Add support for "extended" layouts in the runtimes and add CStruct layout

[jkoritzinsky]

Add support for the ExtendedLayout flag, the System.Runtime.InteropServices.ExtendedLayoutAttribute and the CStruct layout from #100896.

Contributes to #100896

Leaving other layouts from #100896 for later PRs to minimize diffs.

[Copilot]

Pull Request Overview

This PR adds support for an “extended” layout flag in the runtimes, introduces the ExtendedLayoutAttribute and CStruct layout kind, and wires through layout info across types and metadata emitters.

• Add IsExtendedLayout and GetExtendedLayoutInfo APIs to core metadata types
• Implement ComputeCStructFieldLayout and dispatch in field layout algorithms
• Update metadata headers, ILASM grammar, and reflection emit to recognize extended layout

Reviewed Changes

Copilot reviewed 45 out of 45 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/CompilerMetadataFieldLayoutAlgorithm.cs	Dispatch ComputeExtendedFieldLayout before explicit layout
src/coreclr/tools/Common/TypeSystem/Interop/IL/PInvokeDelegateWrapper.cs	Stub out IsExtendedLayout & GetExtendedLayoutInfo
src/coreclr/tools/Common/TypeSystem/Interop/IL/NativeStructType.cs	Override extended layout members and simplify GetClassLayout
src/coreclr/tools/Common/TypeSystem/Interop/IL/MarshalUtils.cs	Treat CStruct as blittable
src/coreclr/tools/Common/TypeSystem/Interop/IL/InlineArrayType.cs	Stub out IsExtendedLayout & GetExtendedLayoutInfo
src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs	Parse ExtendedLayoutAttribute and implement IsExtendedLayout
src/coreclr/tools/Common/TypeSystem/Common/TypeWithRepeatedFields.cs	Forward extended layout members
src/coreclr/tools/Common/TypeSystem/Common/TypeSystemHelpers.cs	Include IsExtendedLayout in HasLayout
src/coreclr/tools/Common/TypeSystem/Common/MetadataType.cs	Add abstract IsExtendedLayout, GetExtendedLayoutInfo, and supporting types
src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs	Implement ComputeCStructFieldLayout and ComputeExtendedFieldLayout
src/coreclr/tools/Common/TypeSystem/Common/InstantiatedType.Metadata.cs	Override extended layout members
src/coreclr/tools/Common/TypeSystem/Canon/CanonTypes.Metadata.cs	Stub out extended layout members
src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/InteropServices/LayoutKind.cs	Add LayoutKind.Extended enum
src/coreclr/md/compiler/custattr_emit.cpp	Handle tdExtendedLayout in custom attr emitter
src/coreclr/inc/il_kywd.h	Add IL keyword extended
src/coreclr/inc/corhdr.h	Define tdExtendedLayout and IsTdExtendedLayout
src/coreclr/ilasm/prebuilt/asmparse.grammar	Recognize extended in grammar
src/coreclr/ilasm/asmparse.y	Add EXTENDED_ token and grammar rules
src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeCustomAttributeData.cs	Map ExtendedLayout to LayoutKind.Extended
src/coreclr/System.Private.CoreLib/src/System/Reflection/Emit/RuntimeTypeBuilder.cs	Remove obsolete layout-mask validation

Comments suppressed due to low confidence (1)

src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs:502

• There are no unit tests covering the new CStruct layout path (ComputeCStructFieldLayout and ComputeExtendedFieldLayout). Adding tests for valid field offsets, alignment boundaries, and error conditions will help catch regressions.

private static ComputedInstanceFieldLayout ComputeCStructFieldLayout(MetadataType type, int numInstanceFields)

[jkotas]

What else needs to be updated?

• Ecma-335-Augments.md?
• https://github.com/jbevain/cecil/?

[jkoritzinsky]

I'll update the Ecma augments doc. It looks like we don't actually need to do any updates on Cecil to ensure correct behavior. Cecil reads layout info directly from the attributes bits, handles masked values right, and doesn't do any validation of valid or invalid values.

Cecil could use nice APIs to read if a type is ExtendedLayout, but it isn't required. Based on my tests, ILSpy will need to react to accurately represent the bits in their decompilation, but that's expected.

[jkotas]

This looks good to me otherwise.

Is your Roslyn PR on track for .NET 10? We should make sure that either both this PR and Roslyn PR are in .NET 10, or neither.

[jkoritzinsky]

I'm waiting on @jaredpar to let me know if Roslyn will have the resources to review the PR over there for this release.