Enable Unused Return Value checker

Author: sandwwraith

benchmark/src/jmh/kotlin/kotlinx/benchmarks/json/ImplicitNullsBenchmark.kt

@@ -82,37 +82,30 @@ open class ImplicitNullsBenchmark {
     private val serializer = Values.serializer()
 
     @Benchmark
-    fun decodeNoNulls() {
+    fun decodeNoNulls() =
         Json.decodeFromString(serializer, jsonNoNulls)
-    }
 
     @Benchmark
-    fun decodeNoNullsImplicit() {
+    fun decodeNoNullsImplicit() =
         jsonImplicitNulls.decodeFromString(serializer, jsonNoNulls)
-    }
 
     @Benchmark
-    fun decodeNulls() {
+    fun decodeNulls() =
         Json.decodeFromString(serializer, jsonWithNulls)
-    }
 
     @Benchmark
-    fun decodeNullsImplicit() {
+    fun decodeNullsImplicit() =
         jsonImplicitNulls.decodeFromString(serializer, jsonWithNulls)
-    }
 
     @Benchmark
-    fun decodeAbsenceImplicit() {
+    fun decodeAbsenceImplicit() =
         jsonImplicitNulls.decodeFromString(serializer, jsonWithAbsence)
-    }
 
     @Benchmark
-    fun encodeNulls() {
+    fun encodeNulls() =
         Json.encodeToString(serializer, valueWithNulls)
-    }
 
     @Benchmark
-    fun encodeNullsImplicit() {
+    fun encodeNullsImplicit() =
         jsonImplicitNulls.encodeToString(serializer, valueWithNulls)
-    }
 }

buildSrc/src/main/kotlin/global-compiler-options.gradle.kts

@@ -15,6 +15,7 @@ val globalCompilerArgs
         "-P", "plugin:org.jetbrains.kotlinx.serialization:disableIntrinsic=false",
         "-Xreport-all-warnings",
         "-Xrender-internal-diagnostic-names",
+        "-Xreturn-value-checker=full",
     )
 
 val kotlin_Werror_override: String? by project

buildSrc/src/main/kotlin/source-sets-conventions.gradle.kts

@@ -76,6 +76,8 @@ kotlin {
             freeCompilerArgs.add("-Xsuppress-version-warnings")
         }
         freeCompilerArgs.add("-Xexpect-actual-classes")
+        // for some reason, IDE does not enable feature in test source sets without this line:
+        freeCompilerArgs.add("-Xreturn-value-checker=full")
     }
 
     sourceSets {

core/commonMain/src/kotlinx/serialization/internal/Tagged.kt

@@ -158,6 +158,7 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
         tagStack.add(name)
     }
 
+    @IgnorableReturnValue
     protected fun popTag(): Tag =
         if (tagStack.isNotEmpty())
             tagStack.removeAt(tagStack.lastIndex)
@@ -316,6 +317,7 @@ public abstract class TaggedDecoder<Tag : Any?> : Decoder, CompositeDecoder {
 
     private var flag = false
 
+    @IgnorableReturnValue
     protected fun popTag(): Tag {
         val r = tagStack.removeAt(tagStack.lastIndex)
         flag = true

core/commonTest/src/kotlinx/serialization/SerializersLookupEnumTest.kt

@@ -10,8 +10,7 @@ import kotlinx.serialization.internal.EnumSerializer
 import kotlinx.serialization.test.*
 import kotlin.test.*
 
-// This is unimplemented functionality that should be
-@Suppress("RemoveExplicitTypeArguments") // This is exactly what's being tested
+@Suppress("RETURN_VALUE_NOT_USED") // assertIs, KT-82363
 class SerializersLookupEnumTest {
     @Serializable(with = EnumExternalObjectSerializer::class)
     enum class EnumExternalObject

core/commonTest/src/kotlinx/serialization/features/SealedInterfacesSerializationTest.kt

@@ -68,9 +68,9 @@ class SealedInterfacesSerializationTest {
         assertEquals(listOf("ImplA", "ImplB"), subclasses)
     }
 
-    private fun SerialDescriptor.isDummy() = serialName == "Dummy"
+    private fun SerialDescriptor.isDummy() = assertEquals(serialName, "Dummy")
 
-    private fun SerialDescriptor.isPolymorphic() = kind == PolymorphicKind.OPEN
+    private fun SerialDescriptor.isPolymorphic() = assertEquals(kind, PolymorphicKind.OPEN)
 
     operator fun SerialDescriptor.get(i: Int) = getElementDescriptor(i)
 

core/commonTest/src/kotlinx/serialization/internal/ObjectSerializerTest.kt

@@ -10,7 +10,7 @@ import kotlinx.serialization.*
 class ObjectSerializerTest {
     @Test
     fun testSequentialDecoding() {
-        SimpleObject.serializer().deserialize(DummySequentialDecoder())
+        assertSame(SimpleObject, SimpleObject.serializer().deserialize(DummySequentialDecoder()))
     }
 
     @Serializable

core/commonTest/src/kotlinx/serialization/internal/TuplesTest.kt

@@ -12,7 +12,7 @@ class TuplesTest {
     fun testSequentialDecodingKeyValue() {
         val decoder = DummySequentialDecoder()
         val serializer = MapEntrySerializer(Unit.serializer(), Unit.serializer())
-        serializer.deserialize(decoder)
+        assertNotNull(serializer.deserialize(decoder))
         assertEquals(decoder.beginStructureCalled, decoder.endStructureCalled)
     }
 }

core/commonTest/src/kotlinx/serialization/test/TestHelpers.kt

@@ -38,7 +38,7 @@ inline fun jvmOnly(test: () -> Unit) {
     if (isJvm()) test()
 }
 
-inline fun <reified T : Throwable> assertFailsWithMessage(message: String, block: () -> Unit) {
+inline fun <reified T : Throwable> assertFailsWithMessage(message: String, block: () -> Any?) {
     val exception = assertFailsWith(T::class, null, block)
     assertTrue(exception.message!!.contains(message), "Expected message '${exception.message}' to contain substring '$message'")
 }

core/jvmMain/src/kotlinx/serialization/internal/Caching.kt

@@ -18,7 +18,7 @@ import kotlin.reflect.KTypeProjection
  * and fallback to ConcurrentHashMap-based cache.
  */
 private val useClassValue = try {
-    Class.forName("java.lang.ClassValue")
+    val _ = Class.forName("java.lang.ClassValue")
     true
 } catch (_: Throwable) {
     false