TypeLoadException when a ref struct using explicit layout has fields of type ref struct

[lithiumtoast]

Description

Context: Found in .NET 9 when experimenting with ref struct in P/Invoke scenarios. I want to make it clear that I'm not using "ref fields" but rather only "ref structs"; the fields of the struct are not ref (pointer) but the type is of ref struct (stack-only).

Reproduction Steps

using System;
using System.Runtime.InteropServices;
					
public class Program
{
    [StructLayout(LayoutKind.Explicit, Size = 8, Pack = 8)]
    public ref struct Foo
    {
        [FieldOffset(0)]
        public Bar A;
        [FieldOffset(4)]
        public Bar B;
    }

    public ref struct Bar
    {
        public uint Value;
    }
	
    public static void Main()
    {
	var e = default(Foo);
	e.A.Value = 1;
	e.B.Value = 2;
	Console.WriteLine("1 + 2 = " + (e.A.Value + e.B.Value));
    }
}

Expected behavior

A ref struct of explicit layout can have fields that are positioned using FieldOffset attribute where the type of the field is a ref struct without creating a runtime exception.

Actual behavior

When a ref struct is using explicit struct layout, a System.TypeLoadException can occur regarding the field offset of the struct's field which the type is another ref struct.

Unhandled exception. System.TypeLoadException: Could not load type 'Foo' from assembly 'r2oeekw4.exe, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' because it contains an object field at offset 4 that is incorrectly aligned or overlapped by a non-object field.
   at Program.Main()

Regression?

No response

Known Workarounds

• Removing the ref from the nested struct.

Configuration

• .NET 9
• macOS Ventura 13.6.5
• Apple M1 (osx-arm64)
• Not specific to any configuration.

Other information

Hypothesis: The word ref in the context of a struct (stack-only) is confused to be in the context of a field (pointer); the field is mistaken for an object type with rules about field alignment when it's actually a struct.

[jkotas]

The offending imprecise check was introduced in dotnet/coreclr#25200 .