FIR checker/IDE: Add checker and quickfix for VAL_WITH_SETTER.

idea/idea-completion/testData/basic/common/CompletionInSetter.kt

@@ -1,3 +1,4 @@
+// FIR_COMPARISON
 val a: Int = 1
     get() {
       return field

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

@@ -48,6 +48,7 @@ class MainKtQuickFixRegistrar : KtQuickFixRegistrar() {
         registerPsiQuickFixes(KtFirDiagnostic.VarOverriddenByVal::class, ChangeVariableMutabilityFix.VAR_OVERRIDDEN_BY_VAL_FACTORY)
         registerPsiQuickFixes(KtFirDiagnostic.VarAnnotationParameter::class, ChangeVariableMutabilityFix.VAR_ANNOTATION_PARAMETER_FACTORY)
         registerPsiQuickFixes(KtFirDiagnostic.InapplicableLateinitModifier::class, ChangeVariableMutabilityFix.LATEINIT_VAL_FACTORY)
+        registerPsiQuickFixes(KtFirDiagnostic.ValWithSetter::class, ChangeVariableMutabilityFix.VAL_WITH_SETTER_FACTORY)
     }
 
     override val list: KtQuickFixesList = KtQuickFixesList.createCombined(

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

@@ -944,6 +944,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.VAL_WITH_SETTER) { firDiagnostic ->
+        ValWithSetterImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.EXPECTED_DECLARATION_WITH_BODY) { firDiagnostic ->
         ExpectedDeclarationWithBodyImpl(
             firDiagnostic as FirPsiDiagnostic<*>,

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

@@ -666,6 +666,10 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ExpectedPrivateDeclaration::class
     }
 
+    abstract class ValWithSetter : KtFirDiagnostic<KtPropertyAccessor>() {
+        override val diagnosticClass get() = ValWithSetter::class
+    }
+
     abstract class ExpectedDeclarationWithBody : KtFirDiagnostic<KtDeclaration>() {
         override val diagnosticClass get() = ExpectedDeclarationWithBody::class
     }

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

@@ -1075,6 +1075,13 @@ internal class ExpectedPrivateDeclarationImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ValWithSetterImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ValWithSetter(), KtAbstractFirDiagnostic<KtPropertyAccessor> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class ExpectedDeclarationWithBodyImpl(
     firDiagnostic: FirPsiDiagnostic<*>,
     override val token: ValidityToken,

idea/testData/quickfix/variables/changeMutability/valWithSetter.kt

@@ -3,3 +3,4 @@ class A() {
     val a: Int = 0
         <caret>set(v: Int) {}
 }
+/* FIR_COMPARISON */

idea/testData/quickfix/variables/changeMutability/valWithSetter.kt.after

@@ -3,3 +3,4 @@ class A() {
     var a: Int = 0
         set(v: Int) {}
 }
+/* FIR_COMPARISON */