Bump Explicit-layout value types with no fields to at minimum 1 byte …

…size. (dotnet#63975)

src/coreclr/vm/methodtablebuilder.cpp

@@ -8648,6 +8648,14 @@ MethodTableBuilder::HandleExplicitLayout(
                 BuildMethodTableThrowException(IDS_CLASSLOAD_GENERAL);
             }
         }
+
+        if (!numInstanceFieldBytes.IsOverflow() && numInstanceFieldBytes.Value() == 0)
+        {
+            // If we calculate a 0-byte size here, we should have also calculated a 0-byte size
+            // in the initial layout algorithm.
+            _ASSERTE(GetLayoutInfo()->IsZeroSized());
+            numInstanceFieldBytes = S_UINT32(1);
+        }
     }
 
     // The GC requires that all valuetypes containing orefs be sized to a multiple of TARGET_POINTER_SIZE.

src/tests/Loader/classloader/explicitlayout/Regressions/empty/explicitStruct_empty.cs

@@ -0,0 +1,60 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Xunit;
+
+[StructLayout(LayoutKind.Explicit)]
+public struct S
+{
+}
+
+[StructLayout(LayoutKind.Explicit)]
+public struct S2
+{
+}
+
+[StructLayout(LayoutKind.Explicit)]
+public class C
+{
+}
+
+[StructLayout(LayoutKind.Explicit)]
+public class C2
+{
+}
+
+public class Test_explicitStruct_empty
+{
+    // Mark as no-inlining so any test failures will show the right stack trace even after
+    // we consolidate test assemblies.
+    [Fact]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void EmptyExplicitStructCanBeLoadedAndCreated()
+    {
+        S s = new S();
+    }
+
+    [Fact]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void EmptyExplicitStructCanBeLoadedAndCreatedThroughReflection()
+    {
+        object s = Activator.CreateInstance(Type.GetType("S2"));
+    }
+
+    [Fact]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void EmptyExplicitClassCanBeLoadedAndCreated()
+    {
+        C c = new C();
+    }
+
+    [Fact]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void EmptyExplicitClassCanBeLoadedAndCreatedThroughReflection()
+    {
+        object c = Activator.CreateInstance(Type.GetType("C2"));
+    }
+}

src/tests/Loader/classloader/explicitlayout/Regressions/empty/explicitStruct_empty.csproj

@@ -0,0 +1,5 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <ItemGroup>
+    <Compile Include="explicitStruct_empty.cs" />
+  </ItemGroup>
+</Project>