Add ExtendedLayoutAttribute support for F# compiler

src/Compiler/AbstractIL/il.fs

@@ -2519,6 +2519,7 @@ type ILTypeDefLayout =
     | Auto
     | Sequential of ILTypeDefLayoutInfo
     | Explicit of ILTypeDefLayoutInfo (* REVIEW: add field info here *)
+    | Extended
 
 and ILTypeDefLayoutInfo =
     {
@@ -2632,6 +2633,10 @@ 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)
+        // This is defined by the ECMA-335 spec for extended layout types
+        enum<TypeAttributes> (0x18)
 
 let convertEncoding encoding =
     match encoding with

src/Compiler/AbstractIL/il.fsi

@@ -1458,6 +1458,7 @@ 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

@@ -2044,6 +2044,8 @@ 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
 

src/Compiler/AbstractIL/ilreflect.fs

@@ -2106,6 +2106,7 @@ 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

@@ -2883,6 +2883,7 @@ 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

@@ -3416,6 +3416,7 @@ module EstablishTypeDefinitionCores =
             let hasMeasureableAttr = HasFSharpAttribute g g.attrib_MeasureableAttribute attrs
 
             let structLayoutAttr = TryFindFSharpInt32Attribute g g.attrib_StructLayoutAttribute attrs
+            let hasExtendedLayoutAttr = HasFSharpAttributeOpt g g.attrib_ExtendedLayoutAttribute_opt attrs
             let hasAllowNullLiteralAttr = TryFindFSharpBoolAttribute g g.attrib_AllowNullLiteralAttribute attrs = Some true
 
             if hasAbstractAttr then 
@@ -3436,7 +3437,13 @@ module EstablishTypeDefinitionCores =
 
             let structLayoutAttributeCheck allowed = 
                 let explicitKind = int32 System.Runtime.InteropServices.LayoutKind.Explicit
+                // LayoutKind.Extended will have enum value 1 in future .NET versions (currently 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
@@ -3446,6 +3453,22 @@ module EstablishTypeDefinitionCores =
                     else
                         errorR (Error(FSComp.SR.tcGenericTypesCannotHaveStructLayout(), m))
                 | None -> ()
+
+            let extendedLayoutAttributeCheck () =
+                if hasExtendedLayoutAttr then
+                    // Check runtime support
+                    match g.attrib_ExtendedLayoutAttribute_opt with
+                    | None -> 
+                        errorR (Error(FSComp.SR.tcRuntimeDoesNotSupportExtendedLayoutTypes(), m))
+                    | Some _ -> ()
+
+                    // 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))
 
             let hiddenReprChecks hasRepr =
                  structLayoutAttributeCheck false
@@ -3684,17 +3707,20 @@ 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
@@ -3703,6 +3729,7 @@ module EstablishTypeDefinitionCores =
                               | SynTypeDefnKind.Delegate (ty, arity) -> 
                                   noSealedAttributeCheck FSComp.SR.tcTypesAreAlwaysSealedDelegate
                                   structLayoutAttributeCheck false
+                                  noExtendedLayoutAttributeCheck()
                                   noAllowNullLiteralAttributeCheck()
                                   noAbstractClassAttributeCheck()
                                   noFieldsCheck userFields

src/Compiler/CodeGen/IlxGen.fs

@@ -11564,60 +11564,64 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                             tdef
 
                     let tdLayout, tdEncoding =
-                        match TryFindFSharpAttribute g g.attrib_StructLayoutAttribute tycon.Attribs with
-                        | Some(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
-                        | Some(Attrib(_, _, _, _, _, _, m)) ->
-                            errorR (Error(FSComp.SR.ilStructLayoutAttributeCouldNotBeDecoded (), m))
-                            ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
-
-                        | _ when
-                            (match ilTypeDefKind with
-                             | HasFlag ILTypeDefAdditionalFlags.ValueType -> true
-                             | _ -> false)
-                            ->
+                        // Check for ExtendedLayoutAttribute first
+                        if HasFSharpAttributeOpt g g.attrib_ExtendedLayoutAttribute_opt tycon.Attribs then
+                            ILTypeDefLayout.Extended, ILDefaultPInvokeEncoding.Ansi
+                        else
+                            match TryFindFSharpAttribute g g.attrib_StructLayoutAttribute tycon.Attribs with
+                            | Some(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)
+                                        }
 
-                            // All structs are sequential by default
-                            // Structs with no instance fields get size 1, pack 0
-                            if
-                                tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
-                                ||
-                                // Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
-                                // In that case we generate a dummy field instead
-                                (cenv.options.workAroundReflectionEmitBugs && not tycon.TyparsNoRange.IsEmpty)
-                            then
-                                ILTypeDefLayout.Sequential { Size = None; Pack = None }, ILDefaultPInvokeEncoding.Ansi
-                            else
-                                ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi
+                                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
+                            | Some(Attrib(_, _, _, _, _, _, m)) ->
+                                errorR (Error(FSComp.SR.ilStructLayoutAttributeCouldNotBeDecoded (), m))
+                                ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
+
+                            | _ when
+                                (match ilTypeDefKind with
+                                 | HasFlag ILTypeDefAdditionalFlags.ValueType -> true
+                                 | _ -> false)
+                                ->
+
+                                // All structs are sequential by default
+                                // Structs with no instance fields get size 1, pack 0
+                                if
+                                    tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic)
+                                    ||
+                                    // Reflection emit doesn't let us emit 'pack' and 'size' for generic structs.
+                                    // In that case we generate a dummy field instead
+                                    (cenv.options.workAroundReflectionEmitBugs && not tycon.TyparsNoRange.IsEmpty)
+                                then
+                                    ILTypeDefLayout.Sequential { Size = None; Pack = None }, ILDefaultPInvokeEncoding.Ansi
+                                else
+                                    ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }, ILDefaultPInvokeEncoding.Ansi
 
-                        | _ -> ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
+                            | _ -> ILTypeDefLayout.Auto, ILDefaultPInvokeEncoding.Ansi
 
                     // if the type's layout is Explicit, ensure that each field has a valid offset
                     let validateExplicit (fdef: ILFieldDef) =
@@ -11641,6 +11645,7 @@ 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 also disallows FieldOffset
                     | _ -> ()
 
                     let tdef =

src/Compiler/FSComp.txt

@@ -788,6 +788,10 @@ tcTypeAbbreviationHasTypeParametersMissingOnType,"This type abbreviation has one
 935,tcAllowNullTypesMayOnlyInheritFromAllowNullTypes,"Types with the 'AllowNullLiteral' attribute may only inherit from or implement types which also allow the use of the null literal"
 936,tcGenericTypesCannotHaveStructLayout,"Generic types cannot be given the 'StructLayout' attribute"
 937,tcOnlyStructsCanHaveStructLayout,"Only structs and classes without primary constructors may be given the 'StructLayout' attribute"
+3879,tcStructLayoutAndExtendedLayout,"The attributes 'StructLayoutAttribute' and 'ExtendedLayoutAttribute' cannot be used together on the same type"
+3880,tcRuntimeDoesNotSupportExtendedLayoutTypes,"The target runtime does not support extended layout types. ExtendedLayoutAttribute requires a newer runtime."
+3881,tcOnlyStructsCanHaveExtendedLayout,"Only structs may be given the 'ExtendedLayoutAttribute'"
+3882,tcInvalidStructLayoutExtendedKind,"LayoutKind value 1 (Extended) cannot be specified via StructLayoutAttribute. Use ExtendedLayoutAttribute instead."
 938,tcRepresentationOfTypeHiddenBySignature,"The representation of this type is hidden by the signature. It must be given an attribute such as [<Sealed>], [<Class>] or [<Interface>] to indicate the characteristics of the type."
 939,tcOnlyClassesCanHaveAbstract,"Only classes may be given the 'AbstractClass' attribute"
 940,tcOnlyTypesRepresentingUnitsOfMeasureCanHaveMeasure,"Only types representing units-of-measure may be given the 'Measure' attribute"

src/Compiler/TypedTree/TcGlobals.fs

@@ -1487,6 +1487,7 @@ type TcGlobals(
   member val attrib_IsByRefLikeAttribute_opt = tryFindSysAttrib "System.Runtime.CompilerServices.IsByRefLikeAttribute"
   member val attrib_DllImportAttribute = tryFindSysAttrib "System.Runtime.InteropServices.DllImportAttribute"
   member val attrib_StructLayoutAttribute = findSysAttrib "System.Runtime.InteropServices.StructLayoutAttribute"
+  member val attrib_ExtendedLayoutAttribute_opt = tryFindSysAttrib "System.Runtime.InteropServices.ExtendedLayoutAttribute"
   member val attrib_TypeForwardedToAttribute = findSysAttrib "System.Runtime.CompilerServices.TypeForwardedToAttribute"
   member val attrib_ComVisibleAttribute = findSysAttrib "System.Runtime.InteropServices.ComVisibleAttribute"
   member val attrib_ComImportAttribute = tryFindSysAttrib "System.Runtime.InteropServices.ComImportAttribute"

src/Compiler/TypedTree/TcGlobals.fsi

@@ -481,6 +481,8 @@ type internal TcGlobals =
 
     member attrib_StructLayoutAttribute: BuiltinAttribInfo
 
+    member attrib_ExtendedLayoutAttribute_opt: BuiltinAttribInfo option
+
     member attrib_StructuralComparisonAttribute: BuiltinAttribInfo
 
     member attrib_StructuralEqualityAttribute: BuiltinAttribInfo