Incorrect alignment of reference struct within a struct on Mono

[UnityAlex]

Description

When creating a struct that has another struct within and reading/writing the alignment used is incorrect. This leads to incorrect memory being accessed for values .

Reproduction Steps

Build & run the following

using System.Runtime.InteropServices;

public class Program
{
    [StructLayout(LayoutKind.Explicit, Size = 8)]
    public struct StructA
    {
        [FieldOffset(0)]
        public long Field0;
    }

    public struct StructB
    {
        public int Field0;
        // padded 4
        public StructA Field1;
        // no padding
        public int Field2;
    }
    
    public static unsafe void TestAlignmentOfExplicitStruct()
    {
        void* mem = stackalloc byte[24];
        Marshal.WriteInt32((IntPtr)mem, 0, 1);
        Marshal.WriteInt64((IntPtr)mem, 8, 2);
        Marshal.WriteInt32((IntPtr)mem, 16, 3);

        var s = Marshal.PtrToStructure<StructB>((IntPtr)mem);

        Console.WriteLine($"s.Field0 {s.Field0}");
        Console.WriteLine($"s.Field1.Field0 {s.Field1.Field0}");
        Console.WriteLine($"s.Field2 {s.Field2}");
    }
    
    public static void Main()
    {
        TestAlignmentOfExplicitStruct();
    }
}

Expected behavior

Output:

s.Field0 1
s.Field1.Field0 2
s.Field2 3

Actual behavior

Output

s.Field0 1
s.Field1.Field0 8589934592
s.Field2 0

Regression?

Not that I'm aware of.

Known Workarounds

Using explicit struct layout avoids the alignment issue.

Configuration

Version: 8.0.100-preview.3.23178.7
OS and (architecture): Reproduced on both OSX (arm64) and Windows (x64)
Mono only.

Other information

No response

[SamMonoRT]

cc @vargaz