Ok so it looks like StructLayoutAttribute melts away and converts to metadata

source/NSubstitute.Elevated/Weaver/MockInjector.cs

@@ -97,34 +97,27 @@ namespace NSubstitute.Elevated.Weaver
                 return;
             if (type.Name == "<Module>")
                 return;
-            if (type.CustomAttributes.Any(a =>
-                    a.AttributeType.FullName == typeof(CompilerGeneratedAttribute).FullName ||
-                    a.AttributeType.FullName == typeof(StructLayoutAttribute).FullName))
+            if (type.IsExplicitLayout)
+                return;
+            if (type.CustomAttributes.Any(a => a.AttributeType.FullName == typeof(CompilerGeneratedAttribute).FullName))
                 return;
 
             try
             {
-                var patched = false;
                 foreach (var method in type.Methods)
+                    Patch(method);
+
+                void AddField(string fieldName, FieldAttributes fieldAttributes)
                 {
-                    if (Patch(method))
-                        patched = true;
+                    type.Fields.Add(new FieldDefinition(fieldName,
+                            FieldAttributes.Private | FieldAttributes.NotSerialized | fieldAttributes,
+                            type.Module.TypeSystem.Object));
                 }
 
-                if (patched)
-                {
-                    void AddField(string fieldName, FieldAttributes fieldAttributes)
-                    {
-                        type.Fields.Add(new FieldDefinition(fieldName,
-                                FieldAttributes.Private | FieldAttributes.NotSerialized | fieldAttributes,
-                                type.Module.TypeSystem.Object));
-                    }
+                AddField(InjectedMockStaticDataName, FieldAttributes.Static);
+                AddField(InjectedMockDataName, 0);
 
-                    AddField(InjectedMockStaticDataName, FieldAttributes.Static);
-                    AddField(InjectedMockDataName, 0);
-
-                    AddMockCtor(type);
-                }
+                AddMockCtor(type);
             }
             catch (Exception e)
             {
@@ -179,14 +172,12 @@ namespace NSubstitute.Elevated.Weaver
             type.Methods.Add(ctor);
         }
 
-        bool Patch(MethodDefinition method)
+        void Patch(MethodDefinition method)
         {
             if (method.IsCompilerControlled || method.IsConstructor || method.IsAbstract)
-                return false;
+                return;
 
             // $$$ DOWIT
-
-            return true;
         }
     }
 }

tests/NSubstitute.Elevated.Tests/ElevatedWeaverTests.cs

@@ -6,7 +6,6 @@ using Shouldly;
 
 namespace NSubstitute.Elevated.Tests
 {
-
     [TestFixture]
     public class ElevatedWeaverTests
     {
@@ -32,7 +31,7 @@ namespace NSubstitute.Elevated.Tests
 
                 class ClassWithGeneratedNestedType
                 {
-	                public IEnumerable<int> Foo()           // this causes a state machine type to be generated which shouldn't be patched
+                    public IEnumerable<int> Foo()           // this causes a state machine type to be generated which shouldn't be patched
                         { yield return 1; }
                 }
 
@@ -78,9 +77,8 @@ namespace NSubstitute.Elevated.Tests
         [Test]
         public void PrivateNestedTypes_ShouldNotPatch()
         {
-            var type = m_FixtureTestAssembly.GetType("ShouldNotPatch.ClassWithPrivateNestedType");
-            var nestedType = type.NestedTypes.Single(t => t.Name == "PrivateNested");
-            MockInjector.IsPatched(nestedType).ShouldBeFalse();
+            var type = m_FixtureTestAssembly.GetType("ShouldNotPatch.ClassWithPrivateNestedType/PrivateNested");
+            MockInjector.IsPatched(type).ShouldBeFalse();
         }
 
         [Test]
@@ -101,17 +99,15 @@ namespace NSubstitute.Elevated.Tests
         [Test]
         public void PublicNestedClasses_ShouldPatch()
         {
-            var type = m_FixtureTestAssembly.GetType("ShouldPatch.ClassWithNestedTypes");
-            var nestedType = type.NestedTypes.Single(t => t.Name == "PublicNested");
-            MockInjector.IsPatched(nestedType).ShouldBeTrue();
+            var type = m_FixtureTestAssembly.GetType("ShouldPatch.ClassWithNestedTypes/PublicNested");
+            MockInjector.IsPatched(type).ShouldBeTrue();
         }
 
         [Test]
         public void InternalNestedClasses_ShouldPatch()
         {
-            var type = m_FixtureTestAssembly.GetType("ShouldPatch.ClassWithNestedTypes");
-            var nestedType = type.NestedTypes.Single(t => t.Name == "InternalNested");
-            MockInjector.IsPatched(nestedType).ShouldBeTrue();
+            var type = m_FixtureTestAssembly.GetType("ShouldPatch.ClassWithNestedTypes/InternalNested");
+            MockInjector.IsPatched(type).ShouldBeTrue();
         }
     }
 }