Remove -allow-illegal-crossing (ocaml-flambda#3516)

Now that we have [@@unsafe_allow_any_mode_crossing], which is a more precise way
of unsafely mode crossing types, we can remove the -allow-illegal-crossing flag,
which was always intended to be temporary, and all its usages.

---------

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

Author: glittershark

driver/main_args.ml

@@ -759,10 +759,6 @@ in
   \    allows a set of extensions, and every successive universe includes \n\
   \    the previous one."
 
-let mk_allow_illegal_crossing f =
-  "-allow-illegal-crossing", Arg.Unit f,
-  "Type declarations will not be checked along the portability or contention axes"
-
 let mk_dump_dir f =
   "-dump-dir", Arg.String f,
   "redirects any file(s) that would be outputted as a result of other flags\n\
@@ -960,7 +956,6 @@ module type Common_options = sig
   val _extension : string -> unit
   val _no_extension : string -> unit
   val _extension_universe : string -> unit
-  val _allow_illegal_crossing : unit -> unit
   val _noassert : unit -> unit
   val _nolabels : unit -> unit
   val _nostdlib : unit -> unit
@@ -1249,7 +1244,6 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
-    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_for_pack_byt F._for_pack;
     mk_g_byt F._g;
     mk_no_g F._no_g;
@@ -1381,7 +1375,6 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
-    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_noassert F._noassert;
     mk_noinit F._noinit;
     mk_nolabels F._nolabels;
@@ -1476,7 +1469,6 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
-    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_for_pack_opt F._for_pack;
     mk_g_opt F._g;
     mk_no_g F._no_g;
@@ -1669,7 +1661,6 @@ module Make_opttop_options (F : Opttop_options) = struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
-    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_no_float_const_prop F._no_float_const_prop;
     mk_noassert F._noassert;
     mk_noinit F._noinit;
@@ -1778,7 +1769,6 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
-    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_noassert F._noassert;
     mk_nolabels F._nolabels;
     mk_nostdlib F._nostdlib;
@@ -1890,7 +1880,6 @@ module Default = struct
     let _no_extension s = Language_extension.(disable_of_string_exn s)
     let _extension_universe s =
       Language_extension.(set_universe_and_enable_all_of_string_exn s)
-    let _allow_illegal_crossing = set Clflags.allow_illegal_crossing
     let _noassert = set noassert
     let _nolabels = set classic
     let _nostdlib = set no_std_include

driver/main_args.mli

@@ -34,7 +34,6 @@ module type Common_options = sig
   val _extension : string -> unit
   val _no_extension : string -> unit
   val _extension_universe : string -> unit
-  val _allow_illegal_crossing : unit -> unit
   val _noassert : unit -> unit
   val _nolabels : unit -> unit
   val _nostdlib : unit -> unit

otherlibs/stdlib_alpha/capsule.ml

@@ -302,10 +302,12 @@ module Mutex = struct
     ; mutex : M.t
     ; mutable poisoned : bool
     }
+  [@@unsafe_allow_any_mode_crossing]
 
-  (* CR: illegal mode crossing on the current version of the compiler,
-     but should be legal. *)
   type packed : value mod portable uncontended = P : 'k t -> packed
+  [@@unsafe_allow_any_mode_crossing
+    "CR layouts v2.8: illegal mode crossing on the current version of the compiler, but \
+     should be legal."]
 
   let[@inline] name t = t.name
 
@@ -357,8 +359,12 @@ module Rwlock = struct
     ; rwlock : Rw.t
     ; mutable poisoned : bool
     }
+  [@@unsafe_allow_any_mode_crossing]
 
   type packed : value mod portable uncontended = P : 'k t -> packed
+  [@@unsafe_allow_any_mode_crossing
+    "CR layouts v2.8: This can go away once we have proper mode crossing \
+     inference for GADT constructors "]
 
   exception Poisoned
 

otherlibs/stdlib_alpha/capsule.mli

@@ -180,6 +180,9 @@ module Mutex : sig
         the capsule ['k] using {!create_with_mutex}. *)
 
     type packed : value mod portable uncontended = P : 'k t -> packed
+    [@@unsafe_allow_any_mode_crossing
+      "CR layouts v2.8: This can go away once we have proper mode crossing \
+       inference for GADT constructors "]
     (** [packed] is the type of a mutex for some unknown capsule.
         Unpacking one provides a ['k Mutex.t] together with a fresh
         existential type brand for ['k]. *)
@@ -219,6 +222,9 @@ module Rwlock : sig
         the capsule ['k] using {!create_with_rwlock} *)
 
     type packed : value mod portable uncontended = P : 'k t -> packed
+    [@@unsafe_allow_any_mode_crossing
+      "CR layouts v2.8: This can go away once we have proper mode crossing \
+       inference for GADT constructors "]
     (** [packed] is the type of a reader-writer lock for some unknown capsule.
         Unpacking one provides a ['k Rwlock.t] together with a fresh existential type
         brand for ['k]. *)

otherlibs/stdlib_alpha/dune

@@ -24,9 +24,7 @@
    -safe-string
    -strict-formats
    -extension-universe
-   alpha
-   ;; Capsule API can't be defined legally.
-   -allow-illegal-crossing))
+   alpha))
  (ocamlopt_flags
   (:include %{project_root}/ocamlopt_flags.sexp))
  (foreign_stubs

testsuite/tests/capsule-api/rwlock_capsule.ml

@@ -1,6 +1,6 @@
 (* TEST
  include stdlib_alpha;
- flags = "-extension-universe alpha -allow-illegal-crossing";
+ flags = "-extension-universe alpha";
  runtime5;
  { bytecode; }
  { native; }
@@ -19,10 +19,12 @@ type ('a, 'k) _data : value mod portable uncontended = ('a, 'k) Capsule.Data.t
 type _packed :  value mod portable uncontended = Capsule.Rwlock.packed
 
 (* CR: without [with] syntax and mode crossing inference, we need to depend on
-  [allow-illegal-crossing] to determine that 'a myref crosses portabilility.
-  This only holds when 'a also crosses portability *)
+   [@@unsafe_allow_any_mode_crossing] to determine that 'a myref crosses portabilility.
+   This only holds when 'a also crosses portability *)
 
 type 'a myref : value mod portable = { mutable v : 'a}
+[@@unsafe_allow_any_mode_crossing
+  "CR layouts v2.8: This can go away once we have with-kinds"]
 
 
 module RwCell = struct