Allow .mli only modules

Closes #9

src/gen_rules.ml

@@ -301,14 +301,17 @@ module Gen(P : Params) = struct
   end
 
   let all_rules = ref []
-  let known_targets_by_dir_so_far = ref Path.Map.empty
+  let known_targets_by_src_dir_so_far = ref Path.Map.empty
 
   let add_rule build =
     let rule = Build_interpret.Rule.make build in
     all_rules := rule :: !all_rules;
-    known_targets_by_dir_so_far :=
+    known_targets_by_src_dir_so_far :=
-      List.fold_left rule.targets ~init:!known_targets_by_dir_so_far ~f:(fun acc target ->
+      List.fold_left rule.targets ~init:!known_targets_by_src_dir_so_far
-        let path = Build_interpret.Target.path target in
+        ~f:(fun acc target ->
+          match Path.extract_build_context (Build_interpret.Target.path target) with
+          | None -> acc
+          | Some (_, path) ->
             let dir = Path.parent path in
             let fn = Path.basename path in
             let files =
@@ -324,7 +327,7 @@ module Gen(P : Params) = struct
       | None -> String_set.empty
       | Some dir -> File_tree.Dir.files dir
     in
-    match Path.Map.find src_path !known_targets_by_dir_so_far with
+    match Path.Map.find src_path !known_targets_by_src_dir_so_far with
     | None -> sources
     | Some set -> String_set.union sources set
 
@@ -1454,11 +1457,25 @@ module Gen(P : Params) = struct
     let impls = parse_one_set ml_files  in
     let intfs = parse_one_set mli_files in
     String_map.merge impls intfs ~f:(fun name ml_fname mli_fname ->
+      let ml_fname =
         match ml_fname with
         | None ->
-        die "module %s in %s doesn't have a corresponding .ml file"
+          let mli_fname = Option.value_exn mli_fname in
+          let ml_fname = String.sub mli_fname ~pos:0 ~len:(String.length mli_fname - 1) in
+          Format.eprintf
+            "@{<warning>Warning@}: module %s in %s doesn't have a corresponding\
+             .ml file.\n\
+             I'm setting up a rule for copying %s to %s.\n"
             name (Path.to_string dir)
-      | Some ml_fname ->
+            mli_fname ml_fname;
+          let dir = Path.append ctx.build_dir dir in
+          add_rule
+            (Build.copy
+               ~src:(Path.relative dir mli_fname)
+               ~dst:(Path.relative dir ml_fname));
+          ml_fname
+        | Some ml_fname -> ml_fname
+      in
       Some
         { Module.
           name
@@ -1472,44 +1489,42 @@ module Gen(P : Params) = struct
      +-----------------------------------------------------------------+ *)
 
   let rules { src_dir; ctx_dir; stanzas } =
-    let files = lazy (
+    (* Interpret user rules and other simple stanzas first in order to populate the known
-      let src_files =
+       target table, which is needed for guessing the list of modules. *)
-        match File_tree.find_dir P.file_tree src_dir with
+    List.iter stanzas ~f:(fun stanza ->
-        | None -> String_set.empty
+      let dir = ctx_dir in
-        | Some dir -> File_tree.Dir.files dir
+      match (stanza : Stanza.t) with
-      in
+      | Rule         rule -> user_rule         rule ~dir
-      let files_produced_by_rules =
+      | Ocamllex     conf -> ocamllex_rules    conf ~dir
-        List.concat_map stanzas ~f:(fun stanza ->
+      | Ocamlyacc    conf -> ocamlyacc_rules   conf ~dir
+      | Alias        alias -> alias_rules alias ~dir
+      | Library _ | Executables _ | Provides _ | Install _ -> ());
+    let files = lazy (
+      let files = sources_and_targets_known_so_far ~src_path:src_dir in
+      (* Manually add files generated by the (select ...) dependencies since we haven't
+         interpreted libraries and executables yet. *)
+      List.fold_left stanzas ~init:files ~f:(fun acc stanza ->
         match (stanza : Stanza.t) with
-          | Rule      rule -> rule.targets
-          | Ocamllex  conf -> List.map conf.names ~f:(fun name -> name ^ ".ml")
-          | Ocamlyacc conf -> List.concat_map conf.names ~f:(fun name ->
-            [ name ^ ".ml"; name ^ ".mli" ])
         | Library { buildable; _ } | Executables { buildable; _ } ->
-            List.filter_map buildable.libraries ~f:(function
+          List.fold_left buildable.libraries ~init:acc ~f:(fun acc dep ->
-              | Direct _ -> None
+            match (dep : Jbuild_types.Lib_dep.t) with
-              | Select s -> Some s.result_fn)
+            | Direct _ -> acc
-          | _ -> [])
+            | Select s -> String_set.add s.result_fn acc)
-        |> String_set.of_list
+        | _ -> acc)
-      in
+    ) in
-      String_set.union src_files files_produced_by_rules)
-    in
     let all_modules = lazy (
       guess_modules ~dir:src_dir
         ~files:(Lazy.force files))
     in
-    let some x = Some x in
-    let none () = None in
     List.filter_map stanzas ~f:(fun stanza ->
       let dir = ctx_dir in
       match (stanza : Stanza.t) with
-      | Library      lib  -> library_rules     lib  ~dir ~all_modules:(Lazy.force all_modules) ~files:(Lazy.force files) |> some
+      | Library lib  ->
-      | Executables  exes -> executables_rules exes ~dir ~all_modules:(Lazy.force all_modules) |> some
+        Some (library_rules lib ~dir
-      | Rule         rule -> user_rule         rule ~dir |> none
+                ~all_modules:(Lazy.force all_modules) ~files:(Lazy.force files))
-      | Ocamllex     conf -> ocamllex_rules    conf ~dir |> none
+      | Executables  exes ->
-      | Ocamlyacc    conf -> ocamlyacc_rules   conf ~dir |> none
+        Some (executables_rules exes ~dir ~all_modules:(Lazy.force all_modules))
-      | Alias        alias -> alias_rules alias ~dir |> none
+      | _ -> None)
-      | Provides _ | Install _ -> () |> none)
     |> merge_dot_merlin ~dir:ctx_dir
 
   let () = List.iter P.stanzas ~f:rules