Add quick fix to add .nn (#23598)

An extension to #23461 that adds a quick fix to add a .nn. 
For example, if the code were
```scala
val t: String | Null = ???
val s: String = t
```
the quick fix would transform the code to:
```scala
val t: String | Null = ???
val s: String = t.nn
```

Author: noti0na1

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -67,6 +67,8 @@ object desugar {
    */
   val TrailingForMap: Property.Key[Unit] = Property.StickyKey()
 
+  val WasTypedInfix: Property.Key[Unit] = Property.StickyKey()
+
   /** What static check should be applied to a Match? */
   enum MatchCheck {
     case None, Exhaustive, IrrefutablePatDef, IrrefutableGenFrom
@@ -1720,10 +1722,12 @@ object desugar {
         case _ =>
           Apply(sel, arg :: Nil)
 
-    if op.name.isRightAssocOperatorName then
+    val apply = if op.name.isRightAssocOperatorName then
       makeOp(right, left, Span(op.span.start, right.span.end))
     else
       makeOp(left, right, Span(left.span.start, op.span.end, op.span.start))
+    apply.pushAttachment(WasTypedInfix, ())
+    return apply
   }
 
   /** Translate throws type `A throws E1 | ... | En` to

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

@@ -14,6 +14,7 @@ import printing.Highlighting.*
 import printing.Formatting
 import ErrorMessageID.*
 import ast.Trees
+import ast.desugar
 import config.{Feature, MigrationVersion, ScalaVersion}
 import transform.patmat.Space
 import transform.patmat.SpaceEngine
@@ -300,6 +301,11 @@ 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 ctx.mode.is(Mode.SafeNulls) && 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 +366,26 @@ 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) =
+    inTree match {
+      case Some(tree) if shouldSuggestNN =>
+        val content = tree.source.content().slice(tree.srcPos.startPos.start, tree.srcPos.endPos.end).mkString
+        val replacement = tree match
+          case a @ Apply(_, _) if !a.hasAttachment(desugar.WasTypedInfix) =>
+            content + ".nn"
+          case _ @ (Select(_, _) | Ident(_)) => content + ".nn"
+          case _ => "(" + content + ").nn"
+        List(
+          CodeAction(title = """Add .nn""",
+            description = None,
+            patches = List(
+              ActionPatch(tree.srcPos.sourcePos, replacement)
+            )
+          )
+        )
+      case _ =>
+        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 +179,130 @@ class CodeActionTest extends DottyTest:
       ctxx = ctxx
       )
 
+  @Test def addNN1 =
+    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
+      )
+
+  @Test def addNN5 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """val s: String | Null = ???
+          |val t: String = s match {
+          | case _: String => "foo"
+          | case _ => s
+          |}""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """val s: String | Null = ???
+          |val t: String = s match {
+          | case _: String => "foo"
+          | case _ => s.nn
+          |}""".stripMargin,
+      ctxx = ctxx
+      )
+
+  @Test def addNN6 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """val s: String | Null = ???
+          |val t: String = if (s != null) "foo" else s""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """val s: String | Null = ???
+          |val t: String = if (s != null) "foo" else s.nn""".stripMargin,
+      ctxx = ctxx
+      )
+
+  @Test def addNN7 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """given ctx: String | Null = null
+          |def f(using c: String): String = c
+          |val s: String = f(using ctx)""".stripMargin,
+        title = "Add .nn",
+      expected =
+        """given ctx: String | Null = null
+          |def f(using c: String): String = c
+          |val s: String = f(using ctx.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 =