Fix off-by-one and add tests

Author: HarrisL2

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

@@ -3596,7 +3596,7 @@ class UnnecessaryNN(reason: String, sourcePosition: SourcePosition)(using Contex
 
   override def explain(using Context) = ""
 
-  private val nnSourcePosition = SourcePosition(sourcePosition.source, Span(sourcePosition.span.end, sourcePosition.span.end + 2, sourcePosition.span.end), sourcePosition.outer)
+  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(

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