feat: add in an action to remove duplicated modifiers

Author: ckipp01

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3028,7 +3028,7 @@ object Parsers {
       val name = in.name
       val mod = atSpan(in.skipToken()) { modOfToken(tok, name) }
 
-      if (mods.isOneOf(mod.flags)) syntaxError(RepeatedModifier(mod.flags.flagsString))
+      if (mods.isOneOf(mod.flags)) syntaxError(RepeatedModifier(mod.flags.flagsString, source, mod.span))
       addMod(mods, mod)
     }
 

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

@@ -33,6 +33,7 @@ import dotty.tools.dotc.rewrites.Rewrites.ActionPatch
 import dotty.tools.dotc.util.Spans.Span
 import dotty.tools.dotc.util.SourcePosition
 import scala.jdk.CollectionConverters.*
+import dotty.tools.dotc.util.SourceFile
 
 /**  Messages
   *  ========
@@ -497,7 +498,7 @@ extends SyntaxMsg(ObjectMayNotHaveSelfTypeID) {
   }
 }
 
-class RepeatedModifier(modifier: String)(implicit ctx:Context)
+class RepeatedModifier(modifier: String, source: SourceFile, span: Span)(implicit ctx:Context)
 extends SyntaxMsg(RepeatedModifierID) {
   def msg(using Context) = i"""Repeated modifier $modifier"""
 
@@ -516,6 +517,16 @@ extends SyntaxMsg(RepeatedModifierID) {
         |
         |"""
   }
+
+  override def actions(using Context) =
+    List(
+      CodeAction(title = s"""Remove repeated modifier: "$modifier"""",
+        description =  java.util.Optional.empty(),
+        patches = List(
+          ActionPatch(SourcePosition(source, span), "")
+        ).asJava
+      )
+    ).asJava
 }
 
 class InterpolatedStringError()(implicit ctx:Context)

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

@@ -44,6 +44,17 @@ class CodeActionTest extends DottyTest:
 
       )
 
+  @Test def removeRepeatModifier =
+    checkCodeAction(
+      """|final final class Test
+         |""".stripMargin,
+      """Remove repeated modifier: "final"""",
+      // TODO look into trying to remove the extra space that is left behind
+      """|final  class Test
+         |""".stripMargin
+
+      )
+
   // 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 =