Non_null_value layout (ocaml-flambda#2480)

Implement the `non_null_value` layout, which is a sublayout of `value`.

---------

Co-authored-by: Diana Kalinichenko <dkalinichenko@janestreet.com>

ocaml/lambda/matching.ml

@@ -118,7 +118,7 @@ let jkind_layout_must_be_value loc jkind =
    outlived its usefulness and should be deleted. *)
 let check_record_field_jkind lbl =
   match Jkind.(get_default_value lbl.lbl_jkind), lbl.lbl_repres with
-  | (Value | Immediate | Immediate64), _ -> ()
+  | (Value | Immediate | Immediate64 | Non_null_value), _ -> ()
   | Float64, (Record_ufloat | Record_mixed _) -> ()
   | Float64, (Record_boxed _ | Record_inlined _
              | Record_unboxed | Record_float) ->

ocaml/testsuite/tests/typing-layouts-non-null-value/basics.ml

@@ -0,0 +1,134 @@
+(* TEST
+ flags = "-extension layouts_alpha";
+ expect;
+*)
+type t_non_null_value : non_null_value
+
+[%%expect{|
+type t_non_null_value : non_null_value
+|}]
+
+(* [non_null_value] can be used in regular functions and modules: *)
+
+let non_null_id (x : t_non_null_value) = x
+
+module type S1 = sig
+  val x : t_non_null_value
+  val f : t_non_null_value -> t_non_null_value option
+end;;
+
+[%%expect{|
+val non_null_id : t_non_null_value -> t_non_null_value = <fun>
+module type S1 =
+  sig
+    val x : t_non_null_value
+    val f : t_non_null_value -> t_non_null_value option
+  end
+|}]
+
+
+(* [non_null_value] is a sublayout of [value]: *)
+
+let id_value : ('a : value). 'a -> 'a = fun x -> x
+
+let id_non_null_value : ('a : non_null_value). 'a -> 'a = fun x -> id_value x
+
+module type S2 = sig
+  type t : value
+  val f : t -> t
+end
+
+module F (X : sig type t : non_null_value val f : t -> t end) : S2 = X;;
+
+[%%expect{|
+val id_value : 'a -> 'a = <fun>
+val id_non_null_value : ('a : non_null_value). 'a -> 'a = <fun>
+module type S2 = sig type t : value val f : t -> t end
+module F : functor (X : sig type t : non_null_value val f : t -> t end) -> S2
+|}]
+
+(* [value] is not a sublayout of [non_null_value]: *)
+
+let id_value' : ('a : value). 'a -> 'a = fun x -> id_non_null_value x;;
+
+[%%expect{|
+Line 1, characters 41-69:
+1 | let id_value' : ('a : value). 'a -> 'a = fun x -> id_non_null_value x;;
+                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: This definition has type 'b -> 'b which is less general than
+         'a. 'a -> 'a
+       The layout of 'a is value, because
+         of the annotation on the universal variable 'a.
+       But the layout of 'a must be a sublayout of non_null_value, because
+         of the definition of id_non_null_value at line 3, characters 4-21.
+|}]
+
+module type S3 = sig
+  type t : non_null_value
+  val f : t -> t
+end
+
+module F (X : sig type t : value val f : t -> t end) : S3 = X;;
+
+[%%expect{|
+module type S3 = sig type t : non_null_value val f : t -> t end
+Line 6, characters 60-61:
+6 | module F (X : sig type t : value val f : t -> t end) : S3 = X;;
+                                                                ^
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t = X.t val f : t -> t end
+       is not included in
+         S3
+       Type declarations do not match:
+         type t = X.t
+       is not included in
+         type t : non_null_value
+       The layout of the first is value, because
+         of the definition of t at line 6, characters 18-32.
+       But the layout of the first must be a sublayout of non_null_value, because
+         of the definition of t at line 2, characters 2-25.
+|}]
+
+(* Something else like [float64] is also not a sublayout of [non_null_value]: *)
+
+let id_float_64 : ('a : float64). 'a -> 'a = fun x -> id_non_null_value x;;
+
+[%%expect{|
+Line 1, characters 72-73:
+1 | let id_float_64 : ('a : float64). 'a -> 'a = fun x -> id_non_null_value x;;
+                                                                            ^
+Error: This expression has type ('a : float64)
+       but an expression was expected of type ('b : non_null_value)
+       The layout of 'a is non_null_value, because
+         of the definition of id_non_null_value at line 3, characters 4-21.
+       But the layout of 'a must overlap with float64, because
+         of the annotation on the universal variable 'a.
+|}]
+
+module type S4 = sig
+  type t : non_null_value
+  val f : t -> t
+end
+
+module F (X : sig type t : float64 val f : t -> t end) : S4 = X;;
+
+[%%expect{|
+module type S4 = sig type t : non_null_value val f : t -> t end
+Line 6, characters 62-63:
+6 | module F (X : sig type t : float64 val f : t -> t end) : S4 = X;;
+                                                                  ^
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t = X.t val f : t -> t end
+       is not included in
+         S4
+       Type declarations do not match:
+         type t = X.t
+       is not included in
+         type t : non_null_value
+       The layout of the first is float64, because
+         of the definition of t at line 6, characters 18-34.
+       But the layout of the first must be a sublayout of non_null_value, because
+         of the definition of t at line 2, characters 2-25.
+|}]

ocaml/testsuite/tests/typing-layouts/annots.ml

@@ -31,6 +31,16 @@ Error: Layout void is more experimental than allowed by the enabled layouts exte
        You must enable -extension layouts_alpha to use this feature.
 |}]
 
+type t_non_null_value : non_null_value;;
+
+[%%expect{|
+Line 1, characters 24-38:
+1 | type t_non_null_value : non_null_value;;
+                            ^^^^^^^^^^^^^^
+Error: Layout non_null_value is more experimental than allowed by the enabled layouts extension.
+       You must enable -extension layouts_alpha to use this feature.
+|}]
+
 (***************************************)
 (* Test 1: annotation on type variable *)
 

ocaml/testsuite/tests/typing-layouts/basics.ml

@@ -31,6 +31,15 @@ Error: Layout void is more experimental than allowed by the enabled layouts exte
        You must enable -extension layouts_alpha to use this feature.
 |}];;
 
+type t_non_null_value : non_null_value;;
+[%%expect{|
+Line 1, characters 24-38:
+1 | type t_non_null_value : non_null_value;;
+                            ^^^^^^^^^^^^^^
+Error: Layout non_null_value is more experimental than allowed by the enabled layouts extension.
+       You must enable -extension layouts_alpha to use this feature.
+|}]
+
 (******************************************************************)
 (* Test 1: Allow non-representable function args/returns in types *)