Add quick fix to remove unnecessary .nn

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

@@ -229,6 +229,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case PointlessAppliedConstructorTypeID // errorNumber: 213
   case IllegalContextBoundsID // errorNumber: 214
   case NamedPatternNotApplicableID // errorNumber: 215
+  case UnnecessaryNN // errorNumber: 216
 
   def errorNumber = ordinal - 1
 

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

@@ -3549,3 +3549,24 @@ final class NamedPatternNotApplicable(selectorType: Type)(using Context) extends
     i"Named patterns cannot be used with $selectorType, because it is not a named tuple or case class"
 
   override protected def explain(using Context): String = ""
+
+/**  @param reason            The reason for the unnecessary null. The warning given to the user will be i""""Unncessary .nn: $reason"""
+   *  @param sourcePosition   The sourcePosition of the qualifier
+   */
+class UnnecessaryNN(reason: String, sourcePosition: SourcePosition)(using Context) extends SyntaxMsg(UnnecessaryNN) {
+  override def msg(using Context) = i"""Unnecessary .nn: $reason"""
+
+  override def explain(using Context) = ""
+
+  private val nnSourcePosition = SourcePosition(sourcePosition.source, Span(sourcePosition.span.end, sourcePosition.span.end + 3, sourcePosition.span.end), sourcePosition.outer)
+
+  override def actions(using Context) =
+    List(
+      CodeAction(title = """Remove unnecessary .nn""",
+        description = None,
+        patches = List(
+          ActionPatch(nnSourcePosition, "")
+        )
+      )
+    )
+}

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1093,8 +1093,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         if symbol.exists && symbol.owner == defn.ScalaPredefModuleClass && symbol.name == nme.nn then
           tree match
           case Apply(_, args) =>
-            if(args.head.tpe.isNotNull) then report.warning("Unnecessary .nn: qualifier is already not null", tree)
-            if pt.admitsNull then report.warning("Unnecessary .nn: expected type admits null", tree)
+            if(args.head.tpe.isNotNull) then report.warning(UnnecessaryNN("qualifier is already not null", args.head.sourcePos), tree)
+            if pt.admitsNull then report.warning(UnnecessaryNN("expected type admits null", args.head.sourcePos), tree)
           case _ =>
       }
     }

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

@@ -4,6 +4,7 @@ import dotty.tools.DottyTest
 import dotty.tools.dotc.rewrites.Rewrites
 import dotty.tools.dotc.rewrites.Rewrites.ActionPatch
 import dotty.tools.dotc.util.SourceFile
+import dotty.tools.dotc.core.Contexts._
 
 import scala.annotation.tailrec
 import scala.jdk.CollectionConverters.*
@@ -149,14 +150,43 @@ class CodeActionTest extends DottyTest:
          afterPhase = "patternMatcher"
       )
 
+  @Test def removeNN =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """|val s: String|Null = "foo".nn
+           |""".stripMargin,
+         title = "Remove unnecessary .nn",
+      expected =
+        """|val s: String|Null = "foo"
+           |""".stripMargin,
+      ctxx = ctxx
+      )
+
+
+  @Test def removeNN2 =
+    val ctxx = newContext
+    ctxx.setSetting(ctxx.settings.YexplicitNulls, true)
+    checkCodeAction(
+      code =
+        """val s: String|Null = null.nn
+           |""".stripMargin,
+         title = "Remove unnecessary .nn",
+      expected =
+        """val s: String|Null = null
+           |""".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 =
     val rep = new StoreReporter(null) with UniqueMessagePositions with HideNonSensicalMessages
     initialCtx.setReporter(rep).withoutColors
 
-  private def checkCodeAction(code: String, title: String, expected: String, afterPhase: String = "typer") =
-    ctx = newContext
+  private def checkCodeAction(code: String, title: String, expected: String, afterPhase: String = "typer", ctxx: Context = newContext) =
+    ctx = ctxx
     val source = SourceFile.virtual("test", code).content
     val runCtx = checkCompile(afterPhase, code) { (_, _) => () }
     val diagnostics = runCtx.reporter.removeBufferedMessages