KT-67706 [FIR] Ignore DesugaredAugmentedAssign source in UnusedChecker

In the cases like `foo.bar += 10`, there is a `val <receiver> = foo.bar`
variable which is also reported from `variablesWithoutReads`

However, this variable has a fake source pointing to the `foo.bar`
`KtDotQualifiedExpression` expression, which is impossible to report
with `UNUSED_VARIABLE`, because it accepts only `KtNamedDeclaration`s

^KT-67706 Fixed

Author: fedochet

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extended/UnusedChecker.kt

@@ -62,6 +62,10 @@ object UnusedChecker : AbstractFirPropertyInitializationChecker(MppCheckerKind.C
     private val FirPropertySymbol.ignoreWarnings: Boolean
         get() = name == SpecialNames.UNDERSCORE_FOR_UNUSED_VAR ||
                 source == null ||
+                // if <receiver> variable is reported as unused,
+                // then the assignment itself is a dead code because of its RHS expression,
+                // which will be eventually reported
+                source?.kind is KtFakeSourceElementKind.DesugaredAugmentedAssign ||
                 source?.elementType == KtNodeTypes.DESTRUCTURING_DECLARATION ||
                 initializerSource?.kind == KtFakeSourceElementKind.DesugaredForLoop
 

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeInAssignment.fir.kt

@@ -3,6 +3,15 @@ fun testAssignment() {
     <!UNREACHABLE_CODE!><!ASSIGNED_VALUE_IS_NEVER_READ!>a<!> =<!> todo()
 }
 
+class Foo {
+    var property: Int = 0
+}
+
+fun testClassPropertyAssignment(foo: Foo) {
+    foo.property = 1
+    foo<!UNREACHABLE_CODE!>.property =<!> todo()
+}
+
 fun testVariableDeclaration() {
     <!UNREACHABLE_CODE!>val <!UNUSED_VARIABLE!>a<!> =<!> todo()
 }
@@ -14,5 +23,10 @@ fun testPlusAssign() {
     a <!UNREACHABLE_CODE!>+=<!> todo()
 }
 
+fun testClassPropertyPlusAssign(foo: Foo) {
+    foo.property += 1
+    foo<!UNREACHABLE_CODE!>.property<!> += todo() as Int
+}
+
 
 fun todo(): Nothing = throw Exception()

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeInAssignment.kt

@@ -3,6 +3,15 @@ fun testAssignment() {
     <!UNREACHABLE_CODE!>a =<!> todo()
 }
 
+class Foo {
+    var property: Int = 0
+}
+
+fun testClassPropertyAssignment(foo: Foo) {
+    foo.property = 1
+    foo<!UNREACHABLE_CODE!>.property =<!> todo()
+}
+
 fun testVariableDeclaration() {
     <!UNREACHABLE_CODE!>val <!UNUSED_VARIABLE!>a<!> =<!> todo()
 }
@@ -14,5 +23,10 @@ fun testPlusAssign() {
     a <!UNREACHABLE_CODE!>+=<!> todo()
 }
 
+fun testClassPropertyPlusAssign(foo: Foo) {
+    foo.property += 1
+    foo.property <!UNREACHABLE_CODE!>+=<!> todo() as Int
+}
+
 
 fun todo(): Nothing = throw Exception()

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeInAssignment.txt

@@ -1,6 +1,17 @@
 package
 
 public fun testAssignment(): kotlin.Unit
+public fun testClassPropertyAssignment(/*0*/ foo: Foo): kotlin.Unit
+public fun testClassPropertyPlusAssign(/*0*/ foo: Foo): kotlin.Unit
 public fun testPlusAssign(): kotlin.Unit
 public fun testVariableDeclaration(): kotlin.Unit
 public fun todo(): kotlin.Nothing
+
+public final class Foo {
+    public constructor Foo()
+    public final var property: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+