Add ExtendedLayoutAttribute support for F# compiler

[Copilot]

ExtendedLayoutAttribute Support Implementation - COMPLETE ✅

Phase 1: IL Layer Changes ✅

• Add Extended case to ILTypeDefLayout type in il.fsi and il.fs
• Update convertLayout function in il.fs to handle Extended → TypeAttributes 0x18
• Update typeLayoutOfFlags in ilread.fs to read 0x18 flags
• Update GenTypeDefPass3 in ilwrite.fs to handle Extended layout
• Update typeAttributesOfTypeLayout in ilreflect.fs for Extended
• Update splitTypeLayout in ilprint.fs for Extended

Phase 2: Compiler Infrastructure ✅

• Add attrib_ExtendedLayoutAttribute_opt to TcGlobals.fs and TcGlobals.fsi
• Add error messages to FSComp.txt (errors 3879-3882) for new validations

Phase 3: Type Checking & Validation ✅

• Add attribute detection in CheckDeclarations.fs
• Add validation: ExtendedLayout only on structs (error 3881)
• Add validation: ExtendedLayout + StructLayout conflict check (error 3879)
• Add validation: Runtime support check (error 3880)
• Add validation: Reject LayoutKind.Extended (value 1) via StructLayoutAttribute (error 3882)
• Extend FieldOffset validation for ExtendedLayout types

Phase 4: IL Generation ✅

• Update IlxGen.fs to detect ExtendedLayoutAttribute and set ILTypeDefLayout.Extended
• ExtendedLayoutAttribute automatically preserved (not filtered like StructLayout)
• Build and core tests passed successfully
• Formatting check passed

Phase 5: Testing ✅

• Create type checking tests (ExtendedLayout on struct, class, interface, delegate)
• Create conflict tests (ExtendedLayout + StructLayout)
• Create FieldOffset validation test
• Create LayoutKind.Extended rejection test
• Create IL verification test with 'extended' layout keyword
• Fix tests with mock attribute (runtime doesn't have ExtendedLayoutAttribute yet)
• All test files created and integrated into Basic.fs

Quality Assurance ✅

• Code review completed - all feedback addressed
• CodeQL security check passed - no vulnerabilities detected
• Formatting verified
• Build successful
• Added runtime issue reference for extendedLayoutKind value clarification

Implementation Complete

All tasks have been successfully completed. The F# compiler now supports ExtendedLayoutAttribute with:

• Proper IL generation (TypeAttributes 0x18)
• Comprehensive validation
• Full test coverage
• Security review passed
• Code comments clarified per review feedback

Original prompt

ExtendedLayoutAttribute Support for F# Compiler

Summary

Add support for System.Runtime.InteropServices.ExtendedLayoutAttribute, a new .NET runtime feature that allows specifying extended type layouts (like C structs/unions) for interop scenarios. When a user applies [<ExtendedLayout(ExtendedLayoutKind.CStruct)>] to a struct, the compiler must:

1. Recognize the attribute and set the correct TypeAttributes.ExtendedLayout bit (0x18) in metadata
2. Validate attribute usage (no combining with StructLayoutAttribute, no FieldOffset on fields, runtime support check)
3. Keep the attribute on the type (it's a real user-written attribute, not a pseudo-attribute)

The compiler intentionally does not validate the specific ExtendedLayoutKind values or field types - those semantics are owned by the runtime and may evolve.

Reference: #19190

---

Architecture Overview

The layout information flows through the system as follows:

User writes [<ExtendedLayout(...)>] on struct
        ↓
CheckDeclarations.fs: Detect attribute, validate, keep on tycon.Attribs
        ↓
IlxGen.fs: Read attributes, determine ILTypeDefLayout.Extended
        ↓
il.fs (convertLayout): ILTypeDefLayout.Extended → TypeAttributes 0x18
        ↓
ILTypeDef created with: attributes containing 0x18, layout = Extended
        ↓
ilwrite.fs: Write TypeAttributes (with 0x18) to TypeDef table Flags column

---

Implementation Tasks

Task 1: Add Attribute Reference to TcGlobals

Files:

• src/Compiler/TypedTree/TcGlobals.fs
• src/Compiler/TypedTree/TcGlobals.fsi

Location in TcGlobals.fs: Near line 1489, where attrib_StructLayoutAttribute is defined.

Add:

member val attrib_ExtendedLayoutAttribute_opt = tryFindSysAttrib "System.Runtime.InteropServices.ExtendedLayoutAttribute"

Location in TcGlobals.fsi: Near line 477, where attrib_StructLayoutAttribute is declared.

Add:

member attrib_ExtendedLayoutAttribute_opt: BuiltinAttribInfo option

---

Task 2: Add ILTypeDefLayout.Extended Case

Files:

• src/Compiler/AbstractIL/il.fs
• src/Compiler/AbstractIL/il.fsi

Location in il.fsi: Around line 1456, the ILTypeDefLayout type definition.

Change from:

type ILTypeDefLayout =
    | Auto
    | Sequential of ILTypeDefLayoutInfo
    | Explicit of ILTypeDefLayoutInfo

To:

type ILTypeDefLayout =
    | Auto
    | Sequential of ILTypeDefLayoutInfo
    | Explicit of ILTypeDefLayoutInfo
    | Extended

Location in il.fs: Around line 2517, the ILTypeDefLayout type definition. Same change.

Location in il.fs: Around line 2630, the convertLayout function.

Change from:

let convertLayout layout =
    match layout with
    | ILTypeDefLayout.Auto -> TypeAttributes.AutoLayout
    | ILTypeDefLayout.Sequential _ -> TypeAttributes.SequentialLayout
    | ILTypeDefLayout.Explicit _ -> TypeAttributes.ExplicitLayout

To:

let convertLayout layout =
    match layout with
    | ILTypeDefLayout.Auto -> TypeAttributes.AutoLayout
    | ILTypeDefLayout.Sequential _ -> TypeAttributes.SequentialLayout
    | ILTypeDefLayout.Explicit _ -> TypeAttributes.ExplicitLayout
    | ILTypeDefLayout.Extended -> enum<TypeAttributes>(0x18)

---

Task 3: Update IL Reading

File: src/Compiler/AbstractIL/ilread.fs

Location: Around line 2040, the typeLayoutOfFlags function.

Change from:

and typeLayoutOfFlags (ctxt: ILMetadataReader) mdv flags tidx =
    let f = (flags &&& 0x00000018)
    if f = 0x00000008 then
        ILTypeDefLayout.Sequential(seekReadClassLayout ctxt mdv tidx)
    elif f = 0x00000010 then
        ILTypeDefLayout.Explicit(seekReadClassLayout ctxt mdv tidx)
    else
        ILTypeDefLayout.Auto

To:

and typeLayoutOfFlags (ctxt: ILMetadataReader) mdv flags tidx =
    let f = (flags &&& 0x00000018)
    if f = 0x00000008 then
        ILTypeDefLayout.Sequential(seekReadClassLayout ctxt mdv tidx)
    elif f = 0x00000010 then
        ILTypeDefLayout.Explicit(seekReadClassLayout ctxt mdv tidx)
    elif f = 0x00000018 then
        ILTypeDefLayout.Extended
    else
        ILTypeDefLayout.Auto

---

Task 4: Update IL Writing

File: src/Compiler/AbstractIL/ilwrite.fs

Location: Around line 2884, in GenTypeDefPass3, the ClassLayout handling.

Change from:

match tdef.Layout with
| ILTypeDefLayout.Auto -> ()
| ILTypeDefLayout.Sequential layout | ILTypeDefLayout.Explicit layout ->
    if Option.isSome layout.Pack || Option.isSome layout.Size then
        AddUnsharedRow cenv TableNames.ClassLayout
            (UnsharedRow
                [| UShort (defaultArg layout.Pack (uint16 0x0))
                   ULong (defaultArg layout.Size 0x0)
                   SimpleIndex (TableNames.TypeDef, tidx) |]) |> ignore

To:

match tdef.Layout with
| ILTypeDefLayout.Auto -> ()
| ILTypeDefLayo...

</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

*This pull request was created from Copilot chat.*
>

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more [Copilot coding agent tips](https://gh.io/copilot-coding-agent-tips) in the docs.

[T-Gro]

Note to self: This waits for NET11 alpha to have the attribute available.

[github-actions]

❗ Release notes required

You can open this PR in browser to add release notes: open in github.dev

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md