[FIR] Add a modifiers diagnostic for the backing field

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

@@ -2430,6 +2430,20 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INAPPLICABLE_BACKING_FIELD_VISIBILITY) { firDiagnostic ->
+        InapplicableBackingFieldVisibilityImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.INAPPLICABLE_BACKING_FIELD_MODIFIER) { firDiagnostic ->
+        InapplicableBackingFieldModifierImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS) { firDiagnostic ->
         AbstractPropertyInPrimaryConstructorParametersImpl(
             firDiagnostic as FirPsiDiagnostic,

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

@@ -1719,6 +1719,16 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = PropertyMustHaveSetter::class
     }
 
+    abstract class InapplicableBackingFieldVisibility : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = InapplicableBackingFieldVisibility::class
+        abstract val modifier: KtModifierKeywordToken
+    }
+
+    abstract class InapplicableBackingFieldModifier : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = InapplicableBackingFieldModifier::class
+        abstract val modifier: KtModifierKeywordToken
+    }
+
     abstract class AbstractPropertyInPrimaryConstructorParameters : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = AbstractPropertyInPrimaryConstructorParameters::class
     }

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

@@ -2788,6 +2788,22 @@ internal class LocalVariableWithTypeParametersImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class InapplicableBackingFieldVisibilityImpl(
+    override val modifier: KtModifierKeywordToken,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.InapplicableBackingFieldVisibility(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
+internal class InapplicableBackingFieldModifierImpl(
+    override val modifier: KtModifierKeywordToken,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.InapplicableBackingFieldModifier(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class ExpectedDeclarationWithBodyImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,