Proof of concept: support directives.

Allow to specify directives emitted verbatim at the top of generated js files.

See #5840

Author: cristianoc

jscomp/common/js_config.ml

@@ -29,6 +29,8 @@ type jsx_module = React
 type jsx_mode = Classic | Automatic
 
 let no_version_header = ref false
+
+let directives = ref []
 let cross_module_inline = ref false
 let diagnose = ref false
 

jscomp/common/js_config.mli

@@ -32,6 +32,9 @@ type jsx_mode = Classic | Automatic
 val no_version_header : bool ref
 (** set/get header *)
 
+val directives : string list ref
+(** directives printed verbatims just after the version header *)
+
 (** return [package_name] and [path] 
     when in script mode: 
 *)

jscomp/core/js_dump_program.ml

@@ -105,6 +105,10 @@ let pp_deps_program ~(output_prefix : string)
   if not !Js_config.no_version_header then (
     P.string f Bs_version.header;
     P.newline f);
+  !Js_config.directives |> List.iter (fun prim ->
+    P.string f prim;
+    P.newline f);
+
   if deps_program_is_empty program then P.string f empty_explanation
     (* This is empty module, it won't be referred anywhere *)
   else

jscomp/frontend/bs_builtin_ppx.ml

@@ -431,6 +431,10 @@ let structure_item_mapper (self : mapper) (str : Parsetree.structure_item) :
                   [ { pvb_pat; pvb_expr; pvb_attributes; pvb_loc } ] );
           })
   | Pstr_attribute ({ txt = "bs.config" | "config" }, _) -> str
+  | Pstr_attribute ({ txt = "directive" },
+     PStr [ { pstr_desc = Pstr_eval ({ pexp_desc = Pexp_constant (Pconst_string (d, _)) }, _) } ]) ->
+      Js_config.directives := d :: !Js_config.directives;
+      str
   | _ -> default_mapper.structure_item self str
 
 let local_module_name =

jscomp/test/build.ninja

@@ -0,0 +1,10 @@
+first directive;
+second directive;
+'use strict';
+
+var Belt_Array = require("../../lib/js/belt_Array.js");
+
+var a = Belt_Array.forEach;
+
+exports.a = a;
+/* No side effect */

jscomp/test/directives.js

@@ -0,0 +1,5 @@
+@@directive("first directive;")
+@@directive("second directive;")
+
+let a = Belt.Array.forEach
+