Support layout annotations (ocaml-flambda#1417)

* Add layout annotations to the language.

This commit also includes various improvements to jane-syntax.

Documentation coming in a separate commit.

This commit has been rebased to include the following
commits:

Pprintast now depends on Printast

Initial implementation of layout annotations

This was taken from ccasin/ocaml-jst#1,
but rebased and cleaned. There are a handful of failing tests,
still, but they're all newly introduced in this patch.

Rename ~reason to ~context for annotations (only)

Move layout annotations into aliases

This now prevents e.g. [(int : immediate)] but allows
[int as ('a : immediate)], as discussed with the type-systems
group.

Add control over printing verbosity

Move alias over to jane-syntax architecture

Move variable layout annotation into jane-syntax

Support for annotations on _

Check layout annotation during conversion

Handle type parameters uniformly

Convert Ptyp_poly to use jane-syntax architecture

Comments from Antal

Move annotations on extension constructors into JS

Combine unboxed constants with other layouts

Newtypes are now in jane-syntax.

Constructor declarations in jane-syntax

Make testsuite pass

Some minor cleanup of old XXXs.

Some cleanup from review

A little cleanup in Jane_syntax from review

Fix compilation with dune-based build

Incorporate test case from old PR, improving msg

Respond to Antal's review

More review fixes

Move test that now passes

Responses to comments

rename test

Remove "assert false" in favor of "Misc.fatal_error"

Add comment about generic parameters

transl_type_param_layout --> get_type_param_layout

Rename error constructors

Remove module that will interfere with merging

* Remove call to make_params in enter_type

* Keep minimizer compatible

* Format chameleon

chamelon/compat.jst.ml

@@ -234,3 +234,12 @@ let mk_value_binding ~vb_pat ~vb_expr ~vb_attributes =
     vb_loc = Location.none;
     vb_sort = Layouts.Sort.value;
   }
+
+let mkTtyp_any = Ttyp_var (None, None)
+let mkTtyp_var s = Ttyp_var (Some s, None)
+
+let is_type_name_used desc typ_name =
+  match desc with
+  | Ttyp_alias (_, Some s, _) -> s = typ_name
+  | Ttyp_constr (_, li, _) -> Longident.last li.txt = typ_name
+  | _ -> false

chamelon/compat.mli

@@ -51,6 +51,10 @@ val mkTexp_match :
   expression * computation case list * partial ->
   expression_desc
 
+val mkTtyp_any : core_type_desc
+val mkTtyp_var : string -> core_type_desc
+val is_type_name_used : core_type_desc -> string -> bool
+
 type matched_expression_desc =
   | Texp_ident of
       Path.t

chamelon/compat.upstream.ml

@@ -154,3 +154,12 @@ let mk_constructor_description cstr_name =
 
 let mk_value_binding ~vb_pat ~vb_expr ~vb_attributes =
   { vb_pat; vb_expr; vb_attributes; vb_loc = Location.none }
+
+let mkTtyp_any = Ttyp_any
+let mkTtyp_var s = Ttyp_var s
+
+let is_type_name_used desc typ_name =
+  match desc with
+  | Ttyp_alias (_, s) -> s = typ_name
+  | Ttyp_constr (_, li, _) -> Longident.last li.txt = typ_name
+  | _ -> false

chamelon/minimizer/dummy.ml

@@ -10,7 +10,7 @@ let dummy_type_expr = newty2 ~level:0 (mkTvar (Some "a"))
 
 let dummy_core_type =
   {
-    ctyp_desc = Ttyp_any;
+    ctyp_desc = mkTtyp_any;
     ctyp_type = dummy_type_expr;
     ctyp_env = Env.empty;
     ctyp_loc = Location.none;
@@ -19,7 +19,7 @@ let dummy_core_type =
 
 let a_typ =
   {
-    ctyp_desc = Ttyp_var "a";
+    ctyp_desc = mkTtyp_var "a";
     ctyp_type = newty2 ~level:0 (mkTvar (Some "a"));
     ctyp_env = Env.empty;
     ctyp_loc = Location.none;

chamelon/minimizer/removedeadcode.ml

@@ -40,11 +40,7 @@ let is_used_typ str typ_name =
       Tast_mapper.default with
       typ =
         (fun mapper ct ->
-          (match ct.ctyp_desc with
-          | Ttyp_alias (_, s) -> if s = typ_name then is_used := true
-          | Ttyp_constr (_, li, _) ->
-              if Longident.last li.txt = typ_name then is_used := true
-          | _ -> ());
+          if is_type_name_used ct.ctyp_desc typ_name then is_used := true;
           if not !is_used then Tast_mapper.default.typ mapper ct else ct);
       expr =
         (fun mapper e ->