Change core annotations retention:

    * SerialInfo, Transient, Required: binary -> runtime
    * Contextual: runtime -> binary

Fixes #1081

core/commonMain/src/kotlinx/serialization/Annotations.kt

@@ -114,22 +114,22 @@ public annotation class Serializer(
  * ```
  */
 @Target(AnnotationTarget.PROPERTY, AnnotationTarget.CLASS)
-@Retention(AnnotationRetention.BINARY)
+// @Retention(AnnotationRetention.RUNTIME) still runtime, but KT-41082
 public annotation class SerialName(val value: String)
 
 /**
  * Indicates that property must be present during deserialization process, despite having a default value.
  */
 @Target(AnnotationTarget.PROPERTY)
-@Retention(AnnotationRetention.BINARY)
+// @Retention(AnnotationRetention.RUNTIME) still runtime, but KT-41082
 public annotation class Required
 
 /**
  * Marks this property invisible for the whole serialization process, including [serial descriptors][SerialDescriptor].
  * Transient properties should have default values.
  */
 @Target(AnnotationTarget.PROPERTY)
-@Retention(AnnotationRetention.BINARY)
+// @Retention(AnnotationRetention.RUNTIME) still runtime, but KT-41082
 public annotation class Transient
 
 /**
@@ -167,6 +167,7 @@ public annotation class ContextualSerialization(vararg val forClasses: KClass<*>
  * @see UseContextualSerialization
  */
 @Target(AnnotationTarget.PROPERTY, AnnotationTarget.TYPE)
+@Retention(AnnotationRetention.BINARY)
 public annotation class Contextual
 
 /**
@@ -176,6 +177,7 @@ public annotation class Contextual
  * @see ContextSerializer
  */
 @Target(AnnotationTarget.FILE)
+@Retention(AnnotationRetention.BINARY)
 public annotation class UseContextualSerialization(vararg val forClasses: KClass<*>)
 
 /**

core/jvmTest/src/kotlinx/serialization/RetentionTest.kt

@@ -0,0 +1,18 @@
+package kotlinx.serialization
+
+import org.junit.Test
+import kotlin.reflect.full.*
+import kotlin.test.*
+
+class RetentionTest {
+
+    @Serializable
+    class F(@SerialName("?") val a: Int, @Transient val b: Int = 42, @Required val c: Int)
+
+    @Test
+    fun testRetention() {
+        assertEquals("?", F::a.findAnnotation<SerialName>()?.value)
+        assertNotNull(F::b.findAnnotation<Transient>())
+        assertNotNull(F::c.findAnnotation<Required>())
+    }
+}