Add Js binary linking mode

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

src/binary_kind.ml

@@ -5,6 +5,7 @@ type t =
   | Exe
   | Object
   | Shared_object
+  | Js
 
 let decode =
   let open Dune_lang.Decoder in
@@ -13,18 +14,20 @@ let decode =
     ; "exe"           , return Exe
     ; "object"        , return Object
     ; "shared_object" , return Shared_object
+    ; "js"            , Syntax.since Stanza.syntax (1, 9) >>> return Js
     ]
 
 let to_string = function
   | C -> "c"
   | Exe -> "exe"
   | Object -> "object"
   | Shared_object -> "shared_object"
+  | Js -> "js"
 
 let pp fmt t =
   Format.pp_print_string fmt (to_string t)
 
 let encode t =
   Dune_lang.unsafe_atom_of_string (to_string t)
 
-let all = [C; Exe; Object; Shared_object]
+let all = [C; Exe; Object; Shared_object; Js]

src/binary_kind.mli

@@ -7,6 +7,7 @@ type t =
   | Exe
   | Object
   | Shared_object
+  | Js
 
 include Dune_lang.Conv with type t := t
 

src/exe.ml

@@ -77,6 +77,8 @@ module Linkage = struct
       | Native , Object        -> ".exe" ^ ctx.ext_obj
       | Byte   , Shared_object -> ".bc"  ^ ctx.ext_dll
       | Native , Shared_object ->          ctx.ext_dll
+      | Byte   , Js            -> ".bc.js"
+      | Native , Js            -> Errors.fail m.loc "Javascript generation only supports bytecode!"
     in
     let flags =
       match m.kind with
@@ -95,7 +97,7 @@ module Linkage = struct
           else
             so_flags_unix
         in
-        match real_mode with
+        begin match real_mode with
         | Native ->
           (* The compiler doesn't pass these flags in native mode. This
              looks like a bug in the compiler. *)
@@ -104,6 +106,8 @@ module Linkage = struct
           @ so_flags
         | Byte ->
           so_flags
+        end
+      | Js -> []
     in
     { ext
     ; mode = real_mode