RFC-1090 for unmanaged generic struct types (#480)

* Added an RFC for fsharp/fslang-suggestions#692 * Simplify summary * Added more verbose error message example; Change wording in the detailed design * Added more examples; Added description of the unmanaged types; Added supportedd types section; Updated motivation section * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Address RFC comments; Added modreq requirement; Added more examples; Re-phrased some definitions * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Update RFCs/FS-1090-Generic-struct-type-whose-fields-are-all-unmanaged-types-is-unmanaged.md Co-authored-by: Phillip Carter <pcarter@fastmail.com> * Address PR comments. Co-authored-by: Phillip Carter <pcarter@fastmail.com>
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+# F# RFC FS-1088 - (Generic struct type whose fields are all unmanaged types is unmanaged)
+
+The design suggestion [Generic struct type whose fields are all unmanaged types is unmanaged](https://github.com/fsharp/fslang-suggestions/issues/692) has been marked "approved in principle".
+This RFC covers the detailed proposal for this suggestion.
+
+* [x] Approved in principle
+* [x] [Suggestion](https://github.com/fsharp/fslang-suggestions/issues/692)
+* [ ] Implementation: [In progress](https://github.com/dotnet/fsharp/pull/6064)
+
+# Summary
+
+[summary]: #summary
+
+Allow generic structs to be `unmanaged` at construction if all fields are `unmanaged` types.
+
+# Motivation
+
+[motivation]: #motivation
+
+* Make it easier to author low-level code.
+* Improve interoperability with C#.
+
+# Detailed design
+
+[design]: #detailed-design
+
+Currently, it is not possible to define a generic struct with the `unmanaged` constraint and substitute the type with another generic struct, even if its type is also constrained to be `unmanaged`.
+
+Consider the following code:
+
+```fsharp
+[<Struct>]
+type MyStruct(x: int, y: int) =
+    member _.X = x
+    member _.Y = y
+
+[<Struct>]
+type MyStructGeneric<'T when 'T: unmanaged>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+
+[<Struct>]
+type MyStructGenericWithNoConstraint<'T>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+
+[<Struct>]
+type Test<'T when 'T: unmanaged> =
+    val element: 'T
+
+let works = Test<int>()
+let works2 = Test<MyStruct>()
+
+// error FS0001: A generic construct requires that the type 'MyStructGeneric<int>' is an unmanaged type
+let error = Test<MyStructGeneric<int>>()
+
+// error FS0001: A generic construct requires that the type 'MyStructGenericWithNoConstraint<int>' is an unmanaged type
+let error = Test<MyStructGenericWithNoConstraint<int>>()
+```
+
+Prior to this RFC, the example above will fail to compile with:
+
+```less
+let error = Test<MyStructGeneric<int>>()
+  ------------^^^^^^^^^^^^^^^
+
+stdin(6,13): error FS0001: A generic construct requires that the type 'MyStructGeneric<int>' is an unmanaged type
+```
+
+This proposal aims to resolve this inconsistency by treating any generic struct type as `unmanaged` if all its fields are unmanaged.
+
+For example, consider the following code:
+
+```fsharp
+[<Struct>]
+type MyStructGeneric<'T when 'T: unmanaged>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+
+[<Struct>]
+type MyStructGenericWithNoConstraint<'T>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+```
+
+Instances of both `MyStructGeneric<'T>` and `MyStructGenericWithNoConstraint<'T>` will be treated as unmanaged type as long as `'T` is being unmanaged.
+
+In other words, any generic struct type, with all of its fielads are known to be unmanaged, can be considered unmanaged, _with_ or _without_ the `unmanaged` constraint on type parameter(s)
+
+## Adjusted definition of an unmanaged type
+
+__The existing definition of an unmanaged type is any type that isn't a reference-type and contains no fields whose type is not an unmanaged type:__
+
+* `sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool`.
+* Any enum-type.
+* Any pointer-type.
+* Any non-generic user-defined struct type that contains fields of unmanaged-types only.
+
+__We adjust this definition by modifying the last section:__
+
+* Any user-defined struct type that can be statically determined to be 'unmanaged' at construction.
+
+Examples:
+
+```fsharp
+// Always unmanaged (forced by constraint).
+[<Struct>]
+type MyStructGeneric<'T when 'T: unmanaged>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+
+// Can be considered unmanaged, as long as 'T is unmanaged.
+[<Struct>]
+type MyStructGenericWithNoConstraint<'T>(x: 'T, y: 'T) =
+    member _.X = x
+    member _.Y = y
+
+// Can be considerd unmanaged, as long as 'T is unmanaged.
+// Note, that despite constructor having managed argument (obj), it is not used as part of the backing field.
+[<Struct>]
+type MyStructGenericWithUnusedUnmanagedParameter<'T>(x: 'T, y: 'T, z: obj) =
+    member _.X = x
+    member _.Y = y
+
+// Not unmanaged, since it has a field with a managed type.
+[<Struct>]
+type MyStructGenericWithUnmanagedField<'T>(x: 'T, y: 'T, z: obj) =
+    member _.X = x
+    member _.Y = y
+    member _.Z = z
+```
+
+## Implementation details
+
+* Any existing generic struct type will be considered unmanaged when all of its fields are unmanaged types.
+* `IsUnmanagedAttribute` should be emitted on type arguments with the `unmanaged` constraints (for interop).
+* Treat type arguments with `IsUnmanagedAttribute` as if they have the `unmanaged` constraint (for interop).
+* [`UnmanagedType`](https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.unmanagedtype?view=netcore-3.1) modreq should be emitted on type argument (for interop).
+* An `unmanaged` constraint cannot be used together with `not struct` constraint (not the case right now).
+
+## Supported types
+
+* Generic struct types whose fields are unmanaged
+* Struct records with unmanaged fields
+* Anonymous struct records with unmanaged fields
+* Struct tuples with unmanaged values
+* Struct unions (single and multi-case) whose fields are all unmanaged
+
+## Examples
+
+The following examples are now possible with this feature:
+
+Basic examples:
+
+```fsharp
+[<Struct>]
+type S<'T, 'U> =
+
+    [<DefaultValue(false)>] val X : 'T
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let passing () =
+    test (S<float, int>())
+
+let passing2 () =
+    test (S<float, obj>()) // passes as the 'obj' type arg is not used as part of the backing field of a struct
+
+let passing3<'T when 'T : unmanaged> () =
+    test (S<'T, obj>())
+
+let not_passing () =
+    test (S<obj, int>())
+
+let not_passing2<'T> () =
+    test (S<'T, obj>())
+```
+
+Constructor:
+
+```fsharp
+[<Struct>]
+type S<'T> =
+
+    val X : 'T
+
+    new (x) = { X = x }
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let passing () =
+    test (S(1))
+
+let not_passing () =
+    test (S(obj()))
+```
+
+Nested generics:
+
+```fsharp
+[<Struct>]
+type W<'T> = { x: 'T }
+
+[<Struct>]
+type S<'T> = { x: W<'T> }
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let passing () =
+    test Unchecked.defaultof<S<int>>
+
+let not_passing () =
+    test Unchecked.defaultof<S<obj>>
+```
+
+Union types:
+
+```fsharp
+[<Struct>]
+type Container<'a> = Container of 'a
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let passing () = test (Container 1)
+
+let not_passing () = test (Container "string")
+```
+
+Tuples:
+
+```fsharp
+let x = struct(1,2,3)
+
+let y = struct("s", "t", "r", "i", "n", "g")
+
+let z = struct(1, 2, 3, "s", "t", "r")
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let passing () = test (x)
+
+let not_passing () = test (y)
+
+let not_passing2 () = test (z)
+```
+
+Records:
+
+```fsharp
+[<Struct>]
+type Point = { X: float; Y: float; Z: float; }
+
+[<Struct>]
+type Person = { Name: string; Age: int; }
+
+let test (x: 'T when 'T : unmanaged) = ()
+
+let mypoint = { X = 1.0; Y = 1.0; Z = -1.0; }
+
+let passing () = test (mypoint)
+
+let passing2 () = test (struct {| A= 1 |})
+
+let person = { Name = "Joe"; Age = 42 }
+
+let not_passing () = test (person)
+
+let not_passing2 () = test (struct {| S = "str" |})
+```
+
+## Documentation
+
+'Unmanaged Constraint' part of [F# constraints documentation](https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/generics/constraints) should be updated to reflect new supported cases.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The major drawback of the feature is that it serves a relatively small number of developers, since most of the F# projects are not focused on a low-level programming.
+
+# Alternatives
+[alternatives]: #alternatives
+
+There are currently no alternatives or workarounds for this.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.