Add quick fix to add .nn

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -300,6 +300,11 @@ extends NotFoundMsg(MissingIdentID) {
 class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tree], addenda: => String*)(using Context)
   extends TypeMismatchMsg(found, expected)(TypeMismatchID):
 
+  private val shouldSuggestNN =
+    if expected.isValueType then
+      found frozen_<:< OrNull(expected)
+    else false
+
   def msg(using Context) =
     // replace constrained TypeParamRefs and their typevars by their bounds where possible
     // and the bounds are not f-bounds.
@@ -360,6 +365,28 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
     val treeStr = inTree.map(x => s"\nTree:\n\n${x.show}\n").getOrElse("")
     treeStr + "\n" + super.explain
 
+  override def actions(using Context) =
+    if shouldSuggestNN then
+      inTree match {
+        case Some(tree) if tree != null =>
+          val content = tree.source.content().slice(tree.srcPos.startPos.start, tree.srcPos.endPos.end).mkString
+          val replacement = tree match
+            case Apply(fun, args) => "(" + content + ").nn"
+            case _ => content + ".nn"
+          List(
+            CodeAction(title = """Add .nn""",
+            description = None,
+            patches = List(
+              ActionPatch(tree.srcPos.sourcePos, replacement)
+            )
+            )
+          )
+        case _ =>
+          List()
+      }
+    else
+      List()
+
 end TypeMismatch
 
 class NotAMember(site: Type, val name: Name, selected: String, proto: Type, addendum: => String = "")(using Context)

compiler/test/dotty/tools/dotc/reporting/CodeActionTest.scala

@@ -179,6 +179,79 @@ class CodeActionTest extends DottyTest:
       ctxx = ctxx
       )
 
+  @Test def addNN =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """val s: String|Null = ???
+          | val t: String = s""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """val s: String|Null = ???
+          | val t: String = s.nn""".stripMargin,
+      ctxx = ctxx
+      )
+
+  @Test def addNN2 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = s q s""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = (s q s).nn""".stripMargin,
+      ctxx = ctxx
+      )
+
+  @Test def addNN3 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String, s3: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = s q (s, s)""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String, s3: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = (s q (s, s)).nn""".stripMargin,
+      ctxx = ctxx
+      )
+
+  @Test def addNN4 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String, s3: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = s.q(s, s)""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """implicit class infixOpTest(val s1: String) extends AnyVal {
+          |  def q(s2: String, s3: String): String | Null = null
+          |}
+          | val s: String = ???
+          | val t: String = (s.q(s, s)).nn""".stripMargin,
+      ctxx = ctxx
+      )
   // Make sure we're not using the default reporter, which is the ConsoleReporter,
   // meaning they will get reported in the test run and that's it.
   private def newContext =