Value classes not using encodeInlineElement and inefficient boxing of primitive values

[Chuckame]

Describe the bug
Given a serializable data class containing 2 value class fields (one having a primitive type like int, the other with a structure), both fields are serialized using encodeSerializableElement instead of encodeInlineElement, forcing primitive values to be boxed, and also doesn't follow the expected as said in the docs:

    /**
     * [...]
     * 
     * Namely, for the `@Serializable @JvmInline value class MyInt(val my: Int)`,
     * and `@Serializable class MyData(val myInt: MyInt)` the following sequence is used:
     * ```
     * thisEncoder.encodeInlineElement(MyData.serializer.descriptor, 0).encodeInt(my)
     * ```
     *
     * This method provides an opportunity for the optimization to avoid boxing of a carried value
     * and its invocation should be equivalent to the following:
     * ```
     * thisEncoder.encodeSerializableElement(MyData.serializer.descriptor, 0, MyInt.serializer(), myInt)
     * ```
     *
     * [...]
     *
     * Note that this function returns [Encoder] instead of the [CompositeEncoder]
     * because value classes always have the single property.
     * 
     * [...]
     */
    public fun encodeInlineElement(
        descriptor: SerialDescriptor,
        index: Int
    ): Encoder

To Reproduce

class ReproduceBug {
    @Test
    fun structureContainingValueClassFieldsShouldCall_encodeInlineElement() {
        val value = Structure(
            BooleanValue(true),
            BooleanValue(false),
            SubStructureValue(SubStructure(42)),
        )
        val encoder = MyEncoder()

        encoder.encodeSerializableValue(Structure.serializer(), value)

        assertEquals(actual = encoder.encodeInlineElementCalled, expected = true)
    }

    @Serializable
    data class Structure(
        val primitiveField: BooleanValue,
        val primitiveNullableField: BooleanValue?,
        val field: SubStructureValue,
    )

    @Serializable
    @JvmInline
    value class BooleanValue(val value: Boolean)

    @Serializable
    @JvmInline
    value class SubStructureValue(val value: SubStructure)

    @Serializable
    data class SubStructure(
        val subField: Int,
    )

    class MyEncoder: AbstractEncoder() {
        var encodeInlineElementCalled = false

        override val serializersModule: SerializersModule
            get() = EmptySerializersModule()

        // I removed the `final` modifier in the `AbstractEncoder` to allow overriding it for reproducing the bug and avoid the useless noise of all the other overrides
        override fun encodeInlineElement(descriptor: SerialDescriptor, index: Int): Encoder {
            encodeInlineElementCalled = true
            return super.encodeInlineElement(descriptor, index)
        }

        override fun encodeValue(value: Any) {
        }
    }
}

And here is the generated serializer (only the interesting part) retrieved using Kotlin bytecode IDEA tooling:

output.encodeSerializableElement(serialDesc, 0, (SerializationStrategy)ReproduceBug.BooleanValue.$serializer.INSTANCE, ReproduceBug.BooleanValue.box-impl(self.primitiveField));
output.encodeNullableSerializableElement(serialDesc, 1, (SerializationStrategy)ReproduceBug.BooleanValue.$serializer.INSTANCE, self.primitiveNullableField);
output.encodeSerializableElement(serialDesc, 2, (SerializationStrategy)ReproduceBug.SubStructureValue.$serializer.INSTANCE, ReproduceBug.SubStructureValue.box-impl(self.field));

Expected behavior
Should call encodeInlineElement and generate the following (not sure for the nullable value class' field):

output.encodeInlineElement(serialDesc, 0, (SerializationStrategy)ReproduceBug.BooleanValue.$serializer.INSTANCE).encodeBoolean(self.primitiveField);
output.encodeNullableSerializableElement(serialDesc, 1, (SerializationStrategy)ReproduceBug.BooleanValue.$serializer.INSTANCE, self.primitiveNullableField);
output.encodeInlineElement(serialDesc, 2, (SerializationStrategy)ReproduceBug.SubStructureValue.$serializer.INSTANCE).encodeSerializableValue((SerializationStrategy)ReproduceBug.SubStructure.$serializer.INSTANCE, self.field);

Environment

• Kotlin version: 1.9.22 & 2.0.0
• Library version: 1.7.0 & dev (commit 6886e345e06a48c4b0f9014bf85047a1e2f96a29)
• Kotlin platforms: jvm (and surely the other platforms, but not tested)
• Gradle version: 8.7