[FIR] Add a modifiers diagnostic for the backing field

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -877,6 +877,12 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val PROPERTY_FIELD_DECLARATION_MISSING_INITIALIZER by error<KtBackingField>()
         val PROPERTY_MUST_HAVE_GETTER by error<KtProperty>()
         val PROPERTY_MUST_HAVE_SETTER by error<KtProperty>()
+        val INAPPLICABLE_BACKING_FIELD_VISIBILITY by error<PsiElement> {
+            parameter<KtModifierKeywordToken>("modifier")
+        }
+        val INAPPLICABLE_BACKING_FIELD_MODIFIER by error<PsiElement> {
+            parameter<KtModifierKeywordToken>("modifier")
+        }
         val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error<KtModifierListOwner>(PositioningStrategy.ABSTRACT_MODIFIER)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -484,6 +484,8 @@ object FirErrors {
     val PROPERTY_FIELD_DECLARATION_MISSING_INITIALIZER by error0<KtBackingField>()
     val PROPERTY_MUST_HAVE_GETTER by error0<KtProperty>()
     val PROPERTY_MUST_HAVE_SETTER by error0<KtProperty>()
+    val INAPPLICABLE_BACKING_FIELD_VISIBILITY by error1<PsiElement, KtModifierKeywordToken>()
+    val INAPPLICABLE_BACKING_FIELD_MODIFIER by error1<PsiElement, KtModifierKeywordToken>()
     val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error0<KtModifierListOwner>(SourceElementPositioningStrategies.ABSTRACT_MODIFIER)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt

@@ -60,6 +60,11 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirLocalVariableTypeParametersSyntaxChecker,
         )
 
+    override val backingFieldCheckers: Set<FirBackingFieldChecker>
+        get() = setOf(
+            FirBackingFieldModifiersChecker,
+        )
+
     override val classCheckers: Set<FirClassChecker>
         get() = setOf(
             FirOverrideChecker,

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirBackingFieldModifiersChecker.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.analysis.checkers.declaration
+
+import org.jetbrains.kotlin.fir.analysis.checkers.FirModifier
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.getModifierList
+import org.jetbrains.kotlin.fir.analysis.checkers.toVisibilityOrNull
+import org.jetbrains.kotlin.fir.analysis.diagnostics.*
+import org.jetbrains.kotlin.fir.declarations.FirBackingField
+import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.lexer.KtTokens
+
+object FirBackingFieldModifiersChecker : FirBackingFieldChecker() {
+    private val ALLOWED_MODIFIERS = setOf(
+        KtTokens.PRIVATE_KEYWORD,
+        KtTokens.INTERNAL_KEYWORD,
+    )
+
+    override fun check(declaration: FirBackingField, context: CheckerContext, reporter: DiagnosticReporter) {
+        val modifiers = declaration.source
+            ?.getModifierList()
+            ?.modifiers
+            ?: return
+
+        modifiers.forEach {
+            if (it.token in ALLOWED_MODIFIERS) {
+                return@forEach
+            }
+
+            val diagnostic = reportInapplicable(it)
+            reporter.reportOn(it.source, diagnostic, it.token, context)
+        }
+    }
+
+    private fun reportInapplicable(modifier: FirModifier<*>): FirDiagnosticFactory1<KtModifierKeywordToken> {
+        return if (modifier.token.toVisibilityOrNull() != null) {
+            FirErrors.INAPPLICABLE_BACKING_FIELD_VISIBILITY
+        } else {
+            FirErrors.INAPPLICABLE_BACKING_FIELD_MODIFIER
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt

@@ -268,6 +268,8 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MANY_LAMBDA_EXPRE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MISPLACED_TYPE_PARAMETER_CONSTRAINTS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MISSING_VAL_ON_ANNOTATION_PARAMETER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_BACKING_FIELD_MODIFIER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_BACKING_FIELD_VISIBILITY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MODIFIER_FORM_FOR_NON_BUILT_IN_SUSPEND
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MULTIPLE_VARARG_PARAMETERS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.MUST_BE_INITIALIZED
@@ -1287,6 +1289,16 @@ class FirDefaultErrorMessages {
                 PROPERTY_MUST_HAVE_SETTER,
                 "This property needs a custom setter, because it's type is not a subtype of the backing field's type"
             )
+            map.put(
+                INAPPLICABLE_BACKING_FIELD_VISIBILITY,
+                "Explicit backing field declarations may only be private or internal, but not ''{0}''",
+                TO_STRING
+            )
+            map.put(
+                INAPPLICABLE_BACKING_FIELD_MODIFIER,
+                "Modifier ''{0}'' is not applicable to explicit backing field declarations",
+                TO_STRING
+            )
             map.put(ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS, "This property cannot be declared abstract")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING, "Type parameters for local variables are deprecated")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS, "Local variables are not allowed to have type parameters")

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,