diff --git a/compiler/src/dotty/tools/dotc/profile/Profiler.scala b/compiler/src/dotty/tools/dotc/profile/Profiler.scala
index 6df88d9e28f0..0533b6bdaec3 100644
--- a/compiler/src/dotty/tools/dotc/profile/Profiler.scala
+++ b/compiler/src/dotty/tools/dotc/profile/Profiler.scala
@@ -1,5 +1,6 @@
 package dotty.tools.dotc.profile
 
+import scala.annotation.*
 import scala.language.unsafeNulls
 
 import java.io.{FileWriter, PrintWriter}
@@ -123,6 +124,7 @@ private [profile] class RealProfiler(reporter : ProfileReporter)(using Context)
   }
   private def readHeapUsage() = RealProfiler.memoryMx.getHeapMemoryUsage.getUsed
 
+  @nowarn
   private def doGC: Unit = {
     System.gc()
     System.runFinalization()
diff --git a/library/src/scala/runtime/LazyVals.scala b/library/src/scala/runtime/LazyVals.scala
index f3fe52beb9ee..7a66639a826a 100644
--- a/library/src/scala/runtime/LazyVals.scala
+++ b/library/src/scala/runtime/LazyVals.scala
@@ -1,9 +1,12 @@
 package scala.runtime
 
+import scala.annotation.*
+
 /**
  * Helper methods used in thread-safe lazy vals.
  */
 object LazyVals {
+  @nowarn
   private[this] val unsafe: sun.misc.Unsafe =
       classOf[sun.misc.Unsafe].getDeclaredFields.nn.find { field =>
         field.nn.getType == classOf[sun.misc.Unsafe] && {
@@ -100,6 +103,7 @@ object LazyVals {
   }
 
   def getOffset(clz: Class[_], name: String): Long = {
+    @nowarn
     val r = unsafe.objectFieldOffset(clz.getDeclaredField(name))
     if (debug)
       println(s"getOffset($clz, $name) = $r")
@@ -107,6 +111,7 @@ object LazyVals {
   }
 
   def getOffsetStatic(field: java.lang.reflect.Field) =
+    @nowarn
     val r = unsafe.objectFieldOffset(field)
     if (debug)
       println(s"getOffset(${field.getDeclaringClass}, ${field.getName}) = $r")
diff --git a/project/Build.scala b/project/Build.scala
index 30aa05b9ee5d..d89f67f0be35 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -169,9 +169,10 @@ object Build {
       "-feature",
       "-deprecation",
       "-unchecked",
-      "-Xfatal-warnings",
+      //"-Wconf:cat=deprecation&msg=Unsafe:s",    // example usage
+      "-Xfatal-warnings",                         // -Werror in modern usage
       "-encoding", "UTF8",
-      "-language:implicitConversions"
+      "-language:implicitConversions",
     ),
 
     (Compile / compile / javacOptions) ++= Seq("-Xlint:unchecked", "-Xlint:deprecation"),