Add FS-1008 (#81)

Author: roboz0r

spec/rfc-status.md

@@ -18,7 +18,7 @@
 | F# 4.1 | FS-1005 | [Underscores in Numeric Literals](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1005-underscores-in-numeric-literals.md) | [completed](https://github.com/fsharp/fslang-spec/pull/29/commits/2bd80c8cdb2f89ccadc9dac038568d3089d8e1dc) |
 | F# 4.1 | FS-1006 | [Struct Tuples](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1006-struct-tuples.md) | [completed](https://github.com/fsharp/fslang-spec/pull/30/commits) |
 | F# 4.1 | FS-1007 | [Additional Option Module Functions](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1007-additional-Option-module-functions.md) | This specifies some details of the `FSharp.Core` library, which is not part of this spec (except for the basic items listed in [§18](https://fsharp.github.io/fslang-spec/the-f-library-fsharpcoredll/)) |
-| F# 4.1 | FS-1008 | [Struct Records](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1008-struct-records.md) | |
+| F# 4.1 | FS-1008 | [Struct Records](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1008-struct-records.md) | [completed](https://github.com/fsharp/fslang-spec/pull/81) |
 | F# 4.1 | FS-1009 | [Mutually Referential Types and Modules in Single Scope](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1009-mutually-referential-types-and-modules-single-scope.md) | |
 | F# 4.1 | FS-1010 | [Add Map.count](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1010-add-map-count.md) | This specifies some details of the `FSharp.Core` library, which is not part of this spec (except for the basic items listed in [§18](https://fsharp.github.io/fslang-spec/the-f-library-fsharpcoredll/)) |
 | F# 4.1 | FS-1012 | [Caller Info Attributes](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1012-caller-info-attributes.md) | [completed](https://github.com/fsharp/fslang-spec/pull/74) |

spec/type-definitions.md

@@ -490,6 +490,8 @@ be given the `AbstractClass` attribute.
 
 Record types are implicitly marked serializable unless the `AutoSerializable(false)` attribute is used.
 
+Record types are reference types unless the `Struct` attribute is used (see [§](type-definitions.md#struct-type-definitions)).
+
 ### Members in Record Types
 
 Record types may declare members ([§](type-definitions.md#members)), overrides, and interface implementations. Like all types
@@ -1255,17 +1257,17 @@ because the size of `BadStruct2` would be infinite:
 
 ```fsharp
 [<Struct>]
-type BadStruct 2 =
+type BadStruct2 =
     val data : float;
-    val rest : BadStruct 2
+    val rest : BadStruct2
     new (data, rest) = { data = data; rest = rest }
 ```
 
 Likewise, the implied size of the following struct would be infinite:
 
 ```fsharp
 [<Struct>]
-type BadStruct 3 (data : float, rest : BadStruct 3 ) =
+type BadStruct3 (data : float, rest : BadStruct3 ) =
     member s.Data = data
     member s.Rest = rest
 ```
@@ -1295,6 +1297,18 @@ abnormal value.
 Public struct types for use from other CLI languages should be designed with the
 existence of the default zero-initializing constructor in mind.
 
+[Record Type Defintions](#record-type-definitions) may also use the `[<Struct>]` attribute to change their representation from a reference type to a value type:
+
+```fsharp
+[<Struct>]
+type Vector3 = { X: float; Y: float; Z: float }
+```
+
+Record structs have the following limitations:
+
+- Unlike normal F# structs you cannot call the default constructor
+- When marked with `[<CLIMutable>]` attribute, a default constructor is not created because it already exists implicitly
+
 ## Enum Type Definitions
 
 Occasionally the need arises to represent a type that compiles as a CLI enumeration type. An _enum