Make various primitives non_null_value (ocaml-flambda#2604)

* `non_null_value_creation_reason`

* Immediates are non-null

* Primitives are non-null

* Extensible variants are non-null

* Boxed variants are non-null

* Boxed records are non-null

* Tuples are non-null

* Polymorphic variants are non-null

* Arrows are non-null

* First-class modules are non-null

* Basic tests

* More tests

* More tests

* Leave `lazy_t` nullable for now

* More basic tests

* More tests

* Format

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

Co-authored-by: Richard Eisenberg <[email protected]>

* stable -> upstream_compatible

* Update comments and tests

* Display `non_null_value` as `value` (ocaml-flambda#2613)

* Naive hack

* Promote tests

* More hacks

* More hacks + promote tests

* Test hiding

* Stub `Or_null` module

* Test sublayot error

* Formatting

---------

Co-authored-by: Diana Kalinichenko <[email protected]>

---------

Co-authored-by: Diana Kalinichenko <[email protected]>
Co-authored-by: Richard Eisenberg <[email protected]>

ocaml/otherlibs/alpha/dune

@@ -39,7 +39,7 @@
 (rule
  (deps
   (glob_files ../stdlib_alpha/*.{ml,mli}))
- (targets alpha.ml alpha.mli)
+ (targets alpha.ml alpha.mli or_null.ml or_null.mli)
  (action
   (bash
    "cp ../stdlib_alpha/*.{ml,mli} .; mv stdlib_alpha.ml alpha.ml; mv stdlib_alpha.mli alpha.mli")))

ocaml/otherlibs/stdlib_alpha/or_null.ml

@@ -0,0 +1,15 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                                 OCaml                                  *)
+(*                                                                        *)
+(*               Diana Kalinichenko, Jane Street, New York                *)
+(*                                                                        *)
+(*   Copyright 2024 Jane Street Group LLC                                 *)
+(*                                                                        *)
+(*   All rights reserved.  This file is distributed under the terms of    *)
+(*   the GNU Lesser General Public License version 2.1, with the          *)
+(*   special exception on linking described in the file LICENSE.          *)
+(*                                                                        *)
+(**************************************************************************)
+
+type ('a : non_null_value) t = 'a option

ocaml/otherlibs/stdlib_alpha/or_null.mli

@@ -0,0 +1,16 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                                 OCaml                                  *)
+(*                                                                        *)
+(*               Diana Kalinichenko, Jane Street, New York                *)
+(*                                                                        *)
+(*   Copyright 2024 Jane Street Group LLC                                 *)
+(*                                                                        *)
+(*   All rights reserved.  This file is distributed under the terms of    *)
+(*   the GNU Lesser General Public License version 2.1, with the          *)
+(*   special exception on linking described in the file LICENSE.          *)
+(*                                                                        *)
+(**************************************************************************)
+
+(** Unboxed option type. Unimplemented. *)
+type ('a : non_null_value) t

ocaml/otherlibs/stdlib_alpha/stdlib_alpha.ml

@@ -0,0 +1 @@
+module Or_null = Or_null

ocaml/otherlibs/stdlib_alpha/stdlib_alpha.mli

@@ -0,0 +1 @@
+module Or_null = Or_null

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

@@ -90,7 +90,7 @@ Line 1, characters 9-33:
              ^^^^^^^^^^^^^^^^^^^^^^^^
 Error: This alias is bound to type 'a -> int
        but is used as an instance of type ('b : void)
-       The layout of 'a -> int is value, because
+       The layout of 'a -> int is non_null_value, because
          it's a function type.
        But the layout of 'a -> int must be a sublayout of void, because
          of the annotation on the type variable 'b.
@@ -104,7 +104,7 @@ Line 1, characters 27-31:
 1 | type t = 'a -> int as (_ : void)
                                ^^^^
 Error: Bad layout annotation:
-         The layout of 'a -> int is value, because
+         The layout of 'a -> int is non_null_value, because
            it's a function type.
          But the layout of 'a -> int must be a sublayout of void, because
            of the annotation on the wildcard _ at line 1, characters 27-31.

ocaml/testsuite/tests/typing-layouts-err-msg/value.ml

@@ -121,7 +121,7 @@ type r : void = {a:string}
 Line 1, characters 0-26:
 1 | type r : void = {a:string}
     ^^^^^^^^^^^^^^^^^^^^^^^^^^
-Error: The layout of type r is value, because
+Error: The layout of type r is non_null_value, because
          it's a boxed record type.
        But the layout of type r must be a sublayout of void, because
          of the annotation on the declaration of the type r.
@@ -133,7 +133,7 @@ type v : void = A of t_value
 Line 1, characters 0-28:
 1 | type v : void = A of t_value
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Error: The layout of type v is value, because
+Error: The layout of type v is non_null_value, because
          it's a boxed variant type.
        But the layout of type v must be a sublayout of void, because
          of the annotation on the declaration of the type v.
@@ -145,7 +145,7 @@ type attr : void = ..
 Line 1, characters 0-21:
 1 | type attr : void = ..
     ^^^^^^^^^^^^^^^^^^^^^
-Error: The layout of type attr is value, because
+Error: The layout of type attr is non_null_value, because
          it's an extensible variant type.
        But the layout of type attr must be a sublayout of void, because
          of the annotation on the declaration of the type attr.
@@ -159,8 +159,8 @@ Line 1, characters 40-45:
                                             ^^^^^
 Error: This expression has type string but an expression was expected of type
          ('a : void)
-       The layout of string is value, because
-         it is the primitive value type string.
+       The layout of string is non_null_value, because
+         it is the primitive non-null value type string.
        But the layout of string must be a sublayout of void, because
          of the annotation on the type variable 'a.
 |}];;
@@ -187,7 +187,7 @@ Line 1, characters 40-45:
                                             ^^^^^
 Error: This expression has type 'b * 'c
        but an expression was expected of type ('a : void)
-       The layout of 'a * 'b is value, because
+       The layout of 'a * 'b is non_null_value, because
          it's a tuple type.
        But the layout of 'a * 'b must be a sublayout of void, because
          of the annotation on the type variable 'a.
@@ -206,7 +206,7 @@ Line 2, characters 36-37:
                                         ^
 Error: This expression has type [ `A of int | `B ]
        but an expression was expected of type 'a t = ('a : void)
-       The layout of [ `A of int | `B ] is value, because
+       The layout of [ `A of int | `B ] is non_null_value, because
          it's a polymorphic variant type.
        But the layout of [ `A of int | `B ] must be a sublayout of void, because
          of the definition of t at line 1, characters 0-22.
@@ -222,7 +222,7 @@ Line 2, characters 31-32:
                                    ^
 Error: This expression has type int -> int
        but an expression was expected of type 'a t = ('a : void)
-       The layout of int -> int is value, because
+       The layout of int -> int is non_null_value, because
          it's a function type.
        But the layout of int -> int must be a sublayout of void, because
          of the definition of t at line 1, characters 0-22.
@@ -248,7 +248,7 @@ Line 4, characters 17-39:
                      ^^^^^^^^^^^^^^^^^^^^^^
 Error: This expression has type (module X_int)
        but an expression was expected of type 'a t = ('a : void)
-       The layout of (module X_int) is value, because
+       The layout of (module X_int) is non_null_value, because
          it's a first-class module type.
        But the layout of (module X_int) must be a sublayout of void, because
          of the definition of t at line 1, characters 0-22.

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

@@ -0,0 +1,129 @@
+(* TEST
+ flags = "-extension-universe alpha";
+ expect;
+*)
+
+module Fake_or_null : sig
+  type ('a : non_null_value) t : value
+
+  val none : 'a t
+  val some : 'a -> 'a t
+end = struct
+  type ('a : non_null_value) t = 'a option
+
+  let none = None
+  let some x = Some x
+end
+;;
+
+[%%expect{|
+module Fake_or_null :
+  sig
+    type ('a : non_null_value) t : value
+    val none : ('a : non_null_value). 'a t
+    val some : ('a : non_null_value). 'a -> 'a t
+  end
+|}]
+
+let _ = Fake_or_null.some (Fake_or_null.none)
+;;
+
+[%%expect{|
+Line 1, characters 26-45:
+1 | let _ = Fake_or_null.some (Fake_or_null.none)
+                              ^^^^^^^^^^^^^^^^^^^
+Error: This expression has type 'a Fake_or_null.t
+       but an expression was expected of type ('b : non_null_value)
+       The layout of 'a Fake_or_null.t is value, because
+         of the definition of t at line 2, characters 2-38.
+       But the layout of 'a Fake_or_null.t must be a sublayout of non_null_value, because
+         of the definition of some at line 5, characters 2-23.
+|}]
+
+
+(* Built-in containers accept nullable values: *)
+
+let _ = [ Fake_or_null.none ]
+
+let _ = [| Fake_or_null.some 3 |]
+
+let _ = [: Fake_or_null.some "test " :]
+
+let _ = Some (Fake_or_null.some 4.2)
+
+let _ = lazy (Fake_or_null.none)
+;;
+
+[%%expect{|
+- : 'a Fake_or_null.t list = [<abstr>]
+- : int Fake_or_null.t array = [|<abstr>|]
+- : string Fake_or_null.t iarray = [:<abstr>:]
+- : float Fake_or_null.t option = Some <abstr>
+- : 'a Fake_or_null.t lazy_t = lazy <abstr>
+|}]
+
+module M1 : sig
+  type 'a t
+
+  val mk : 'a -> 'a t
+end = struct
+  type 'a t = 'a
+
+  let mk x = x
+end
+
+(* CR layouts v3.0: abstract types and type parameters to
+   abstract types should default to non-null: *)
+
+let _ = Fake_or_null.some (M1.mk 2)
+;;
+[%%expect{|
+module M1 : sig type 'a t val mk : 'a -> 'a t end
+Line 14, characters 26-35:
+14 | let _ = Fake_or_null.some (M1.mk 2)
+                               ^^^^^^^^^
+Error: This expression has type int M1.t
+       but an expression was expected of type ('a : non_null_value)
+       The layout of int M1.t is value, because
+         of the definition of t at line 2, characters 2-11.
+       But the layout of int M1.t must be a sublayout of non_null_value, because
+         of the definition of some at line 5, characters 2-23.
+|}]
+
+let _ = M1.mk (Fake_or_null.some 5)
+;;
+
+[%%expect{|
+- : int Fake_or_null.t M1.t = <abstr>
+|}]
+
+let my_id1 x = x
+let my_id2 (x : 'a) = x
+let my_id3 : 'a . 'a -> 'a = fun x -> x
+let my_id4 (type a) (x : a) = x
+;;
+
+[%%expect{|
+val my_id1 : 'a -> 'a = <fun>
+val my_id2 : 'a -> 'a = <fun>
+val my_id3 : 'a -> 'a = <fun>
+val my_id4 : 'a -> 'a = <fun>
+|}]
+
+(* By default, type variables in functions are nullable: *)
+
+let _ = my_id1 (Fake_or_null.some 1)
+
+let _ = my_id2 (Fake_or_null.some 2)
+
+let _ = my_id3 (Fake_or_null.some 3)
+
+let _ = my_id4 (Fake_or_null.some 4)
+;;
+
+[%%expect{|
+- : int Fake_or_null.t = <abstr>
+- : int Fake_or_null.t = <abstr>
+- : int Fake_or_null.t = <abstr>
+- : int Fake_or_null.t = <abstr>
+|}]