Make the extension method nn inline.

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -104,6 +104,30 @@ class InlineBytecodeTests extends DottyBytecodeTest {
     }
   }
 
+  @Test def inlineNn = {
+    val source =
+      s"""
+         |class Foo {
+         |  def meth1(x: Int | Null): Int = x.nn
+         |  def meth2(x: Int | Null): Int = scala.runtime.Scala3RunTime.nn(x)
+         |}
+         """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn = dir.lookupName("Foo.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+      val meth1 = getMethod(clsNode, "meth1")
+      val meth2 = getMethod(clsNode, "meth2")
+
+      val instructions1 = instructionsFromMethod(meth1)
+      val instructions2 = instructionsFromMethod(meth2)
+
+      assert(instructions1 == instructions2,
+        "`nn` was not properly inlined in `meth1`\n" +
+        diffInstructions(instructions1, instructions2))
+    }
+  }
+
   @Test def i4947 = {
     val source = """class Foo {
                    |  transparent inline def track[T](inline f: T): T = {

library/src/dotty/DottyPredef.scala

@@ -79,7 +79,6 @@ object DottyPredef {
    *
    *  Note that `.nn` performs a checked cast, so if invoked on a null value it'll throw an NPE.
    */
-  extension [T](x: T | Null) def nn: x.type & T =
-    if (x == null) throw new NullPointerException("tried to cast away nullability, but value is null")
-    else x.asInstanceOf[x.type & T]
+  extension [T](x: T | Null) inline def nn: x.type & T =
+    scala.runtime.Scala3RunTime.nn(x)
 }

library/src/scala/runtime/Scala3RunTime.scala

@@ -10,4 +10,12 @@ object Scala3RunTime:
   def assertFailed(): Nothing =
     throw new java.lang.AssertionError("assertion failed")
 
+  /** Called by the inline extension def `nn`.
+   *
+   *  Extracted to minimize the bytecode size at call site.
+   */
+  def nn[T](x: T | Null): x.type & T =
+    if (x == null) throw new NullPointerException("tried to cast away nullability, but value is null")
+    else x.asInstanceOf[x.type & T]
+
 end Scala3RunTime