Add ExtendedLayoutAttribute support for F# compiler

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -187,6 +187,7 @@
 * Lines starting with `#:` are now ignored ([Language suggestion 1440](https://github.com/fsharp/fslang-suggestions/issues/1440), [RFC FS-1337](https://github.com/fsharp/fslang-design/pull/830), [PR #20212](https://github.com/dotnet/fsharp/pull/20212))
 
 ### Breaking Changes
+* Add `ExtendedLayoutAttribute` support for future .NET runtime interop. `ILTypeDefLayout` has a new `Extended` case. ([Issue #19190](https://github.com/dotnet/fsharp/issues/19190), [PR #19194](https://github.com/dotnet/fsharp/pull/19194))
 
 * `FSharp.Compiler.Syntax.SynInterpolatedStringPart.FillExpr` now carries a `SynInterpolationFormatting` value (separating .NET alignment/format from printf specifiers) instead of an `Ident option`. ([PR #19971](https://github.com/dotnet/fsharp/pull/19971))
 * Optimizer: don't inline named functions in debug builds ([PR #19548](https://github.com/dotnet/fsharp/pull/19548)

src/Compiler/AbstractIL/il.fs

@@ -2572,6 +2572,7 @@ type ILTypeDefLayout =
     | Auto
     | Sequential of ILTypeDefLayoutInfo
     | Explicit of ILTypeDefLayoutInfo (* REVIEW: add field info here *)
+    | Extended
 
 and ILTypeDefLayoutInfo =
     {
@@ -2685,6 +2686,10 @@ let convertLayout layout =
     | ILTypeDefLayout.Auto -> TypeAttributes.AutoLayout
     | ILTypeDefLayout.Sequential _ -> TypeAttributes.SequentialLayout
     | ILTypeDefLayout.Explicit _ -> TypeAttributes.ExplicitLayout
+    | ILTypeDefLayout.Extended ->
+        // Extended layout is represented by TypeAttributes value 0x18 (both Sequential and Explicit bits set)
+        // See: https://github.com/dotnet/runtime/issues/102727
+        enum<TypeAttributes> (0x18)
 
 let convertEncoding encoding =
     match encoding with

src/Compiler/AbstractIL/il.fsi

@@ -1498,6 +1498,7 @@ type ILTypeDefLayout =
     | Auto
     | Sequential of ILTypeDefLayoutInfo
     | Explicit of ILTypeDefLayoutInfo
+    | Extended
 
 type internal ILTypeDefLayoutInfo =
     { Size: int32 option

src/Compiler/AbstractIL/ilprint.fs

@@ -776,6 +776,7 @@ let splitTypeLayout =
     | ILTypeDefLayout.Auto -> "auto", (fun _os () -> ())
     | ILTypeDefLayout.Sequential info -> "sequential", (fun os () -> output_type_layout_info os info)
     | ILTypeDefLayout.Explicit info -> "explicit", (fun os () -> output_type_layout_info os info)
+    | ILTypeDefLayout.Extended -> "extended", (fun _os () -> ())
 
 let goutput_fdefs tref env os (fdefs: ILFieldDefs) =
     for f in fdefs.AsList() do

src/Compiler/AbstractIL/ilread.fs

@@ -2064,6 +2064,8 @@ and typeLayoutOfFlags (ctxt: ILMetadataReader) mdv flags tidx =
         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
 
@@ -2136,6 +2138,8 @@ and typeDefReader ctxtH : ILTypeDefStored =
         let super = seekReadSuperType ctxt numTypars AsObject extendsIdx
         let layout = typeLayoutOfFlags ctxt mdv flags idx
 
+        // Only Explicit layout has per-field offsets in the FieldLayout metadata table.
+        // Sequential and Extended layouts don't use FieldLayout rows.
         let hasLayout =
             match layout with
             | ILTypeDefLayout.Explicit _ -> true

src/Compiler/AbstractIL/ilreflect.fs

@@ -2138,6 +2138,7 @@ let typeAttributesOfTypeLayout cenv emEnv x =
     | ILTypeDefLayout.Auto -> None
     | ILTypeDefLayout.Explicit p -> (attr 0x02 p)
     | ILTypeDefLayout.Sequential p -> (attr 0x00 p)
+    | ILTypeDefLayout.Extended -> None // No StructLayoutAttribute needed; user's ExtendedLayoutAttribute is preserved
 
 //----------------------------------------------------------------------------
 // buildTypeDefPass1 cenv

src/Compiler/AbstractIL/ilwrite.fs

@@ -2916,6 +2916,7 @@ let rec GenTypeDefPass3 enc cenv (tdef: ILTypeDef) =
         // ClassLayout entry if needed
         match tdef.Layout with
         | ILTypeDefLayout.Auto -> ()
+        | ILTypeDefLayout.Extended -> ()  // No ClassLayout row for Extended; bits are in TypeAttributes
         | ILTypeDefLayout.Sequential layout | ILTypeDefLayout.Explicit layout ->
             if Option.isSome layout.Pack || Option.isSome layout.Size then
                 AddUnsharedRow cenv TableNames.ClassLayout

src/Compiler/Checking/CheckDeclarations.fs

@@ -3547,7 +3547,12 @@ module EstablishTypeDefinitionCores =
                 match attrs with
                 | EntityAttribInt g WellKnownEntityAttributes.StructLayoutAttribute v -> Some v
                 | _ -> None
+            let hasExtendedLayoutAttr =
+                match g.attrib_ExtendedLayoutAttribute_opt with
+                | Some attrib -> HasFSharpAttribute g attrib attrs
+                | None -> false
             let hasAllowNullLiteralAttr = hasFlag entityFlags WellKnownEntityAttributes.AllowNullLiteralAttribute_True
+            let hasStructAttr = hasFlag entityFlags WellKnownEntityAttributes.StructAttribute
 
             if hasAbstractAttr then 
                 tycon.TypeContents.tcaug_abstract <- true
@@ -3567,7 +3572,13 @@ module EstablishTypeDefinitionCores =
 
             let structLayoutAttributeCheck allowed = 
                 let explicitKind = int32 System.Runtime.InteropServices.LayoutKind.Explicit
+                // LayoutKind.Extended has enum value 1 in .NET 11+ (previously unused slot)
+                // It cannot be specified via StructLayoutAttribute - users must use ExtendedLayoutAttribute instead
+                // See: https://github.com/dotnet/runtime/issues/102727
+                let extendedLayoutKind = 1
                 match structLayoutAttr with
+                | Some kind when kind = extendedLayoutKind ->
+                    errorR (Error(FSComp.SR.tcInvalidStructLayoutExtendedKind(), m))
                 | Some kind ->
                     if allowed then 
                         if kind = explicitKind then
@@ -3577,9 +3588,32 @@ module EstablishTypeDefinitionCores =
                     else
                         errorR (Error(FSComp.SR.tcGenericTypesCannotHaveStructLayout(), m))
                 | None -> ()
+
+            let extendedLayoutAttributeCheck () =
+                if hasExtendedLayoutAttr then
+                    // Check not combined with StructLayoutAttribute
+                    if structLayoutAttr.IsSome then
+                        errorR (Error(FSComp.SR.tcStructLayoutAndExtendedLayout(), m))
+
+            let noExtendedLayoutAttributeCheck () =
+                if hasExtendedLayoutAttr then
+                    errorR (Error(FSComp.SR.tcOnlyStructsCanHaveExtendedLayout(), m))
+
+            // Records become value types when marked [<Struct>]. Extended layout is valid on those
+            // (subject to the StructLayout conflict check); reference-typed records still reject it.
+            let recordExtendedLayoutAttributeCheck () =
+                if hasStructAttr then extendedLayoutAttributeCheck ()
+                else noExtendedLayoutAttributeCheck ()
+
+            // A discriminated union (including a [<Struct>] one) carries a case tag plus per-case fields,
+            // which is incompatible with the CStruct/CUnion field layout, so extended layout is never valid.
+            let unionExtendedLayoutAttributeCheck () =
+                if hasExtendedLayoutAttr then
+                    errorR (Error(FSComp.SR.tcExtendedLayoutCannotBeUsedOnUnions(), m))
 
             let hiddenReprChecks hasRepr =
                  structLayoutAttributeCheck false
+                 noExtendedLayoutAttributeCheck()
                  if hasSealedAttr = Some false || (hasRepr && hasSealedAttr <> Some true && not (id.idText = "Unit" && g.compilingFSharpCore) ) then 
                     errorR(Error(FSComp.SR.tcRepresentationOfTypeHiddenBySignature(), m))
                  if hasAbstractAttr then 
@@ -3673,6 +3707,7 @@ module EstablishTypeDefinitionCores =
                 | TyconCoreAbbrevThatIsReallyAUnion (hasMeasureAttr, envinner, id) (unionCaseName, _) ->
 
                     structLayoutAttributeCheck false
+                    unionExtendedLayoutAttributeCheck()
                     noAllowNullLiteralAttributeCheck()
 
                     let hasRQAAttribute = EntityHasWellKnownAttribute cenv.g WellKnownEntityAttributes.RequireQualifiedAccessAttribute tycon
@@ -3689,7 +3724,8 @@ module EstablishTypeDefinitionCores =
                         errorR (Error(FSComp.SR.tcAbbreviatedTypesCannotBeSealed(), m))
                     noAbstractClassAttributeCheck()
                     noAllowNullLiteralAttributeCheck()
-                    if hasMeasureableAttr then 
+                    noExtendedLayoutAttributeCheck()
+                    if hasMeasureableAttr then
                         let kind = if hasMeasureAttr then TyparKind.Measure else TyparKind.Type
                         let theTypeAbbrev, _ = TcTypeOrMeasureAndRecover (Some kind) cenv NoNewTypars CheckCxs ItemOccurrence.UseInType WarnOnIWSAM.No envinner tpenv rhsType
 
@@ -3706,6 +3742,7 @@ module EstablishTypeDefinitionCores =
                     noAbstractClassAttributeCheck()
                     noAllowNullLiteralAttributeCheck()
                     structLayoutAttributeCheck false
+                    unionExtendedLayoutAttributeCheck()
 
                     let hasRQAAttribute = EntityHasWellKnownAttribute cenv.g WellKnownEntityAttributes.RequireQualifiedAccessAttribute tycon
                     let unionCases = TcRecdUnionAndEnumDeclarations.TcUnionCaseDecls cenv envinner innerParent thisTy thisTyInst hasRQAAttribute tpenv addFixup unionCases
@@ -3722,6 +3759,7 @@ module EstablishTypeDefinitionCores =
                     noAbstractClassAttributeCheck()
                     noAllowNullLiteralAttributeCheck()
                     structLayoutAttributeCheck true  // these are allowed for records
+                    recordExtendedLayoutAttributeCheck()
 
                     let check pass =
                         let firstPass = pass = FirstPass
@@ -3879,6 +3917,7 @@ module EstablishTypeDefinitionCores =
                     noSealedAttributeCheck FSComp.SR.tcTypesAreAlwaysSealedAssemblyCode
                     noAllowNullLiteralAttributeCheck()
                     structLayoutAttributeCheck false
+                    noExtendedLayoutAttributeCheck()
                     noAbstractClassAttributeCheck()
                     (TAsmRepr s, None, NoSafeInitInfo), ignore
 
@@ -3949,17 +3988,20 @@ module EstablishTypeDefinitionCores =
                                   if not (isNil slotsigs) then 
                                     errorR (Error(FSComp.SR.tcStructTypesCannotContainAbstractMembers(), m)) 
                                   structLayoutAttributeCheck true
+                                  extendedLayoutAttributeCheck()
 
                                   TFSharpStruct
                               | SynTypeDefnKind.Interface -> 
                                   if hasSealedAttr = Some true then errorR (Error(FSComp.SR.tcInterfaceTypesCannotBeSealed(), m))
                                   structLayoutAttributeCheck false
+                                  noExtendedLayoutAttributeCheck()
                                   noAbstractClassAttributeCheck()
                                   allowNullLiteralAttributeCheck()
                                   noFieldsCheck userFields
                                   TFSharpInterface
                               | SynTypeDefnKind.Class -> 
                                   structLayoutAttributeCheck(not isIncrClass)
+                                  noExtendedLayoutAttributeCheck()
                                   allowNullLiteralAttributeCheck()
                                   for slot in abstractSlots do
                                       if not slot.IsInstanceMember then
@@ -3968,6 +4010,7 @@ module EstablishTypeDefinitionCores =
                               | SynTypeDefnKind.Delegate (ty, arity) -> 
                                   noSealedAttributeCheck FSComp.SR.tcTypesAreAlwaysSealedDelegate
                                   structLayoutAttributeCheck false
+                                  noExtendedLayoutAttributeCheck()
                                   noAllowNullLiteralAttributeCheck()
                                   noAbstractClassAttributeCheck()
                                   noFieldsCheck userFields
@@ -4039,6 +4082,7 @@ module EstablishTypeDefinitionCores =
                     let fieldTy, fields' = TcRecdUnionAndEnumDeclarations.TcEnumDecls cenv envinner tpenv innerParent thisTy decls
                     let kind = TFSharpEnum
                     structLayoutAttributeCheck false
+                    noExtendedLayoutAttributeCheck()
                     noSealedAttributeCheck FSComp.SR.tcTypesAreAlwaysSealedEnum
                     noAllowNullLiteralAttributeCheck()
                     let vid = ident("value__", m)

src/Compiler/CodeGen/IlxGen.fs

@@ -12351,46 +12351,52 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                                     ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi
                             | _ -> ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
 
-                        match tycon.Attribs with
-                        | EntityAttrib g WellKnownEntityAttributes.StructLayoutAttribute (Attrib(_,
-                                                                                                 _,
-                                                                                                 [ AttribInt32Arg layoutKind ],
-                                                                                                 namedArgs,
-                                                                                                 _,
-                                                                                                 _,
-                                                                                                 _)) ->
-                            let decoder = AttributeDecoder namedArgs
-                            let ilPack = decoder.FindInt32 "Pack" 0x0
-                            let ilSize = decoder.FindInt32 "Size" 0x0
-
-                            let tdEncoding =
-                                match (decoder.FindInt32 "CharSet" 0x0) with
-                                (* enumeration values for System.Runtime.InteropServices.CharSet taken from mscorlib.il *)
-                                | 0x03 -> ILDefaultPInvokeEncoding.Unicode
-                                | 0x04 -> ILDefaultPInvokeEncoding.Auto
-                                | _ -> ILDefaultPInvokeEncoding.Ansi
-
-                            let layoutInfo =
-                                if ilPack = 0x0 && ilSize = 0x0 then
-                                    { Size = None; Pack = None }
-                                else
-                                    {
-                                        Size = Some ilSize
-                                        Pack = Some(uint16 ilPack)
-                                    }
-
-                            let tdLayout =
-                                match layoutKind with
-                                (* enumeration values for System.Runtime.InteropServices.LayoutKind taken from mscorlib.il *)
-                                | 0x0 -> ILTypeDefLayout.Sequential layoutInfo
-                                | 0x2 -> ILTypeDefLayout.Explicit layoutInfo
-                                | _ -> ILTypeDefLayout.Auto
-
-                            tdLayout, tdEncoding
-                        | EntityAttrib g WellKnownEntityAttributes.StructLayoutAttribute (Attrib(_, _, _, _, _, _, m)) ->
-                            errorR (Error(FSComp.SR.ilStructLayoutAttributeCouldNotBeDecoded (), m))
-                            ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
-                        | _ -> defaultLayout ()
+                        // Check for ExtendedLayoutAttribute first
+                        match g.attrib_ExtendedLayoutAttribute_opt with
+                        | Some attrib when HasFSharpAttribute g attrib tycon.Attribs ->
+                            ILTypeDefLayout.Extended, ILDefaultPInvokeEncoding.Ansi
+                        | _ ->
+
+                            match tycon.Attribs with
+                            | EntityAttrib g WellKnownEntityAttributes.StructLayoutAttribute (Attrib(_,
+                                                                                                     _,
+                                                                                                     [ AttribInt32Arg layoutKind ],
+                                                                                                     namedArgs,
+                                                                                                     _,
+                                                                                                     _,
+                                                                                                     _)) ->
+                                let decoder = AttributeDecoder namedArgs
+                                let ilPack = decoder.FindInt32 "Pack" 0x0
+                                let ilSize = decoder.FindInt32 "Size" 0x0
+
+                                let tdEncoding =
+                                    match (decoder.FindInt32 "CharSet" 0x0) with
+                                    (* enumeration values for System.Runtime.InteropServices.CharSet taken from mscorlib.il *)
+                                    | 0x03 -> ILDefaultPInvokeEncoding.Unicode
+                                    | 0x04 -> ILDefaultPInvokeEncoding.Auto
+                                    | _ -> ILDefaultPInvokeEncoding.Ansi
+
+                                let layoutInfo =
+                                    if ilPack = 0x0 && ilSize = 0x0 then
+                                        { Size = None; Pack = None }
+                                    else
+                                        {
+                                            Size = Some ilSize
+                                            Pack = Some(uint16 ilPack)
+                                        }
+
+                                let tdLayout =
+                                    match layoutKind with
+                                    (* enumeration values for System.Runtime.InteropServices.LayoutKind taken from mscorlib.il *)
+                                    | 0x0 -> ILTypeDefLayout.Sequential layoutInfo
+                                    | 0x2 -> ILTypeDefLayout.Explicit layoutInfo
+                                    | _ -> ILTypeDefLayout.Auto
+
+                                tdLayout, tdEncoding
+                            | EntityAttrib g WellKnownEntityAttributes.StructLayoutAttribute (Attrib(_, _, _, _, _, _, m)) ->
+                                errorR (Error(FSComp.SR.ilStructLayoutAttributeCouldNotBeDecoded (), m))
+                                ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
+                            | _ -> defaultLayout ()
 
                     // if the type's layout is Explicit, ensure that each field has a valid offset
                     let validateExplicit (fdef: ILFieldDef) =
@@ -12417,6 +12423,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                     match tdLayout with
                     | ILTypeDefLayout.Explicit _ -> List.iter validateExplicit ilFieldDefs
                     | ILTypeDefLayout.Sequential _ -> List.iter validateSequential ilFieldDefs
+                    | ILTypeDefLayout.Extended -> List.iter validateSequential ilFieldDefs // Extended layout manages field layout via the attribute; FieldOffset is not allowed
                     | _ -> ()
 
                     let tdef =