Implement support for ExtendedLayoutKind.CUnion

[jkoritzinsky]

Implement support for the CUnion layout kind mentioned in #100896.

This layout kind places all fields at offset 0, like a C union type. It only supports unmanaged fields with non-auto layout, like CStruct.

Contributes to #100896

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[Copilot]

Pull request overview

This pull request implements support for the CUnion extended layout kind, which allows defining value types with C-style union semantics where all fields are placed at offset 0 and the type's size is determined by the largest field.

Key changes:

• Adds CUnion = 1 to the ExtendedLayoutKind enum across all implementations (CoreCLR, Mono, and TypeSystem)
• Implements field layout logic that places all fields at offset 0 and calculates size as the maximum field size with proper alignment
• Adds comprehensive validation including checks for GC pointers, auto-layout fields, empty unions, and inline arrays
• Includes extensive test coverage with IL type definitions and C# tests

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/ExtendedLayoutKind.cs	Adds CUnion enum value with documentation
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds CUnion enum value to reference assembly
src/coreclr/inc/corhdr.h	Adds CUnion to CorExtendedLayoutKind enum
src/coreclr/vm/class.h	Adds CUnion layout type and InitializeCUnionFieldLayout method
src/coreclr/vm/classlayoutinfo.cpp	Implements CUnion field layout initialization logic
src/coreclr/vm/methodtablebuilder.h	Declares HandleCUnionLayout method
src/coreclr/vm/methodtablebuilder.cpp	Implements CUnion layout handling with validation
src/coreclr/tools/Common/TypeSystem/Common/MetadataType.cs	Adds CUnion to MetadataLayoutKind enum
src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs	Implements CUnion field layout computation
src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs	Handles CUnion kind value parsing from metadata
src/coreclr/tools/Common/TypeSystem/Interop/IL/MarshalUtils.cs	Treats CUnion as blittable like CStruct
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/CompilerMetadataFieldLayoutAlgorithm.cs	Adds CUnion case to layout computation
src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/ReadyToRunMetadataFieldLayoutAlgorithm.cs	Adds CUnion case to layout computation
src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/TestMetadataFieldLayoutAlgorithm.cs	Adds CUnion case to test layout algorithm
src/mono/mono/metadata/class-init.c	Implements CUnion layout logic in Mono runtime
src/tests/Loader/classloader/ExtendedLayout/ExtendedLayoutTypes.il	Defines CUnion test types in IL
src/tests/Loader/classloader/ExtendedLayout/CUnion.cs	Adds comprehensive test cases for CUnion functionality
src/tests/Loader/classloader/ExtendedLayout/ExtendedLayout.csproj	References new CUnion test file

[jkotas]

Are there any places under libraries that can take advantage of this?

[jkoritzinsky]

I was going to open a follow-up issue for us to start using this across the libraries code once we are using an SDK with a new enough Roslyn to support ExtendedLayout.

In particular, ComVariant immediately comes to mind. With a cursory search, it looks like there's a few interop types (like IO_STATUS, CMSG_KEY_AGREE_RECIPIENT_INFO_ORIGINATOR_UNION) that pop as well as the implementation types of [JSImport]/[JSExport].

I'm sure that there's more (most if not all usages of LayoutKind.Explicit in the libraries tree can probably be moved, as well as a number in CoreLib).

[jkotas]

I was going to open a follow-up issue for us to start using this across the libraries code once we are using an SDK with a new enough Roslyn to support ExtendedLayout.

Ah ok, I have not realized that we do not have Roslyn with the supporting changes yet.