FIR: introduce diagnostic NESTED_CLASS_NOT_ALLOWED

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/file/structure/FileStructureElement.kt

@@ -173,7 +173,10 @@ internal class NonReanalyzableDeclarationStructureElement(
                 firFile,
                 onDeclarationEnter = { firDeclaration ->
                     when {
-                        firDeclaration.isGeneratedDeclaration -> DiagnosticCollectorDeclarationAction.SKIP
+                        // Some generated declaration contains structures that we need to check. For example the FIR representation of an
+                        // enum entry initializer, when present, is a generated anonymous object of kind `ENUM_ENTRY`.
+                        firDeclaration.isGeneratedDeclaration ->
+                            DiagnosticCollectorDeclarationAction.SKIP_CURRENT_DECLARATION_AND_CHECK_NESTED
                         firDeclaration is FirFile -> DiagnosticCollectorDeclarationAction.CHECK_CURRENT_DECLARATION_AND_CHECK_NESTED
                         firDeclaration == fir -> {
                             inCurrentDeclaration = true

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -99,6 +99,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.NESTED_CLASS_NOT_ALLOWED) { firDiagnostic ->
+        NestedClassNotAllowedImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.HIDDEN) { firDiagnostic ->
         HiddenImpl(
             firSymbolBuilder.buildSymbol(firDiagnostic.a.fir as FirDeclaration),

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -83,6 +83,11 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = DelegationInInterface::class
     }
 
+    abstract class NestedClassNotAllowed : KtFirDiagnostic<KtNamedDeclaration>() {
+        override val diagnosticClass get() = NestedClassNotAllowed::class
+        abstract val declaration: String
+    }
+
     abstract class Hidden : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = Hidden::class
         abstract val hidden: KtSymbol

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -114,6 +114,14 @@ internal class DelegationInInterfaceImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class NestedClassNotAllowedImpl(
+    override val declaration: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.NestedClassNotAllowed(), KtAbstractFirDiagnostic<KtNamedDeclaration> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class HiddenImpl(
     override val hidden: KtSymbol,
     firDiagnostic: FirPsiDiagnostic<*>,