Migrate ExtendedLayout test types from IL to C#

[Copilot]

Description

The C# compiler now supports the ExtendedLayoutAttribute. Migrated CStruct and CUnion type definitions from IL to C# where possible, keeping types that require both StructLayout and ExtendedLayout attributes in IL.

Changes

CStruct.cs

• Added 9 type definitions using [ExtendedLayout(ExtendedLayoutKind.CStruct)]
• Includes types with primitive fields, reference fields, nested types (with explicit [StructLayout(LayoutKind.Auto)] on NestedAutoLayoutType), empty structs, and byref-like structs

CUnion.cs

• Added 9 type definitions using [ExtendedLayout(ExtendedLayoutKind.CUnion)]
• Includes types with primitive fields, reference fields, nested unions, empty unions, and byref-like unions

ExtendedLayoutTypes.il

• Retained 9 types that must remain in IL:
  • 3 invalid type examples that cannot be expressed in valid C#:
    • NoExtendedAttributeOnType - tests missing attribute
    • InlineArrayOnExtendedLayout - tests invalid attribute combination
    • ExtendedLayoutInvalidKind - tests invalid enum value
  • 6 types requiring both ExtendedLayout and StructLayout/FieldOffset attributes:
    • CStructWithOffsets, CStructWithSize, CStructWithPack
    • CUnionWithOffsets, CUnionWithSize, CUnionWithPack

Example migration:

// Before (IL):
// .class flags(0x18) public value beforefieldinit CUnionMixedSizes
// {
//     .custom instance void [System.Runtime]System.Runtime.InteropServices.ExtendedLayoutAttribute::.ctor(...) = {
//         int32(0x00000001) // CUnion
//     }
//     .field public uint8 byteField
//     .field public int64 longField
// }

// After (C#):
[ExtendedLayout(ExtendedLayoutKind.CUnion)]
public struct CUnionMixedSizes
{
    public byte byteField;
    public short shortField;
    public int intField;
    public long longField;
}

All existing tests remain unchanged - they reference the same type names, now defined in C# where possible and in IL where dual attributes are required.

Original prompt

A C# compiler that supports the new ExtendedLayout attribute has recently been merged into this repo. Please update the ExtendedLayout test suite to use this attribute in C#. In particular, move all types defined in the CStruct section into CStruct.cs and define the types in C#. Do the same for the types in the CUnion section. Please leave the types in the "Invalid type examples" in IL.

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[jkoritzinsky]

@copilot address the feedback from my review

[Copilot]

@copilot address the feedback from my review

Addressed all review feedback in e97f014

[Copilot]

Pull request overview

This PR migrates ExtendedLayout test type definitions from IL to C# now that the C# compiler supports the ExtendedLayoutAttribute. The migration moves type definitions to C# where possible while retaining IL types that require features C# cannot express.

Changes:

• Migrated 9 CStruct types and 9 CUnion types from IL to C# using the ExtendedLayoutAttribute
• Retained 9 IL types that require dual attributes or test invalid type scenarios
• Added shared helper types (NestedSequentialType, NestedAutoLayoutType) in CStruct.cs for use across both test files

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ExtendedLayoutTypes.il	Removed migrated types; retained invalid test types and types requiring both ExtendedLayout and StructLayout/FieldOffset attributes
CStruct.cs	Added 9 CStruct type definitions and 2 shared nested helper types using ExtendedLayoutAttribute
CUnion.cs	Added 9 CUnion type definitions using ExtendedLayoutAttribute