Add attributes to (unsafely) skip jkind check

parsing/builtin_attributes.ml

@@ -504,6 +504,10 @@ let has_unboxed attrs = has_attribute "unboxed" attrs
 
 let has_boxed attrs = has_attribute "boxed" attrs
 
+let has_unsafe_allow_any_kind_in_intf attrs = has_attribute "unsafe_allow_any_kind_in_intf" attrs
+
+let has_unsafe_allow_any_kind_in_impl attrs = has_attribute "unsafe_allow_any_kind_in_impl" attrs
+
 let parse_empty_payload attr =
   match attr.attr_payload with
   | PStr [] -> Some ()
@@ -602,6 +606,15 @@ let zero_alloc_attribute (attr : Parsetree.attribute)  =
         warn_payload attr.attr_loc attr.attr_name.txt
           "Only 'all', 'check', 'check_opt', 'check_all', and 'check_none' are supported")
 
+let attribute_with_ignored_payload name attr =
+  when_attribute_is [name; "ocaml." ^ name] attr ~f:(fun () -> ())
+
+let unsafe_allow_any_kind_in_impl_attribute =
+  attribute_with_ignored_payload "unsafe_allow_any_kind_in_impl"
+
+let unsafe_allow_any_kind_in_intf_attribute =
+  attribute_with_ignored_payload "unsafe_allow_any_kind_in_intf"
+
 let afl_inst_ratio_attribute attr =
   clflags_attribute_with_int_payload attr
     ~name:"afl_inst_ratio" Clflags.afl_inst_ratio
@@ -610,7 +623,8 @@ let parse_standard_interface_attributes attr =
   warning_attribute attr;
   principal_attribute attr;
   noprincipal_attribute attr;
-  nolabels_attribute attr
+  nolabels_attribute attr;
+  unsafe_allow_any_kind_in_intf_attribute attr
 
 let parse_standard_implementation_attributes attr =
   warning_attribute attr;
@@ -621,7 +635,8 @@ let parse_standard_implementation_attributes attr =
   afl_inst_ratio_attribute attr;
   flambda_o3_attribute attr;
   flambda_oclassic_attribute attr;
-  zero_alloc_attribute attr
+  zero_alloc_attribute attr;
+  unsafe_allow_any_kind_in_impl_attribute attr
 
 let has_no_mutable_implied_modalities attrs =
   has_attribute "no_mutable_implied_modalities" attrs

parsing/builtin_attributes.mli

@@ -35,6 +35,8 @@
     - ocaml.tailcall
     - ocaml.tail_mod_cons
     - ocaml.unboxed
+    - ocaml.unsafe_allow_any_kind_in_impl
+    - ocaml.unsafe_allow_any_kind_in_intf
     - ocaml.untagged
     - ocaml.unrolled
     - ocaml.warnerror
@@ -198,6 +200,9 @@ val explicit_arity: Parsetree.attributes -> bool
 val has_unboxed: Parsetree.attributes -> bool
 val has_boxed: Parsetree.attributes -> bool
 
+val has_unsafe_allow_any_kind_in_impl: Parsetree.attributes -> bool
+val has_unsafe_allow_any_kind_in_intf: Parsetree.attributes -> bool
+
 val parse_standard_interface_attributes : Parsetree.attribute -> unit
 val parse_standard_implementation_attributes : Parsetree.attribute -> unit
 

testsuite/tests/typing-layouts/allow_any.ml

@@ -0,0 +1,190 @@
+(* TEST
+ flags = "-extension layouts_beta";
+ expect;
+*)
+
+(* Baseline: if the jkind doesn't match, we should get an error. *)
+module Mismatched_no_attrs : sig
+  type t : float64
+end = struct
+  type t = string
+end
+[%%expect{|
+Lines 3-5, characters 6-3:
+3 | ......struct
+4 |   type t = string
+5 | end
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t = string end
+       is not included in
+         sig type t : float64 end
+       Type declarations do not match:
+         type t = string
+       is not included in
+         type t : float64
+       The layout of the first is value
+         because it is the primitive type string.
+       But the layout of the first must be a sublayout of float64
+         because of the definition of t at line 2, characters 2-18.
+|}]
+
+(* On the other hand, if we set the correct attributes on both the impl and the intf, we
+   shouldn't get an error (though, obviously, this is completely unsound!) *)
+module Mismatched_with_both_attrs : sig
+  type t : float64
+  [@@unsafe_allow_any_kind_in_impl "I love segfaults"]
+end = struct
+  type t = string
+  [@@unsafe_allow_any_kind_in_intf "I love segfaults"]
+end
+[%%expect{|
+module Mismatched_with_both_attrs : sig type t : float64 end
+|}]
+
+(* If we set the attributes but *don't* get a kind mismatch, we ought to be fine *)
+module Matching : sig
+  type t : value
+  [@@unsafe_allow_any_kind_in_impl "I love segfaults"]
+end = struct
+  type t = string
+  [@@unsafe_allow_any_kind_in_intf "I love segfaults"]
+end
+[%%expect{|
+Lines 2-3, characters 2-54:
+2 | ..type t : value
+3 |   [@@unsafe_allow_any_kind_in_impl "I love segfaults"]
+Warning 212 [unnecessary-allow-any-kind]: [@@allow_any_kind_in_intf] and [@@allow_any_kind_in_impl] set on a
+type, but the kind matches. The attributes can be removed.
+
+module Matching : sig type t end
+|}]
+
+(* If the attr is only on the signature we should get an error *)
+module Mismatched_with_attr_on_intf : sig
+  type t : float64
+  [@@unsafe_allow_any_kind_in_impl "I love segfaults"]
+end = struct
+  type t = string
+end
+[%%expect{|
+Lines 4-6, characters 6-3:
+4 | ......struct
+5 |   type t = string
+6 | end
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t = string end
+       is not included in
+         sig type t : float64 end
+       Type declarations do not match:
+         type t = string
+       is not included in
+         type t : float64
+       The layout of the first is value
+         because it is the primitive type string.
+       But the layout of the first must be a sublayout of float64
+         because of the definition of t at lines 2-3, characters 2-54.
+|}]
+
+(* If the attr is only on the struct we should get an error *)
+module Mismatched_with_attr_on_impl : sig
+  type t : float64
+end = struct
+  type t = string
+  [@@unsafe_allow_any_kind_in_intf "I love segfaults"]
+end
+[%%expect{|
+Lines 3-6, characters 6-3:
+3 | ......struct
+4 |   type t = string
+5 |   [@@unsafe_allow_any_kind_in_intf "I love segfaults"]
+6 | end
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t = string end
+       is not included in
+         sig type t : float64 end
+       Type declarations do not match:
+         type t = string
+       is not included in
+         type t : float64
+       The layout of the first is value
+         because it is the primitive type string.
+       But the layout of the first must be a sublayout of float64
+         because of the definition of t at line 2, characters 2-18.
+|}]
+
+(* Some more complex stuff with functors *)
+
+module type S1 = sig
+  type t : value
+end
+
+module type S2 = sig
+  type t : float64
+  [@@unsafe_allow_any_kind_in_impl]
+end
+
+module type S1 = sig
+  type t : value
+  [@@unsafe_allow_any_kind_in_intf]
+end
+
+module F1 (X : S1) : S2 = X
+
+[%%expect{|
+module type S1 = sig type t end
+module type S2 = sig type t : float64 end
+module type S1 = sig type t end
+module F1 : functor (X : S1) -> S2
+|}]
+
+module F2 (X : S2) : S1 = X
+[%%expect{|
+Line 1, characters 26-27:
+1 | module F2 (X : S2) : S1 = X
+                              ^
+Error: Signature mismatch:
+       Modules do not match: sig type t = X.t end is not included in S1
+       Type declarations do not match: type t = X.t is not included in type t
+       The layout of the first is float64
+         because of the definition of t at lines 6-7, characters 2-35.
+       But the layout of the first must be a sublayout of value
+         because of the definition of t at lines 11-12, characters 2-35.
+|}]
+
+(* Non-abstract types can be annotated with [@@unsafe_allow_any_kind_in_intf] too, and get
+   checked against signatures during inclusion. *)
+
+module M1 : sig
+  type t : value = string [@@unsafe_allow_any_kind_in_intf]
+end = struct
+  type t = string
+end
+
+module M2 : S2 = M1
+
+[%%expect{|
+module M1 : sig type t = string end
+module M2 : S2
+|}]
+
+module type S3 = sig
+  type t : value
+  [@@unsafe_allow_any_kind_in_impl]
+end
+
+module M3 : S3 = M1
+(* CR aspsmith: This is somewhat unfortunate, if S3 and M1 are defined far away, but it's
+   unclear how to squash the warning *)
+[%%expect{|
+module type S3 = sig type t end
+Lines 2-3, characters 2-35:
+2 | ..type t : value
+3 |   [@@unsafe_allow_any_kind_in_impl]
+Warning 212 [unnecessary-allow-any-kind]: [@@allow_any_kind_in_intf] and [@@allow_any_kind_in_impl] set on a
+type, but the kind matches. The attributes can be removed.
+
+module M3 : S3
+|}]

typing/includecore.ml

@@ -1338,15 +1338,25 @@ let type_declarations ?(equality = false) ~loc env ~mark name
       rep1 rep2
   in
   let err = match (decl1.type_kind, decl2.type_kind) with
-      (_, Type_abstract _) ->
-       (* Note that [decl2.type_jkind] is an upper bound.
-          If it isn't tight, [decl2] must
-          have a manifest, which we're already checking for equality
-          above. Similarly, [decl1]'s kind may conservatively approximate its
-          jkind, but [check_decl_jkind] will expand its manifest.  *)
-        (match Ctype.check_decl_jkind env decl1 decl2.type_jkind with
-         | Ok _ -> None
-         | Error v -> Some (Jkind v))
+      (_, Type_abstract _) -> begin
+        (* If both the intf has "allow any kind in impl" *and* the impl has "allow any
+           kind in intf", don't check the jkind at all. *)
+        let allow_any =
+          Builtin_attributes.has_unsafe_allow_any_kind_in_impl decl2.type_attributes
+          && Builtin_attributes.has_unsafe_allow_any_kind_in_intf decl1.type_attributes
+        in
+        (* Note that [decl2.type_jkind] is an upper bound. If it isn't tight, [decl2] must
+           have a manifest, which we're already checking for equality above. Similarly,
+           [decl1]'s kind may conservatively approximate its jkind, but [check_decl_jkind]
+           will expand its manifest. *)
+        match Ctype.check_decl_jkind env decl1 decl2.type_jkind  with
+        | Ok _ ->
+          (if allow_any
+           then Location.prerr_warning decl2.type_loc (Warnings.Unnecessary_allow_any_kind));
+          None
+        | Error _ when allow_any -> None
+        | Error v -> Some (Jkind v)
+      end
     | (Type_variant (cstrs1, rep1), Type_variant (cstrs2, rep2)) ->
         if mark then begin
           let mark usage cstrs =

utils/warnings.ml

@@ -129,6 +129,7 @@ type t =
   | Unchecked_zero_alloc_attribute          (* 199 *)
   | Unboxing_impossible                     (* 210 *)
   | Mod_by_top of string                    (* 211 *)
+  | Unnecessary_allow_any_kind              (* 212 *)
 
 (* If you remove a warning, leave a hole in the numbering.  NEVER change
    the numbers of existing warnings.
@@ -217,6 +218,7 @@ let number = function
   | Unchecked_zero_alloc_attribute -> 199
   | Unboxing_impossible -> 210
   | Mod_by_top _ -> 211
+  | Unnecessary_allow_any_kind -> 212
 ;;
 (* DO NOT REMOVE the ;; above: it is used by
    the testsuite/ests/warnings/mnemonics.mll test to determine where
@@ -592,6 +594,11 @@ let descriptions = [
     names = ["mod-by-top"];
     description = "Including the top-most element of an axis in a kind's modifiers is a no-op.";
     since = since 4 14 };
+  { number = 212;
+    names = ["unnecessary-allow-any-kind"];
+    description = "[@@unsafe_allow_any_kind_in_{impl,intf}] attributes included \
+                   on a type and a signature with matching kinds";
+    since = since 5 1 };
 ]
 
 let name_to_number =
@@ -1235,6 +1242,10 @@ let message = function
         "%s is the top-most modifier.\n\
          Modifying by a top element is a no-op."
         modifier
+  | Unnecessary_allow_any_kind ->
+    Printf.sprintf
+      "[@@allow_any_kind_in_intf] and [@@allow_any_kind_in_impl] set on a \n\
+       type, but the kind matches. The attributes can be removed."
 ;;
 
 let nerrors = ref 0

utils/warnings.mli

@@ -135,6 +135,7 @@ type t =
   | Unchecked_zero_alloc_attribute          (* 199 *)
   | Unboxing_impossible                     (* 210 *)
   | Mod_by_top of string                    (* 211 *)
+  | Unnecessary_allow_any_kind              (* 212 *)
 
 type alert = {kind:string; message:string; def:loc; use:loc}