FIR: drop questionable DiagnosticKind.NotRootCause

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

@@ -19,7 +19,6 @@ import org.jetbrains.kotlin.fir.resolve.inference.ConeTypeParameterBasedTypeVari
 import org.jetbrains.kotlin.fir.resolve.inference.ConeTypeVariableForLambdaReturnType
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeArgumentConstraintPosition
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeExpectedTypeConstraintPosition
-import org.jetbrains.kotlin.fir.resolve.inference.model.ConeExplicitTypeParameterConstraintPosition
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeLambdaArgumentConstraintPosition
 import org.jetbrains.kotlin.fir.symbols.impl.FirBackingFieldSymbol
 import org.jetbrains.kotlin.fir.typeContext
@@ -75,7 +74,7 @@ private fun ConeDiagnostic.toFirDiagnostic(
         FirErrors.NO_TYPE_ARGUMENTS_ON_RHS.createOn(qualifiedAccessSource ?: source, this.desiredCount, this.type)
     is ConeSimpleDiagnostic -> when (source.kind) {
         is FirFakeSourceElementKind -> null
-        else -> this.getFactory(source)?.createOn(qualifiedAccessSource ?: source)
+        else -> this.getFactory(source).createOn(qualifiedAccessSource ?: source)
     }
     is ConeInstanceAccessBeforeSuperCall -> FirErrors.INSTANCE_ACCESS_BEFORE_SUPER_CALL.createOn(source, this.target)
     is ConeStubDiagnostic -> null
@@ -322,7 +321,7 @@ private fun ConstraintSystemError.toDiagnostic(
 private val NewConstraintError.lowerConeType: ConeKotlinType get() = lowerType as ConeKotlinType
 private val NewConstraintError.upperConeType: ConeKotlinType get() = upperType as ConeKotlinType
 
-private fun ConeSimpleDiagnostic.getFactory(source: FirSourceElement): FirDiagnosticFactory0? {
+private fun ConeSimpleDiagnostic.getFactory(source: FirSourceElement): FirDiagnosticFactory0 {
     @Suppress("UNCHECKED_CAST")
     return when (kind) {
         DiagnosticKind.Syntax -> FirErrors.SYNTAX
@@ -365,7 +364,6 @@ private fun ConeSimpleDiagnostic.getFactory(source: FirSourceElement): FirDiagno
         DiagnosticKind.UnresolvedSupertype,
         DiagnosticKind.UnresolvedExpandedType,
         DiagnosticKind.Other -> FirErrors.OTHER_ERROR
-        DiagnosticKind.NotRootCause -> null
         else -> throw IllegalArgumentException("Unsupported diagnostic kind: $kind at $javaClass")
     }
 }

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt

@@ -707,8 +707,7 @@ class ExpressionsConverter(
         whenEntryNodes.mapTo(whenEntries) {
             convertWhenEntry(
                 it,
-                subject.takeIf { hasSubject },
-                hasSubjectVariable = subjectVariable != null
+                subject.takeIf { hasSubject }
             )
         }
         return buildWhenExpression {
@@ -756,8 +755,7 @@ class ExpressionsConverter(
      */
     private fun convertWhenEntry(
         whenEntry: LighterASTNode,
-        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?,
-        hasSubjectVariable: Boolean
+        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?
     ): WhenEntry {
         var isElse = false
         var firBlock: FirBlock = buildEmptyExpressionBlock()
@@ -765,8 +763,8 @@ class ExpressionsConverter(
         whenEntry.forEachChildren {
             when (it.tokenType) {
                 WHEN_CONDITION_EXPRESSION -> conditions += convertWhenConditionExpression(it, whenRefWithSubject)
-                WHEN_CONDITION_IN_RANGE -> conditions += convertWhenConditionInRange(it, whenRefWithSubject, hasSubjectVariable)
-                WHEN_CONDITION_IS_PATTERN -> conditions += convertWhenConditionIsPattern(it, whenRefWithSubject, hasSubjectVariable)
+                WHEN_CONDITION_IN_RANGE -> conditions += convertWhenConditionInRange(it, whenRefWithSubject)
+                WHEN_CONDITION_IS_PATTERN -> conditions += convertWhenConditionIsPattern(it, whenRefWithSubject)
                 ELSE_KEYWORD -> isElse = true
                 BLOCK -> firBlock = declarationsConverter.convertBlock(it)
                 else -> if (it.isExpression()) firBlock = declarationsConverter.convertBlock(it)
@@ -809,8 +807,7 @@ class ExpressionsConverter(
 
     private fun convertWhenConditionInRange(
         whenCondition: LighterASTNode,
-        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?,
-        hasSubjectVariable: Boolean
+        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?
     ): FirExpression {
         var isNegate = false
         var firExpression: FirExpression? = null
@@ -838,8 +835,7 @@ class ExpressionsConverter(
                 source = whenCondition.toFirSourceElement()
                 diagnostic = ConeSimpleDiagnostic(
                     "No expression in condition with expression",
-                    // A subject variable without initializer should be highlighted only at the subject expression.
-                    if (hasSubjectVariable) DiagnosticKind.NotRootCause else DiagnosticKind.ExpressionExpected
+                    DiagnosticKind.ExpressionExpected
                 )
             }
         }
@@ -859,8 +855,7 @@ class ExpressionsConverter(
 
     private fun convertWhenConditionIsPattern(
         whenCondition: LighterASTNode,
-        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?,
-        hasSubjectVariable: Boolean
+        whenRefWithSubject: FirExpressionRef<FirWhenExpression>?
     ): FirExpression {
         lateinit var firOperation: FirOperation
         lateinit var firType: FirTypeRef
@@ -881,8 +876,7 @@ class ExpressionsConverter(
                 source = whenCondition.toFirSourceElement()
                 diagnostic = ConeSimpleDiagnostic(
                     "No expression in condition with expression",
-                    // A subject variable without initializer should be highlighted only at the subject expression.
-                    if (hasSubjectVariable) DiagnosticKind.NotRootCause else DiagnosticKind.ExpressionExpected
+                    DiagnosticKind.ExpressionExpected
                 )
             }
         }

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -320,8 +320,7 @@ open class RawFirBuilder(
                 )
             }
             val name = this.getArgumentName()?.asName
-            val expression = this.getArgumentExpression()
-            val firExpression = when (expression) {
+            val firExpression = when (val expression = this.getArgumentExpression()) {
                 is KtConstantExpression, is KtStringTemplateExpression -> {
                     expression.accept(this@Visitor, Unit) as FirExpression
                 }
@@ -580,7 +579,7 @@ open class RawFirBuilder(
             container.argumentList = argumentList
         }
 
-        fun KtClassOrObject.extractSuperTypeListEntriesTo(
+        private fun KtClassOrObject.extractSuperTypeListEntriesTo(
             container: FirClassBuilder,
             delegatedSelfTypeRef: FirTypeRef?,
             delegatedEnumSuperTypeRef: FirTypeRef?,
@@ -1754,9 +1753,7 @@ open class RawFirBuilder(
                                     ?: ktCondition)
                                     .toFirExpression(
                                         "No expression in condition with expression",
-                                        // A subject variable without initializer should be highlighted only at the subject expression.
-                                        if (ktSubjectExpression is KtVariableDeclaration) DiagnosticKind.NotRootCause
-                                        else DiagnosticKind.ExpressionExpected
+                                        DiagnosticKind.ExpressionExpected
                                     )
                                 result = branchBody
                             }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/ConeSimpleDiagnostic.kt

@@ -52,10 +52,4 @@ enum class DiagnosticKind {
     WrongLongSuffix,
 
     Other,
-
-    /**
-     * Special diagnostic kind that is caused by issues at other places in the code. Such diagnostics should not be reported so that user
-     * sees less noise.
-     */
-    NotRootCause
 }

compiler/testData/diagnostics/tests/when/withSubjectVariable/unsupportedVariableDeclarationsInWhenSubject.fir.kt

@@ -12,7 +12,7 @@ fun testSimpleValInWhenSubject() {
 
 fun testValWithoutInitializerWhenSubject() {
     when (<!ILLEGAL_DECLARATION_IN_WHEN_SUBJECT!>val y: Any<!>) {
-        is String -> <!UNINITIALIZED_VARIABLE!>y<!>.<!UNRESOLVED_REFERENCE!>length<!>
+        <!EXPECTED_CONDITION!>is String<!> -> <!UNINITIALIZED_VARIABLE!>y<!>.<!UNRESOLVED_REFERENCE!>length<!>
     }
 }