FIR IDE: add quickfix for NESTED_CLASS_NOT_ALLOWED

idea/idea-fir/src/org/jetbrains/kotlin/idea/quickfix/MainKtQuickFixRegistrar.kt

@@ -44,6 +44,10 @@ class MainKtQuickFixRegistrar : KtQuickFixRegistrar() {
             KtFirDiagnostic.PrivateSetterForAbstractProperty::class,
             RemoveModifierFix.removePrivateModifier
         )
+        registerPsiQuickFixes(
+            KtFirDiagnostic.NestedClassNotAllowed::class,
+            AddModifierFix.addInnerModifier
+        )
     }
 
     private val overrides = KtQuickFixesListBuilder.registerPsiQuickFix {

idea/idea-frontend-independent/src/org/jetbrains/kotlin/idea/quickfix/AddModifierFix.kt

@@ -106,13 +106,12 @@ open class AddModifierFix(
     }
 
     companion object : Factory<AddModifierFix> {
-        val addAbstractModifier: QuickFixesPsiBasedFactory<PsiElement> = AddModifierFix.createFactory(KtTokens.ABSTRACT_KEYWORD)
-        val addAbstractToContainingClass: QuickFixesPsiBasedFactory<PsiElement> =
-            AddModifierFix.createFactory(KtTokens.ABSTRACT_KEYWORD, KtClassOrObject::class.java)
-        val addOpenToContainingClass: QuickFixesPsiBasedFactory<PsiElement> =
-            AddModifierFix.createFactory(KtTokens.OPEN_KEYWORD, KtClassOrObject::class.java)
-        val addFinalToProperty: QuickFixesPsiBasedFactory<PsiElement> =
-            AddModifierFix.createFactory(KtTokens.FINAL_KEYWORD, KtProperty::class.java)
+        val addAbstractModifier = AddModifierFix.createFactory(KtTokens.ABSTRACT_KEYWORD)
+        val addAbstractToContainingClass = AddModifierFix.createFactory(KtTokens.ABSTRACT_KEYWORD, KtClassOrObject::class.java)
+        val addOpenToContainingClass = AddModifierFix.createFactory(KtTokens.OPEN_KEYWORD, KtClassOrObject::class.java)
+        val addFinalToProperty = AddModifierFix.createFactory(KtTokens.FINAL_KEYWORD, KtProperty::class.java)
+        val addInnerModifier = createFactory(KtTokens.INNER_KEYWORD)
+
         private val modalityModifiers: Set<KtModifierKeywordToken> =
             setOf(KtTokens.ABSTRACT_KEYWORD, KtTokens.OPEN_KEYWORD, KtTokens.FINAL_KEYWORD)
 

idea/testData/quickfix/modifiers/nestedAbstractClass.kt

@@ -4,3 +4,4 @@ class A() {
         abstract class <caret>C
     }
 }
+/* FIR_COMPARISON */

idea/testData/quickfix/modifiers/nestedAbstractClass.kt.after

@@ -4,3 +4,4 @@ class A() {
         abstract inner class C
     }
 }
+/* FIR_COMPARISON */

idea/testData/quickfix/modifiers/nestedClassNotAllowed.kt

@@ -5,3 +5,4 @@ class A() {
         }
     }
 }
+/* FIR_COMPARISON */

idea/testData/quickfix/modifiers/nestedClassNotAllowed.kt.after

@@ -5,3 +5,4 @@ class A() {
         }
     }
 }
+/* FIR_COMPARISON */