cb4f..599b09273046 100644
--- a/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
+++ b/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
@@ -36,7 +36,7 @@ class PostProcessor(val frontendAccess: PostProcessorFrontendAccess, val bTypes:
         setInnerClasses(classNode)
         serializeClass(classNode)
       catch
-        case e: java.lang.RuntimeException if e.getMessage != null && e.getMessage.nn.contains("too large!") =>
+        case e: java.lang.RuntimeException if e.getMessage != null && e.getMessage.contains("too large!") =>
           backendReporting.error(em"Could not write class $internalName because it exceeds JVM code size limits. ${e.getMessage}")
           null
         case ex: Throwable =>
@@ -58,8 +58,8 @@ class PostProcessor(val frontendAccess: PostProcessorFrontendAccess, val bTypes:
   }
 
   private def warnCaseInsensitiveOverwrite(clazz: GeneratedClass) = {
-    val name = clazz.classNode.name.nn
-    val lowerCaseJavaName = name.nn.toLowerCase
+    val name = clazz.classNode.name
+    val lowerCaseJavaName = name.toLowerCase
     val clsPos = clazz.position
     caseInsensitively.putIfAbsent(lowerCaseJavaName, (name, clsPos)) match {
       case null => ()
diff --git a/compiler/src/dotty/tools/dotc/Bench.scala b/compiler/src/dotty/tools/dotc/Bench.scala
index cbc490919cfe..26259dcaa28b 100644
--- a/compiler/src/dotty/tools/dotc/Bench.scala
+++ b/compiler/src/dotty/tools/dotc/Bench.scala
@@ -30,7 +30,7 @@ object Bench extends Driver:
       println(s"time elapsed: ${times(curRun)}ms")
       if ctx.settings.Xprompt.value || waitAfter == curRun + 1 then
         print("hit <return> to continue >")
-        System.in.nn.read()
+        System.in.read()
     reporter
 
   def extractNumArg(args: Array[String], name: String, default: Int = 1): (Int, Array[String]) = {
diff --git a/compiler/src/dotty/tools/dotc/Driver.scala b/compiler/src/dotty/tools/dotc/Driver.scala
index 98abe2ac6c38..a401bb689db0 100644
--- a/compiler/src/dotty/tools/dotc/Driver.scala
+++ b/compiler/src/dotty/tools/dotc/Driver.scala
@@ -38,7 +38,7 @@ class Driver {
         finish(compiler, run)
       catch
         case ex: FatalError =>
-          report.error(ex.getMessage.nn) // signals that we should fail compilation.
+          report.error(ex.getMessage) // signals that we should fail compilation.
         case ex: Throwable if ctx.usedBestEffortTasty =>
           report.bestEffortError(ex, "Some best-effort tasty files were not able to be read.")
           throw ex
@@ -117,7 +117,7 @@ class Driver {
         .distinct
       val ctx1 = ctx.fresh
       val fullClassPath =
-        (newEntries :+ ctx.settings.classpath.value).mkString(java.io.File.pathSeparator.nn)
+        (newEntries :+ ctx.settings.classpath.value).mkString(java.io.File.pathSeparator)
       ctx1.setSetting(ctx1.settings.classpath, fullClassPath)
     else ctx
 
diff --git a/compiler/src/dotty/tools/dotc/Run.scala b/compiler/src/dotty/tools/dotc/Run.scala
index 11a0430480d9..ca51788c3a53 100644
--- a/compiler/src/dotty/tools/dotc/Run.scala
+++ b/compiler/src/dotty/tools/dotc/Run.scala
@@ -211,7 +211,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     try
       trackProgress(_.cancel())
     finally
-      Thread.currentThread().nn.interrupt()
+      Thread.currentThread().interrupt()
 
   private def doAdvancePhase(currentPhase: Phase, wasRan: Boolean)(using Context): Unit =
     trackProgress: progress =>
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 44d5e2cf4b88..e891e1b6d402 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -630,7 +630,7 @@ object CaptureSet:
 
     // For debugging: A trace where a set was created. Note that logically it would make more
     // sense to place this variable in Mapped, but that runs afoul of the initializatuon checker.
-    val stack = if debugSets && this.isInstanceOf[Mapped] then (new Throwable).getStackTrace().nn.take(20) else null
+    val stack = if debugSets && this.isInstanceOf[Mapped] then (new Throwable).getStackTrace().take(20) else null
 
     /** The variable from which this variable is derived */
     def source: Var
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index ec134149eb49..500765b04980 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -963,7 +963,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             trace.force(i"rechecking $tree with pt = $pt", recheckr, show = true):
               super.recheck(tree, pt)
         catch case ex: NoCommonRoot =>
-          report.error(ex.getMessage.nn)
+          report.error(ex.getMessage)
           tree.tpe
         finally curEnv = saved
       if tree.isTerm then
diff --git a/compiler/src/dotty/tools/dotc/classpath/ClassPathFactory.scala b/compiler/src/dotty/tools/dotc/classpath/ClassPathFactory.scala
index 080f8d4e63d2..416bcc85a783 100644
--- a/compiler/src/dotty/tools/dotc/classpath/ClassPathFactory.scala
+++ b/compiler/src/dotty/tools/dotc/classpath/ClassPathFactory.scala
@@ -67,7 +67,7 @@ class ClassPathFactory {
         for
           file <- files
           a <- ClassPath.expandManifestPath(file.absolutePath)
-          path = java.nio.file.Paths.get(a.toURI()).nn
+          path = java.nio.file.Paths.get(a.toURI())
           if Files.exists(path)
         yield
           newClassPath(AbstractFile.getFile(path))
diff --git a/compiler/src/dotty/tools/dotc/config/CommandLineParser.scala b/compiler/src/dotty/tools/dotc/config/CommandLineParser.scala
index 2e76561c9913..50aa89779f50 100644
--- a/compiler/src/dotty/tools/dotc/config/CommandLineParser.scala
+++ b/compiler/src/dotty/tools/dotc/config/CommandLineParser.scala
@@ -114,12 +114,12 @@ object CommandLineParser:
   def expandArg(arg: String): List[String] =
     val path = Paths.get(arg.stripPrefix("@"))
     if !Files.exists(path) then
-      System.err.nn.println(s"Argument file ${path.nn.getFileName} could not be found")
+      System.err.println(s"Argument file ${path.getFileName} could not be found")
       Nil
     else
       def stripComment(s: String) = s.indexOf('#') match { case -1 => s case i => s.substring(0, i) }
-      val lines = Files.readAllLines(path).nn
-      val params = lines.asScala.map(stripComment).filter(!_.nn.isEmpty).mkString(" ")
+      val lines = Files.readAllLines(path)
+      val params = lines.asScala.map(stripComment).filter(!_.isEmpty).mkString(" ")
       tokenize(params)
 
   class ParseException(msg: String) extends RuntimeException(msg)
diff --git a/compiler/src/dotty/tools/dotc/config/Printers.scala b/compiler/src/dotty/tools/dotc/config/Printers.scala
index 81fd60497025..4c66e1cdf833 100644
--- a/compiler/src/dotty/tools/dotc/config/Printers.scala
+++ b/compiler/src/dotty/tools/dotc/config/Printers.scala
@@ -5,7 +5,7 @@ import core.Contexts.{Context, ctx}
 object Printers {
 
   class Printer {
-    def println(msg: => String): Unit = System.out.nn.println(msg)
+    def println(msg: => String): Unit = System.out.println(msg)
   }
 
   object noPrinter extends Printer {
diff --git a/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala b/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala
index 72a051ea8154..9472b193ba02 100644
--- a/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala
+++ b/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala
@@ -1,7 +1,6 @@
 package dotty.tools.dotc
 package config
 
-import scala.language.unsafeNulls
 import dotty.tools.dotc.config.PathResolver.Defaults
 import dotty.tools.dotc.config.Settings.{Setting, SettingGroup, SettingCategory, Deprecation}
 import dotty.tools.dotc.config.SourceVersion
diff --git a/compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala b/compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala
index 022916cc9f53..ecb10d30fce8 100644
--- a/compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala
+++ b/compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala
@@ -4,7 +4,6 @@ package config
 import Settings.Setting.ChoiceWithHelp
 import dotty.tools.backend.jvm.BackendUtils.classfileVersionMap
 import dotty.tools.io.{AbstractFile, Directory, JDK9Reflectors, PlainDirectory, NoAbstractFile}
-import scala.language.unsafeNulls
 
 object ScalaSettingsProperties:
 
diff --git a/compiler/src/dotty/tools/dotc/config/ScalaVersion.scala b/compiler/src/dotty/tools/dotc/config/ScalaVersion.scala
index 9f603e6792be..a9fffa7a3bb6 100644
--- a/compiler/src/dotty/tools/dotc/config/ScalaVersion.scala
+++ b/compiler/src/dotty/tools/dotc/config/ScalaVersion.scala
@@ -3,8 +3,6 @@
 package dotty.tools
 package dotc.config
 
-import scala.language.unsafeNulls
-
 import scala.annotation.internal.sharable
 import scala.util.{Try, Success, Failure}
 
@@ -82,20 +80,20 @@ case class SpecificScalaVersion(major: Int, minor: Int, rev: Int, build: ScalaBu
        "The minor and revision parts are optional."
     ))
 
-    def toInt(s: String) = s match {
+    def toInt(s: String | Null) = s match {
       case null | "" => 0
-      case _ => s.toInt
+      case _ => s.nn.toInt
     }
 
     def isInt(s: String) = Try(toInt(s)).isSuccess
 
     import ScalaBuild.*
 
-    def toBuild(s: String) = s match {
+    def toBuild(s: String | Null) = s match {
       case null | "FINAL" => Final
       case s if (s.toUpperCase.startsWith("RC") && isInt(s.substring(2))) => RC(toInt(s.substring(2)))
       case s if (s.toUpperCase.startsWith("M") && isInt(s.substring(1))) => Milestone(toInt(s.substring(1)))
-      case _ => Development(s)
+      case _ => Development(s.nn)
     }
 
     try versionString match {
diff --git a/compiler/src/dotty/tools/dotc/config/Settings.scala b/compiler/src/dotty/tools/dotc/config/Settings.scala
index a5fc6a64aa45..8444d6261721 100644
--- a/compiler/src/dotty/tools/dotc/config/Settings.scala
+++ b/compiler/src/dotty/tools/dotc/config/Settings.scala
@@ -1,8 +1,6 @@
 package dotty.tools.dotc
 package config
 
-import scala.language.unsafeNulls
-
 import core.Contexts.*
 
 import dotty.tools.io.{AbstractFile, Directory, JarArchive, PlainDirectory}
diff --git a/compiler/src/dotty/tools/dotc/core/Comments.scala b/compiler/src/dotty/tools/dotc/core/Comments.scala
index 92160c97973d..5ee1d5856605 100644
--- a/compiler/src/dotty/tools/dotc/core/Comments.scala
+++ b/compiler/src/dotty/tools/dotc/core/Comments.scala
@@ -2,8 +2,6 @@ package dotty.tools
 package dotc
 package core
 
-import scala.language.unsafeNulls
-
 import ast.{ untpd, tpd }
 import Symbols.*, Contexts.*
 import util.{SourceFile, ReadOnlyMap}
diff --git a/compiler/src/dotty/tools/dotc/core/Decorators.scala b/compiler/src/dotty/tools/dotc/core/Decorators.scala
index 29d4b3fa4052..765fbd2907fc 100644
--- a/compiler/src/dotty/tools/dotc/core/Decorators.scala
+++ b/compiler/src/dotty/tools/dotc/core/Decorators.scala
@@ -51,7 +51,7 @@ object Decorators {
         if name.length != 0 then name.getChars(0, name.length, chars, s.length)
         termName(chars, 0, len)
       case name: TypeName => s.concat(name.toTermName)
-      case _ => termName(s.concat(name.toString).nn)
+      case _ => termName(s.concat(name.toString))
 
     def indented(width: Int): String =
       val padding = " " * width
@@ -289,10 +289,10 @@ object Decorators {
           case NonFatal(ex)
           if !ctx.settings.YshowPrintErrors.value =>
             s"... (cannot display due to ${ex.className} ${ex.getMessage}) ..."
-      case _ => String.valueOf(x).nn
+      case _ => String.valueOf(x)
 
     /** Returns the simple class name of `x`. */
-    def className: String = x.getClass.getSimpleName.nn
+    def className: String = x.getClass.getSimpleName
 
   extension [T](x: T)
     def assertingErrorsReported(using Context): T = {
diff --git a/compiler/src/dotty/tools/dotc/core/MacroClassLoader.scala b/compiler/src/dotty/tools/dotc/core/MacroClassLoader.scala
index b6b316ac14d9..c4c41a460a70 100644
--- a/compiler/src/dotty/tools/dotc/core/MacroClassLoader.scala
+++ b/compiler/src/dotty/tools/dotc/core/MacroClassLoader.scala
@@ -19,8 +19,6 @@ object MacroClassLoader {
     ctx.setProperty(MacroClassLoaderKey, makeMacroClassLoader(using ctx))
 
   private def makeMacroClassLoader(using Context): ClassLoader = trace("new macro class loader") {
-    import scala.language.unsafeNulls
-
     val entries = ClassPath.expandPath(ctx.settings.classpath.value, expandStar=true)
     val urls = entries.map(cp => java.nio.file.Paths.get(cp).toUri.toURL).toArray
     val out = Option(ctx.settings.outputDir.value.toURL) // to find classes in case of suspended compilation
diff --git a/compiler/src/dotty/tools/dotc/core/NameOps.scala b/compiler/src/dotty/tools/dotc/core/NameOps.scala
index 415aa049c587..ce726785e7e7 100644
--- a/compiler/src/dotty/tools/dotc/core/NameOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/NameOps.scala
@@ -15,7 +15,7 @@ import nme.*
 object NameOps {
 
   object compactify {
-    lazy val md5: MessageDigest = MessageDigest.getInstance("MD5").nn
+    lazy val md5: MessageDigest = MessageDigest.getInstance("MD5")
 
     inline val CLASSFILE_NAME_CHAR_LIMIT = 240
 
@@ -43,9 +43,9 @@ object NameOps {
         val suffix = s.takeRight(edge)
 
         val cs = s.toArray
-        val bytes = Codec.toUTF8(CharBuffer.wrap(cs).nn)
+        val bytes = Codec.toUTF8(CharBuffer.wrap(cs))
         md5.update(bytes)
-        val md5chars = md5.digest().nn.map(b => (b & 0xFF).toHexString).mkString
+        val md5chars = md5.digest().map(b => (b & 0xFF).toHexString).mkString
 
         prefix + marker + md5chars + marker + suffix
       }
diff --git a/compiler/src/dotty/tools/dotc/core/Names.scala b/compiler/src/dotty/tools/dotc/core/Names.scala
index 3f9667b08067..baf95b9306d3 100644
--- a/compiler/src/dotty/tools/dotc/core/Names.scala
+++ b/compiler/src/dotty/tools/dotc/core/Names.scala
@@ -393,7 +393,7 @@ object Names {
             // because asserts are caught in exception handlers which might
             // cause other failures. In that case the first, important failure
             // is lost.
-            System.err.nn.println("Backend should not call Name#toString, Name#mangledString should be used instead.")
+            System.err.println("Backend should not call Name#toString, Name#mangledString should be used instead.")
             Thread.dumpStack()
             assert(false)
           }
@@ -404,8 +404,8 @@ object Names {
      *  from GenBCode or it also contains one of the whitelisted methods below.
      */
     private def toStringOK = {
-      val trace: Array[StackTraceElement] = Thread.currentThread.nn.getStackTrace.asInstanceOf[Array[StackTraceElement]]
-      !trace.exists(_.getClassName.nn.endsWith("GenBCode")) ||
+      val trace: Array[StackTraceElement] = Thread.currentThread.getStackTrace.asInstanceOf[Array[StackTraceElement]]
+      !trace.exists(_.getClassName.endsWith("GenBCode")) ||
       trace.exists(elem =>
           List(
               "mangledString",
diff --git a/compiler/src/dotty/tools/dotc/core/StdNames.scala b/compiler/src/dotty/tools/dotc/core/StdNames.scala
index d3e198a7e7a7..a981f942016c 100644
--- a/compiler/src/dotty/tools/dotc/core/StdNames.scala
+++ b/compiler/src/dotty/tools/dotc/core/StdNames.scala
@@ -42,7 +42,7 @@ object StdNames {
 
     @sharable
     private val disallowed = java.util.regex.Pattern.compile("""[<>]""").nn
-    def sanitize(str: String): String = disallowed.matcher(str).nn.replaceAll("""\$""").nn
+    def sanitize(str: String): String = disallowed.matcher(str).replaceAll("""\$""").nn
   }
 
   abstract class DefinedNames[N <: Name] {
diff --git a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
index 1c9696da67d1..ce14cad6871c 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
@@ -32,7 +32,7 @@ abstract class TypeError(using creationContext: Context) extends Exception(""):
     || ctx.settings.YdebugCyclic.value
 
   override def fillInStackTrace(): Throwable =
-    if computeStackTrace then super.fillInStackTrace().nn
+    if computeStackTrace then super.fillInStackTrace()
     else this
 
   /** Convert to message. This takes an additional Context, so that we
@@ -246,7 +246,7 @@ class UnpicklingError(denot: Denotation, where: String, cause: Throwable)(using
       case cause: UnpicklingError => ""
       case _ =>
         if ctx.settings.YdebugUnpickling.value then
-          cause.getStackTrace().nn.mkString("\n    ", "\n    ", "")
+          cause.getStackTrace().mkString("\n    ", "\n    ", "")
         else "\n\nRun with -Ydebug-unpickling to see full stack trace."
     em"""Could not read definition $denot$where. Caused by the following exception:
         |$cause$debugUnpickling"""
diff --git a/compiler/src/dotty/tools/dotc/core/TypeEval.scala b/compiler/src/dotty/tools/dotc/core/TypeEval.scala
index 4d5496cff880..a6c308bb2a55 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeEval.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeEval.scala
@@ -94,7 +94,7 @@ object TypeEval:
         val result =
           try op
           catch case e: Throwable =>
-            throw TypeError(em"${e.getMessage.nn}")
+            throw TypeError(em"${e.getMessage}")
         ConstantType(Constant(result))
 
       def fieldsOf: Option[Type] =
diff --git a/compiler/src/dotty/tools/dotc/core/classfile/AbstractFileReader.scala b/compiler/src/dotty/tools/dotc/core/classfile/AbstractFileReader.scala
index 46581d00714e..d4f781e22126 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/AbstractFileReader.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/AbstractFileReader.scala
@@ -3,8 +3,6 @@ package dotc
 package core
 package classfile
 
-import scala.language.unsafeNulls
-
 import java.lang.Float.intBitsToFloat
 import java.lang.Double.longBitsToDouble
 import java.io.{ByteArrayInputStream, DataInputStream}
diff --git a/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala b/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
index 43a2bea27216..c5770d69b002 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
@@ -1,7 +1,6 @@
 package dotty.tools.dotc
 package core.tasty
 
-import scala.language.unsafeNulls
 import scala.collection.immutable.BitSet
 import scala.collection.immutable.TreeMap
 
diff --git a/compiler/src/dotty/tools/dotc/core/tasty/TastyPrinter.scala b/compiler/src/dotty/tools/dotc/core/tasty/TastyPrinter.scala
index 72f6895f122c..ee798e29721e 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/TastyPrinter.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/TastyPrinter.scala
@@ -53,7 +53,7 @@ object TastyPrinter:
       else if arg.endsWith(".tasty") || (allowBetasty && arg.endsWith(".betasty")) then
         val path = Paths.get(arg)
         if Files.exists(path) then
-          printTasty(arg, Files.readAllBytes(path).nn, arg.endsWith(".betasty"))
+          printTasty(arg, Files.readAllBytes(path), arg.endsWith(".betasty"))
         else
           println("File not found: " + arg)
           System.exit(1)
diff --git a/compiler/src/dotty/tools/dotc/parsing/JavaScanners.scala b/compiler/src/dotty/tools/dotc/parsing/JavaScanners.scala
index 3f993195e4f3..64f586ca0410 100644
--- a/compiler/src/dotty/tools/dotc/parsing/JavaScanners.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/JavaScanners.scala
@@ -666,7 +666,7 @@ object JavaScanners {
       val limit: Double =
         if (token == DOUBLELIT) Double.MaxValue else Float.MaxValue
       try {
-        val value: Double = java.lang.Double.valueOf(strVal.toString).nn.doubleValue()
+        val value: Double = java.lang.Double.valueOf(strVal.toString).doubleValue()
         if (value > limit)
           error(em"floating point number too large")
         if (negated) -value else value
diff --git a/compiler/src/dotty/tools/dotc/plugins/Plugins.scala b/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
index a6672d475129..490d1683b304 100644
--- a/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
+++ b/compiler/src/dotty/tools/dotc/plugins/Plugins.scala
@@ -1,8 +1,6 @@
 package dotty.tools.dotc
 package plugins
 
-import scala.language.unsafeNulls
-
 import core.*
 import Contexts.*
 import Decorators.em
@@ -47,13 +45,12 @@ trait Plugins {
     goods map (_.get)
   }
 
-  private var _roughPluginsList: List[Plugin] = uninitialized
+  private var _roughPluginsList: List[Plugin] | Null = null
   protected def roughPluginsList(using Context): List[Plugin] =
     if (_roughPluginsList == null) {
       _roughPluginsList = loadRoughPluginsList
-      _roughPluginsList
     }
-    else _roughPluginsList
+    _roughPluginsList.nn
 
   /** Load all available plugins. Skips plugins that
    *  either have the same name as another one, or which
@@ -99,13 +96,12 @@ trait Plugins {
     plugs
   }
 
-  private var _plugins: List[Plugin] = uninitialized
+  private var _plugins: List[Plugin] | Null = null
   def plugins(using Context): List[Plugin] =
     if (_plugins == null) {
       _plugins = loadPlugins
-      _plugins
     }
-    else _plugins
+    _plugins.nn
 
   /** A description of all the plugins that are loaded */
   def pluginDescriptions(using Context): String =
diff --git a/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala b/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
index b5ed3bdb4fa7..00b22f2b7df3 100644
--- a/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
+++ b/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
@@ -394,7 +394,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: ThisType =>
         nameString(tp.cls) + ".this"
       case SuperType(thistpe: SingletonType, _) =>
-        toTextRef(thistpe).map(_.replaceAll("""\bthis$""", "super").nn)
+        toTextRef(thistpe).map(_.replaceAll("""\bthis$""", "super"))
       case SuperType(thistpe, _) =>
         "Super(" ~ toTextGlobal(thistpe) ~ ")"
       case tp @ ConstantType(value) =>
@@ -628,7 +628,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
     case '"' => "\\\""
     case '\'' => "\\\'"
     case '\\' => "\\\\"
-    case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch).nn
+    case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch)
   }
 
   def toText(const: Constant): Text = const.tag match {
@@ -638,7 +638,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
     case LongTag => literalText(const.longValue.toString + "L")
     case DoubleTag => literalText(const.doubleValue.toString + "d")
     case FloatTag => literalText(const.floatValue.toString + "f")
-    case _ => literalText(String.valueOf(const.value).nn)
+    case _ => literalText(String.valueOf(const.value))
   }
 
   /** Usual target for `Annotation#toText`, overridden in RefinedPrinter */
diff --git a/compiler/src/dotty/tools/dotc/printing/Showable.scala b/compiler/src/dotty/tools/dotc/printing/Showable.scala
index 4480aa9c76a4..490778a7413a 100644
--- a/compiler/src/dotty/tools/dotc/printing/Showable.scala
+++ b/compiler/src/dotty/tools/dotc/printing/Showable.scala
@@ -25,7 +25,7 @@ trait Showable extends Any {
   /** The string representation with each line after the first one indented
    *  by the given given margin (in spaces).
    */
-  def showIndented(margin: Int)(using Context): String = show.replace("\n", "\n" + " " * margin).nn
+  def showIndented(margin: Int)(using Context): String = show.replace("\n", "\n" + " " * margin)
 
   /** The summarized string representation of this showable element.
    *  Recursion depth is limited to some smallish value. Default is
diff --git a/compiler/src/dotty/tools/dotc/printing/Texts.scala b/compiler/src/dotty/tools/dotc/printing/Texts.scala
index 475e2c6900d5..014765b63274 100644
--- a/compiler/src/dotty/tools/dotc/printing/Texts.scala
+++ b/compiler/src/dotty/tools/dotc/printing/Texts.scala
@@ -5,7 +5,7 @@ import scala.annotation.internal.sharable
 object Texts {
 
   @sharable
-  private val ansi = java.util.regex.Pattern.compile("\u001b\\[\\d+m").nn
+  private val ansi = java.util.regex.Pattern.compile("\u001b\\[\\d+m")
 
   sealed abstract class Text {
 
@@ -74,7 +74,7 @@ object Texts {
       else appendIndented(that)(width)
 
     private def lengthWithoutAnsi(str: String): Int =
-      ansi.matcher(str).nn.replaceAll("").nn.length
+      ansi.matcher(str).replaceAll("").length
 
     def layout(width: Int): Text = this match {
       case Str(s, _) =>
diff --git a/compiler/src/dotty/tools/dotc/quoted/Interpreter.scala b/compiler/src/dotty/tools/dotc/quoted/Interpreter.scala
index e34d35065476..dbc2694dc891 100644
--- a/compiler/src/dotty/tools/dotc/quoted/Interpreter.scala
+++ b/compiler/src/dotty/tools/dotc/quoted/Interpreter.scala
@@ -1,8 +1,6 @@
 package dotty.tools.dotc
 package quoted
 
-import scala.language.unsafeNulls
-
 import scala.collection.mutable
 import scala.reflect.ClassTag
 
diff --git a/compiler/src/dotty/tools/dotc/report.scala b/compiler/src/dotty/tools/dotc/report.scala
index e24e6be38b2b..9e79a3d5eef6 100644
--- a/compiler/src/dotty/tools/dotc/report.scala
+++ b/compiler/src/dotty/tools/dotc/report.scala
@@ -84,8 +84,8 @@ object report:
   def bestEffortError(ex: Throwable, msg: String)(using Context): Unit =
     val stackTrace =
       Option(ex.getStackTrace()).map { st =>
-        if st.nn.isEmpty then ""
-        else s"Stack trace: \n ${st.nn.mkString("\n ")}".stripMargin
+        if st.isEmpty then ""
+        else s"Stack trace: \n ${st.mkString("\n ")}".stripMargin
       }.getOrElse("")
     // Build tools and dotty's test framework may check precisely for
     // "Unsuccessful best-effort compilation." error text.
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index 01eb2acfa4de..396f2eadc332 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -92,7 +92,7 @@ abstract class CyclicMsg(errorId: ErrorMessageID)(using Context) extends Message
 
   protected def debugInfo =
     if ctx.settings.YdebugCyclic.value then
-      "\n\nStacktrace:" ++ ex.getStackTrace().nn.mkString("\n    ", "\n    ", "")
+      "\n\nStacktrace:" ++ ex.getStackTrace().mkString("\n    ", "\n    ", "")
     else "\n\n Run with both -explain-cyclic and -Ydebug-cyclic to see full stack trace."
 
   protected def context: String = ex.optTrace match
diff --git a/compiler/src/dotty/tools/dotc/semanticdb/Scala3.scala b/compiler/src/dotty/tools/dotc/semanticdb/Scala3.scala
index f49b00089712..a20a0d181a55 100644
--- a/compiler/src/dotty/tools/dotc/semanticdb/Scala3.scala
+++ b/compiler/src/dotty/tools/dotc/semanticdb/Scala3.scala
@@ -420,7 +420,7 @@ object Scala3:
         else Descriptor.None
 
       def unescapeUnicode =
-        unicodeEscape.replaceAllIn(symbol, m => String.valueOf(Integer.parseInt(m.group(1), 16).toChar).nn)
+        unicodeEscape.replaceAllIn(symbol, m => String.valueOf(Integer.parseInt(m.group(1), 16).toChar))
 
       def isJavaIdent =
         symbol.nonEmpty && isJavaIdentifierStart(symbol.head) && symbol.tail.forall(isJavaIdentifierPart)
diff --git a/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala b/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
index b5b75450272c..0725d2f971e2 100644
--- a/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
+++ b/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
@@ -170,7 +170,7 @@ object GenericSignatures {
     def fullNameInSig(sym: Symbol): Unit = {
       assert(sym.isClass)
       val name = atPhase(genBCodePhase) { sanitizeName(sym.fullName).replace('.', '/') }
-      builder.append('L').nn.append(name)
+      builder.append('L').append(name)
     }
 
     def classSig(sym: Symbol, pre: Type = NoType, args: List[Type] = Nil): Unit = {
@@ -215,7 +215,7 @@ object GenericSignatures {
 
             // TODO revisit this. Does it align with javac for code that can be expressed in both languages?
             val delimiter = if (builder.charAt(builder.length() - 1) == '>') '.' else '$'
-            builder.append(delimiter).nn.append(sanitizeName(sym.name))
+            builder.append(delimiter).append(sanitizeName(sym.name))
           }
           else fullNameInSig(sym)
         }
diff --git a/compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala b/compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala
index f5e0f8c63b58..a89e03f4fa6e 100644
--- a/compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala
+++ b/compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala
@@ -56,9 +56,9 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
 
     if !newlyCreated then
       // If the directory existed before, let's clean it up.
-      dataDir.listFiles.nn
-        .filter(_.nn.getName.nn.startsWith("scoverage"))
-        .foreach(_.nn.delete())
+      dataDir.listFiles
+        .filter(_.getName.startsWith("scoverage"))
+        .foreach(_.delete())
     end if
 
     coverageExcludeClasslikePatterns = ctx.settings.coverageExcludeClasslikes.value.map(_.r.pattern)
@@ -71,13 +71,13 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
   private def isClassIncluded(sym: Symbol)(using Context): Boolean =
     val fqn = sym.fullName.toText(ctx.printerFn(ctx)).show
     coverageExcludeClasslikePatterns.isEmpty || !coverageExcludeClasslikePatterns.exists(
-      _.matcher(fqn).nn.matches
+      _.matcher(fqn).matches
     )
 
   private def isFileIncluded(file: SourceFile)(using Context): Boolean =
     val normalizedPath = file.path.replace(".scala", "")
     coverageExcludeFilePatterns.isEmpty || !coverageExcludeFilePatterns.exists(
-      _.matcher(normalizedPath).nn.matches
+      _.matcher(normalizedPath).matches
     )
 
   override protected def newTransformer(using Context) =
diff --git a/compiler/src/dotty/tools/dotc/transform/Pickler.scala b/compiler/src/dotty/tools/dotc/transform/Pickler.scala
index c8c071064ab8..108277599010 100644
--- a/compiler/src/dotty/tools/dotc/transform/Pickler.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Pickler.scala
@@ -258,7 +258,7 @@ class Pickler extends Phase {
   }
 
   private def computeInternalName(cls: ClassSymbol)(using Context): String =
-    if cls.is(Module) then cls.binaryClassName.stripSuffix(str.MODULE_SUFFIX).nn
+    if cls.is(Module) then cls.binaryClassName.stripSuffix(str.MODULE_SUFFIX)
     else cls.binaryClassName
 
   override def run(using Context): Unit = {
@@ -413,11 +413,11 @@ class Pickler extends Phase {
       )
     if ctx.isBestEffort then
       val outpath =
-        ctx.settings.outputDir.value.jpath.toAbsolutePath.nn.normalize.nn
-          .resolve("META-INF").nn
+        ctx.settings.outputDir.value.jpath.toAbsolutePath.normalize
+          .resolve("META-INF")
           .resolve("best-effort")
       Files.createDirectories(outpath)
-      BestEffortTastyWriter.write(outpath.nn, result)
+      BestEffortTastyWriter.write(outpath, result)
     result
   }
 
diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index a2b403fdae6c..7eea19ba8807 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -860,7 +860,7 @@ object TreeChecker {
           val stack =
             if !ctx.settings.Ydebug.value then "\nstacktrace available when compiling with `-Ydebug`"
             else if err.getStackTrace == null then "  no stacktrace"
-            else err.getStackTrace.nn.mkString("  ", "  \n", "")
+            else err.getStackTrace.mkString("  ", "  \n", "")
 
           report.error(
             em"""Malformed tree was found while expanding macro with -Xcheck-macros.
diff --git a/compiler/src/dotty/tools/dotc/transform/init/Errors.scala b/compiler/src/dotty/tools/dotc/transform/init/Errors.scala
index 85feb609c90a..0b843b4b33fd 100644
--- a/compiler/src/dotty/tools/dotc/transform/init/Errors.scala
+++ b/compiler/src/dotty/tools/dotc/transform/init/Errors.scala
@@ -31,7 +31,7 @@ object Errors:
       report.warning(show, this.pos)
   end Error
 
-  override def toString() = this.getClass.getName.nn
+  override def toString() = this.getClass.getName
 
   /** Access non-initialized field */
   case class AccessNonInit(field: Symbol)(val trace: Trace) extends Error:
diff --git a/compiler/src/dotty/tools/dotc/transform/init/Trace.scala b/compiler/src/dotty/tools/dotc/transform/init/Trace.scala
index ffaccad963af..9a00589197d5 100644
--- a/compiler/src/dotty/tools/dotc/transform/init/Trace.scala
+++ b/compiler/src/dotty/tools/dotc/transform/init/Trace.scala
@@ -51,7 +51,7 @@ object Trace:
       val line =
         if pos.source.exists then
           val loc = "[ " + pos.source.file.name + ":" + (pos.line + 1) + " ]"
-          val code = SyntaxHighlighting.highlight(pos.lineContent.trim.nn)
+          val code = SyntaxHighlighting.highlight(pos.lineContent.trim)
           i"$code\t$loc"
         else
           tree match
@@ -59,7 +59,7 @@ object Trace:
               // The definition can be huge, avoid printing the whole definition.
               defDef.symbol.showFullName
             case _ =>
-              tree.show.split(System.lineSeparator(), 2).nn.head.nn
+              tree.show.split(System.lineSeparator(), 2).head
 
       val positionMarkerLine =
         if pos.exists && pos.source.exists then
@@ -86,7 +86,7 @@ object Trace:
    */
   private def positionMarker(pos: SourcePosition): String =
     val trimmed = pos.source.lineContent(pos.start).takeWhile(c => c.isWhitespace).length
-    val padding = pos.startColumnPadding.substring(trimmed).nn
+    val padding = pos.startColumnPadding.substring(trimmed)
     val carets =
       if (pos.startLine == pos.endLine)
         "^" * math.max(1, pos.endColumn - pos.startColumn)
diff --git a/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala b/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
index 9e40792895c0..5ac96f58011b 100644
--- a/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
@@ -74,7 +74,7 @@ class TypedFormatChecker(partsElems: List[Tree], parts: List[String], args: List
     def loop(remaining: List[String], n: Int): Unit =
       remaining match
         case part0 :: more =>
-          def badPart(t: Throwable): String = "".tap(_ => report.partError(t.getMessage.nn, index = n, offset = 0))
+          def badPart(t: Throwable): String = "".tap(_ => report.partError(t.getMessage, index = n, offset = 0))
           val part = try StringContext.processEscapes(part0) catch badPart
           val matches = formatPattern.findAllMatchIn(part)
 
diff --git a/compiler/src/dotty/tools/dotc/transform/sjs/PrepJSInterop.scala b/compiler/src/dotty/tools/dotc/transform/sjs/PrepJSInterop.scala
index 1b8fdd268ece..595a2e9a70ad 100644
--- a/compiler/src/dotty/tools/dotc/transform/sjs/PrepJSInterop.scala
+++ b/compiler/src/dotty/tools/dotc/transform/sjs/PrepJSInterop.scala
@@ -651,7 +651,7 @@ class PrepJSInterop extends MacroTransform with IdentityDenotTransformer { thisP
           val dotIndex = pathName.indexOf('.')
           val globalRef =
             if (dotIndex < 0) pathName
-            else pathName.substring(0, dotIndex).nn
+            else pathName.substring(0, dotIndex)
           checkGlobalRefName(globalRef)
         }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 0a0356707048..c9cd2b05a94a 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -1259,9 +1259,9 @@ object RefChecks {
       val matches = referencePattern.findAllIn(s)
       for reference <- matches do
         val referenceOffset = matches.start
-        val prefixlessReference = reference.replaceFirst("""\$\{\s*""", "").nn
+        val prefixlessReference = reference.replaceFirst("""\$\{\s*""", "")
         val variableOffset = referenceOffset + reference.length - prefixlessReference.length
-        val variableName = prefixlessReference.replaceFirst("""\s*\}""", "").nn
+        val variableName = prefixlessReference.replaceFirst("""\s*\}""", "")
         f(variableName, variableOffset)
 
   end checkImplicitNotFoundAnnotation
diff --git a/compiler/src/dotty/tools/dotc/util/ParsedComment.scala b/compiler/src/dotty/tools/dotc/util/ParsedComment.scala
index ee56a74d5eb5..acced8dd857c 100644
--- a/compiler/src/dotty/tools/dotc/util/ParsedComment.scala
+++ b/compiler/src/dotty/tools/dotc/util/ParsedComment.scala
@@ -1,7 +1,5 @@
 package dotty.tools.dotc.util
 
-import scala.language.unsafeNulls
-
 import dotty.tools.dotc.core.Comments.{Comment, docCtx}
 import dotty.tools.dotc.core.Contexts.*
 import dotty.tools.dotc.core.Names.TermName
diff --git a/compiler/src/dotty/tools/dotc/util/ShowPickled.scala b/compiler/src/dotty/tools/dotc/util/ShowPickled.scala
index 8d5d0c27ab0e..15d553e703b3 100644
--- a/compiler/src/dotty/tools/dotc/util/ShowPickled.scala
+++ b/compiler/src/dotty/tools/dotc/util/ShowPickled.scala
@@ -113,7 +113,7 @@ object ShowPickled {
     result.toInt
   }
 
-  def printFile(buf: PickleBuffer, out: PrintStream = System.out.nn): Unit = {
+  def printFile(buf: PickleBuffer, out: PrintStream = System.out): Unit = {
     out.println("Version " + buf.readNat() + "." + buf.readNat())
     val index = buf.createIndex
     val entryList = makeEntryList(buf, index)
diff --git a/compiler/src/dotty/tools/io/File.scala b/compiler/src/dotty/tools/io/File.scala
index 59e4a2ee451b..ea48966de6fb 100644
--- a/compiler/src/dotty/tools/io/File.scala
+++ b/compiler/src/dotty/tools/io/File.scala
@@ -8,8 +8,6 @@
 
 package dotty.tools.io
 
-import scala.language.unsafeNulls
-
 import java.io.{File => JavaIoFile, _}
 import java.nio.file.{Files, Paths}
 import java.nio.file.StandardOpenOption.*
diff --git a/compiler/src/dotty/tools/io/FileWriters.scala b/compiler/src/dotty/tools/io/FileWriters.scala
index 5fdf43cfe8e3..849b80c5e745 100644
--- a/compiler/src/dotty/tools/io/FileWriters.scala
+++ b/compiler/src/dotty/tools/io/FileWriters.scala
@@ -66,7 +66,7 @@ object FileWriters {
     def warning(message: Context ?=> Message): Unit = warning(message, NoSourcePosition)
     final def exception(reason: Context ?=> Message, throwable: Throwable): Unit =
       error({
-        val trace = throwable.getStackTrace().nn.mkString("\n  ")
+        val trace = throwable.getStackTrace().mkString("\n  ")
         em"An unhandled exception was thrown in the compiler while\n  ${reason.message}.\n${throwable}\n  $trace"
       }, NoSourcePosition)
   }
@@ -189,7 +189,7 @@ object FileWriters {
     def close(): Unit
 
     protected def classToRelativePath(className: InternalName): String =
-      className.replace('.', '/').nn + ".tasty"
+      className.replace('.', '/') + ".tasty"
   }
 
   object TastyWriter {
@@ -234,7 +234,7 @@ object FileWriters {
         new JarEntryWriter(jarFile, jarManifestMainClass, jarCompressionLevel)
       }
       else if (file.isVirtual) new VirtualFileWriter(file)
-      else if (file.isDirectory) new DirEntryWriter(file.file.toPath.nn)
+      else if (file.isDirectory) new DirEntryWriter(file.file.toPath)
       else throw new IllegalStateException(s"don't know how to handle an output of $file [${file.getClass}]")
   }
 
@@ -248,7 +248,7 @@ object FileWriters {
     val jarWriter: JarOutputStream = {
       import scala.util.Properties.*
       val manifest = new Manifest
-      val attrs = manifest.getMainAttributes.nn
+      val attrs = manifest.getMainAttributes
       attrs.put(MANIFEST_VERSION, "1.0")
       attrs.put(ScalaCompilerVersion, versionNumberString)
       mainClass.foreach(c => attrs.put(MAIN_CLASS, c))
diff --git a/compiler/src/dotty/tools/io/Path.scala b/compiler/src/dotty/tools/io/Path.scala
index 6f97e03ca4d7..39665395c289 100644
--- a/compiler/src/dotty/tools/io/Path.scala
+++ b/compiler/src/dotty/tools/io/Path.scala
@@ -5,8 +5,6 @@
 
 package dotty.tools.io
 
-import scala.language.unsafeNulls
-
 import java.io.RandomAccessFile
 import java.nio.file.*
 import java.net.{URI, URL}
@@ -46,7 +44,7 @@ object Path {
   def fileName(name: String): String = {
     val i = name.lastIndexOf('.')
     if (i < 0) name
-    else name.substring(0, i).nn
+    else name.substring(0, i)
   }
 
   def onlyDirs(xs: Iterator[Path]): Iterator[Directory] = xs.filter(_.isDirectory).map(_.toDirectory)
diff --git a/compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala b/compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala
index 9503177ff738..86ba6708caba 100644
--- a/compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala
+++ b/compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala
@@ -1441,7 +1441,7 @@ object SourceCode {
       case '"' => "\\\""
       case '\'' => "\\\'"
       case '\\' => "\\\\"
-      case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch).nn
+      case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch)
     }
 
     private def escapedString(str: String): String = str flatMap escapedChar
diff --git a/compiler/test-coursier/dotty/tools/coursier/CoursierScalaTests.scala b/compiler/test-coursier/dotty/tools/coursier/CoursierScalaTests.scala
index 86b22009d15a..f44774141947 100644
--- a/compiler/test-coursier/dotty/tools/coursier/CoursierScalaTests.scala
+++ b/compiler/test-coursier/dotty/tools/coursier/CoursierScalaTests.scala
@@ -159,7 +159,7 @@ object CoursierScalaTests:
 
   def csScalaCmd(options: String*): List[String] =
     csScalaCmdWithStdin(options, None)
-  
+
   def csScalaCmdWithStdin(options: Seq[String], stdin: Option[String]): List[String] =
     csCmd("dotty.tools.MainGenericRunner", options, stdin)
 
diff --git a/compiler/test/dotty/tools/backend/jvm/StringConcatTest.scala b/compiler/test/dotty/tools/backend/jvm/StringConcatTest.scala
index 14352106c5e8..613e72b32e52 100644
--- a/compiler/test/dotty/tools/backend/jvm/StringConcatTest.scala
+++ b/compiler/test/dotty/tools/backend/jvm/StringConcatTest.scala
@@ -122,7 +122,7 @@ class StringConcatTest extends DottyBytecodeTest {
            chsq: java.lang.CharSequence,
            chrs: Array[Char]) = {
       val s1 = str + obj + v + z + c + b + s + i + f + l + d + sbuf + chsq + chrs
-      val s2 = String.valueOf(obj).nn + str + v + z + c + b + s + i + f + l + d + sbuf + chsq + chrs
+      val s2 = String.valueOf(obj) + str + v + z + c + b + s + i + f + l + d + sbuf + chsq + chrs
       s1 + "//" + s2
     }
     def sbuf = { val r = new java.lang.StringBuffer(); r.append("sbuf"); r }
diff --git a/compiler/test/dotty/tools/dotc/SettingsTests.scala b/compiler/test/dotty/tools/dotc/SettingsTests.scala
index 996ab22f67b1..ec342db875a6 100644
--- a/compiler/test/dotty/tools/dotc/SettingsTests.scala
+++ b/compiler/test/dotty/tools/dotc/SettingsTests.scala
@@ -1,8 +1,6 @@
 package dotty.tools
 package dotc
 
-import scala.language.unsafeNulls
-
 import reporting.StoreReporter
 import vulpix.TestConfiguration
 
@@ -205,7 +203,7 @@ class SettingsTests {
     }
 
   @Test def `Output setting is overriding existing jar`: Unit =
-    val result = Using.resource(Files.createTempFile("myfile", ".jar").nn){ file =>
+    val result = Using.resource(Files.createTempFile("myfile", ".jar")){ file =>
       object Settings extends SettingGroup:
         val defaultDir = new PlainDirectory(Directory("."))
         val testOutput = OutputSetting(RootSetting, "testOutput", "testOutput", "", defaultDir)
@@ -224,7 +222,7 @@ class SettingsTests {
 
   @Test def `Output setting is respecting previous setting`: Unit =
     val result = Using.resources(
-      Files.createTempFile("myfile", ".jar").nn, Files.createTempFile("myfile2", ".jar").nn
+      Files.createTempFile("myfile", ".jar"), Files.createTempFile("myfile2", ".jar")
     ){ (file1, file2) =>
       object Settings extends SettingGroup:
         val defaultDir = new PlainDirectory(Directory("."))
@@ -250,7 +248,7 @@ class SettingsTests {
     }(Files.deleteIfExists(_), Files.deleteIfExists(_))
 
   @Test def `Output side effect is not present when setting is deprecated`: Unit =
-    val result = Using.resource(Files.createTempFile("myfile", ".jar").nn){ file =>
+    val result = Using.resource(Files.createTempFile("myfile", ".jar")){ file =>
       object Settings extends SettingGroup:
         val defaultDir = new PlainDirectory(Directory("."))
         val testOutput = OutputSetting(RootSetting, "testOutput", "testOutput", "", defaultDir, preferPrevious = true, deprecation = Deprecation.renamed("XtestOutput"))
diff --git a/compiler/test/dotty/tools/dotc/TupleShowTests.scala b/compiler/test/dotty/tools/dotc/TupleShowTests.scala
index 2d76c480b001..88e0587d7d71 100644
--- a/compiler/test/dotty/tools/dotc/TupleShowTests.scala
+++ b/compiler/test/dotty/tools/dotc/TupleShowTests.scala
@@ -71,7 +71,7 @@ class TupleShowTests extends DottyTest:
   /** On Windows the string literal in this test source file will be read with `\n` (b/c of "-encoding UTF8")
   *   but the compiler will correctly emit \r\n as the line separator.
   *   So we align the expected result to faithfully compare test results. */
-  extension (str: String) def normEOL = if EOL == "\n" then str else str.replace("\n", EOL).nn
+  extension (str: String) def normEOL = if EOL == "\n" then str else str.replace("\n", EOL)
 
   def diff(exp: String, obt: String) =
     val min = math.min(exp.length, obt.length)
diff --git a/compiler/test/dotty/tools/dotc/core/tasty/TastyHeaderUnpicklerTest.scala b/compiler/test/dotty/tools/dotc/core/tasty/TastyHeaderUnpicklerTest.scala
index bde4246ef0f0..ddc88803854f 100644
--- a/compiler/test/dotty/tools/dotc/core/tasty/TastyHeaderUnpicklerTest.scala
+++ b/compiler/test/dotty/tools/dotc/core/tasty/TastyHeaderUnpicklerTest.scala
@@ -265,7 +265,7 @@ class TastyHeaderUnpicklerTest {
 object TastyHeaderUnpicklerTest {
 
   def fillHeader(maj: Int, min: Int, exp: Int, compiler: String): TastyBuffer = {
-    val compilerBytes = compiler.getBytes(java.nio.charset.StandardCharsets.UTF_8).nn
+    val compilerBytes = compiler.getBytes(java.nio.charset.StandardCharsets.UTF_8)
     val buf = new TastyBuffer(header.length + 32 + compilerBytes.length)
     for (ch <- header) buf.writeByte(ch.toByte)
     buf.writeNat(maj)
@@ -304,7 +304,7 @@ object TastyHeaderUnpicklerTest {
       fail()
     }
     catch {
-      case err: UnpickleException => assert(err.getMessage.nn.contains(message))
+      case err: UnpickleException => assert(err.getMessage.contains(message))
     }
   }
 
diff --git a/compiler/test/dotty/tools/dotc/transform/PatmatExhaustivityTest.scala b/compiler/test/dotty/tools/dotc/transform/PatmatExhaustivityTest.scala
index 4ed59db5c10e..70f26dfd104e 100644
--- a/compiler/test/dotty/tools/dotc/transform/PatmatExhaustivityTest.scala
+++ b/compiler/test/dotty/tools/dotc/transform/PatmatExhaustivityTest.scala
@@ -35,7 +35,7 @@ class PatmatExhaustivityTest {
         e.printStackTrace(printWriter)
     }
 
-    stringBuffer.toString.trim.nn.replaceAll("\\s+\n", "\n") match {
+    stringBuffer.toString.trim.replaceAll("\\s+\n", "\n") match {
       case "" => Nil
       case s  => s.linesIterator.toSeq
     }
diff --git a/library/src/scala/runtime/LazyVals.scala b/library/src/scala/runtime/LazyVals.scala
index 15220ea2410a..5d4c508d2076 100644
--- a/library/src/scala/runtime/LazyVals.scala
+++ b/library/src/scala/runtime/LazyVals.scala
@@ -26,7 +26,7 @@ object LazyVals {
   }
 
   private val base: Int = {
-    val processors = java.lang.Runtime.getRuntime.nn.availableProcessors()
+    val processors = java.lang.Runtime.getRuntime.availableProcessors()
     8 * processors * processors
   }
 
diff --git a/library/src/scala/runtime/coverage/Invoker.scala b/library/src/scala/runtime/coverage/Invoker.scala
index c35c6c2ec7df..b2159e6873f6 100644
--- a/library/src/scala/runtime/coverage/Invoker.scala
+++ b/library/src/scala/runtime/coverage/Invoker.scala
@@ -51,6 +51,6 @@ object Invoker {
   @nowarn("cat=deprecation")
   def measurementFile(dataDir: String): File = new File(
     dataDir,
-    MeasurementsPrefix + runtimeUUID + "." + Thread.currentThread.nn.getId
+    MeasurementsPrefix + runtimeUUID + "." + Thread.currentThread.getId
   )
 }
diff --git a/library/src/scala/util/FromDigits.scala b/library/src/scala/util/FromDigits.scala
index cb73782829ff..44532649868d 100644
--- a/library/src/scala/util/FromDigits.scala
+++ b/library/src/scala/util/FromDigits.scala
@@ -135,7 +135,7 @@ object FromDigits {
         case ex: NumberFormatException => throw MalformedNumber()
       }
     if (x.isInfinite) throw NumberTooLarge()
-    if (x == 0.0f && !zeroFloat.pattern.matcher(digits).nn.matches) throw NumberTooSmall()
+    if (x == 0.0f && !zeroFloat.pattern.matcher(digits).matches) throw NumberTooSmall()
     x
   }
 
diff --git a/presentation-compiler/src/main/dotty/tools/pc/CompilerSearchVisitor.scala b/presentation-compiler/src/main/dotty/tools/pc/CompilerSearchVisitor.scala
index 035c1062a3e3..21966499476f 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/CompilerSearchVisitor.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/CompilerSearchVisitor.scala
@@ -105,6 +105,6 @@ class CompilerSearchVisitor(
   override def isCancelled: Boolean = false
 
   private def normalizePackage(pkg: String): String =
-    pkg.replace("/", ".").nn.stripSuffix(".")
+    pkg.replace("/", ".").stripSuffix(".")
 
 end CompilerSearchVisitor
diff --git a/presentation-compiler/src/main/dotty/tools/pc/SymbolInformationProvider.scala b/presentation-compiler/src/main/dotty/tools/pc/SymbolInformationProvider.scala
index ccda618078b8..0c08ed63ffe8 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/SymbolInformationProvider.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/SymbolInformationProvider.scala
@@ -105,7 +105,7 @@ object SymbolProvider:
     catch case NonFatal(e) => Nil
 
   private def normalizePackage(pkg: String): String =
-    pkg.replace("/", ".").nn.stripSuffix(".")
+    pkg.replace("/", ".").stripSuffix(".")
 
   private def toSymbols(info: SymbolInfo.SymbolParts)(using Context): List[Symbol] =
     def collectSymbols(denotation: Denotation): List[Symbol] =
diff --git a/presentation-compiler/src/main/dotty/tools/pc/TastyUtils.scala b/presentation-compiler/src/main/dotty/tools/pc/TastyUtils.scala
index 62a947aeb50b..0c2afad5f92a 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/TastyUtils.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/TastyUtils.scala
@@ -40,7 +40,7 @@ object TastyUtils:
   end htmlTasty
 
   private def tastyHtmlPageTitle(file: URI) =
-    val filename = Paths.get(file).nn.getFileName().toString
+    val filename = Paths.get(file).getFileName().toString
     s"TASTy for $filename"
 
   private val standaloneHtmlStyles =
diff --git a/presentation-compiler/src/main/dotty/tools/pc/completions/AmmoniteFileCompletions.scala b/presentation-compiler/src/main/dotty/tools/pc/completions/AmmoniteFileCompletions.scala
index 81337c7d8dcb..a903aa5dea19 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/completions/AmmoniteFileCompletions.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/completions/AmmoniteFileCompletions.scala
@@ -39,10 +39,10 @@ object AmmoniteFileCompletions:
       rawFileName: String
   )(using Context): List[CompletionValue] =
     val fileName: Option[String] = Option(rawFileName)
-      .flatMap(_.split("/").nn.lastOption.map(_.nn.stripSuffix(".amm.sc.scala")))
+      .flatMap(_.split("/").lastOption.map(_.stripSuffix(".amm.sc.scala")))
 
     val split: List[String] = Option(rawPath)
-      .fold(Nil)(_.split("\\$file").nn.toList.map(_.nn))
+      .fold(Nil)(_.split("\\$file").toList.map(_.nn))
 
     val editRange = selector.headOption.map { sel =>
       if sel.sourcePos.span.isZeroExtent then posRange
@@ -71,10 +71,10 @@ object AmmoniteFileCompletions:
         // drop / or \
         val current = workspace.resolve(script.drop(1))
         val importPath = translateImportToPath(select).drop(1)
-        val currentPath = current.nn.getParent().nn.resolve(importPath).nn.toAbsolutePath()
+        val currentPath = current.getParent().resolve(importPath).toAbsolutePath()
         val parentTextEdit =
-          if query.exists(_.nn.isEmpty()) &&
-            Files.exists(currentPath.nn.getParent()) && Files.isDirectory(
+          if query.exists(_.isEmpty()) &&
+            Files.exists(currentPath.getParent()) && Files.isDirectory(
               currentPath
             )
           then List(parent)
@@ -84,7 +84,7 @@ object AmmoniteFileCompletions:
           .iterator().nn
           .asScala
           .toList
-          .filter(path => !fileName.contains(path.nn.getFileName().toString.stripSuffix(".sc")))
+          .filter(path => !fileName.contains(path.getFileName().toString.stripSuffix(".sc")))
           .collect {
             case file if matches(file) =>
               CompletionValue.FileSystemMember(
diff --git a/project/Build.scala b/project/Build.scala
index db1cf16588aa..9170760514f4 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -986,7 +986,7 @@ object Build {
     // compiler is updated.
     // Then, the next step is to enable flexible types by default and reduce the use of
     // `unsafeNulls`.
-    scalacOptions ++= Seq("-Yno-flexible-types"),
+    // scalacOptions ++= Seq("-Yno-flexible-types"),
     packageAll := {
       (`scala3-compiler` / packageAll).value ++ Seq(
         "scala3-compiler" -> (Compile / packageBin).value.getAbsolutePath,
@@ -1377,10 +1377,10 @@ object Build {
     .asScala3PresentationCompiler(NonBootstrapped)
   lazy val `scala3-presentation-compiler-bootstrapped` = project.in(file("presentation-compiler"))
     .asScala3PresentationCompiler(Bootstrapped)
-    .settings(
-      // Add `-Yno-flexible-types` flag for bootstrap, see comments for `bootstrappedDottyCompilerSettings`
-      Compile / scalacOptions +=  "-Yno-flexible-types"
-    )
+    // .settings(
+    //   // Add `-Yno-flexible-types` flag for bootstrap, see comments for `bootstrappedDottyCompilerSettings`
+    //   Compile / scalacOptions +=  "-Yno-flexible-types"
+    // )
 
   def scala3PresentationCompiler(implicit mode: Mode): Project = mode match {
     case NonBootstrapped => `scala3-presentation-compiler`