namespace System.Runtime.InteropServices
{
    public enum LayoutKind
    {
        Custom
    }

    [AttributeUsage(AttributeTargets.Struct)]
    public sealed class CustomLayoutAttribute : Attribute
    {
        public CustomLayoutAttribute(CustomLayoutKind kind) {}
        
        // Only valid for CustomLayoutKind.SwiftEnum
        public int RequiredDiscriminatorBits { get; set; }
    }

    public enum CustomLayoutKind
    {
        Sequential, // C-style struct
        Union, // C-style union
        SwiftStruct, // Swift struct
        SwiftEnum // Swift enumeration
    }
}

namespace System.Runtime.InteropServices.Swift
{
    // Represents a pointer to a Swift object (for the purposes of calculating spare bits)
    public readonly unsafe struct SwiftObject
    {
        // Would be implemented to mask out the spare bits
        public void* Value { get; }
    }

    // Represents a bool value in Swift (for the purposes of calculating spare bits)
    public readonly unsafe struct SwiftBool
    {
        // Would be implemented to mask out the spare bits
        // or assign while preserving spare bits
        public bool Value { get; }
    }
}

[StructLayout(LayoutKind.Custom)]
[CustomLayout(CustomLayoutKind.SwiftStruct)]
public struct InnerStruct
{
     public short F0; // offset 0
     public sbyte F1; // offset 2
}

[StructLayout(LayoutKind.Custom)]
[CustomLayout(CustomLayoutKind.SwiftStruct)]
public struct OuterStruct
{
    public ulong F0; // offset 0
    public long F1; // offset 8
    public InnerStruct F2; // offset 16
    public sbyte F3; // offset 19
}

[StructLayout(LayoutKind.Custom)]
[CustomLayout(CustomLayoutKind.SwiftEnum, RequiredDescriminatorBits = 1)]
public struct MyOptional<T> where T : unmanaged
{
    [StructLayout(LayoutKind.Custom)]
    [CustomLayout(CustomLayoutKind.SwiftStruct)]
    private struct Some_Payload
    {
          public T Value;
    }

    private Some_Payload Some;

    // An API surface to describe the different cases and provide a C# API around accessing them, out of the scope of this API proposal
}


[StructLayout(LayoutKind.Custom)]
[CustomLayout(CustomLayoutKind.SwiftEnum, RequiredDescriminatorBits = 1)]
public struct ParsedResult
{
     [StructLayout(LayoutKind.Custom)]
     [CustomLayout(CustomLayoutKind.SwiftStruct)]
     private struct ParsedObject_Payload
     {
            public SwiftPointer payload_0;
     }
     [StructLayout(LayoutKind.Custom)]
     [CustomLayout(CustomLayoutKind.SwiftStruct)]
     private struct ParsedBool_Payload
     {
            public SwiftBool payload_0;
     }

     private ParsedObject_Payload ParsedObject;
     private ParsedBool_Payload SwiftBool;
    // An API surface to describe the different cases and provide a C# API around accessing them, out of the scope of this API proposal
}