Make the extension method nn inline.

And delegate its body to a runtime method in Scala3RunTime.

With this change, all the methods of DottyPredef are inline
methods. We will therefore be able to patch them on Predef in the
future, rather than having DottyPredef.

Author: sjrd

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 +10,10 @@ object Scala3RunTime:
   def assertFailed(): Nothing =
     throw new java.lang.AssertionError("assertion failed")
 
+  // Called by the inline extension def nn. Extract 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