Add stable hashCode()/equals() calculation to PrimitiveSerialDescriptor (#2136)

Fixes #2135

Author: vgv

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

@@ -58,6 +58,13 @@ internal class PrimitiveSerialDescriptor(
     override fun getElementDescriptor(index: Int): SerialDescriptor = error()
     override fun getElementAnnotations(index: Int): List<Annotation> = error()
     override fun toString(): String = "PrimitiveDescriptor($serialName)"
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is PrimitiveSerialDescriptor) return false
+        if (serialName == other.serialName && kind == other.kind) return true
+        return false
+    }
+    override fun hashCode() = serialName.hashCode() + 31 * kind.hashCode()
     private fun error(): Nothing = throw IllegalStateException("Primitive descriptor does not have elements")
 }
 

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

@@ -0,0 +1,29 @@
+package kotlinx.serialization
+
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.internal.PrimitiveSerialDescriptor
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotSame
+
+class PrimitiveSerialDescriptorTest {
+
+    @Test
+    fun testEqualsImplemented() {
+        val first = PrimitiveSerialDescriptor("test_name", PrimitiveKind.LONG)
+        val second = PrimitiveSerialDescriptor("test_name", PrimitiveKind.LONG)
+
+        assertNotSame(first, second)
+        assertEquals(first, second)
+    }
+
+    @Test
+    fun testHashCodeStability() {
+        val first = PrimitiveSerialDescriptor("test_name", PrimitiveKind.LONG)
+        val second = PrimitiveSerialDescriptor("test_name", PrimitiveKind.LONG)
+
+        assertNotSame(first, second)
+        assertEquals(first.hashCode(), second.hashCode())
+    }
+
+}