[flow][refactor][EZ] Only return one copy of builtins from Merge_js.merge_lib_files

Summary:
In the case of `EmptyMasterContext`, the builtins will be empty any ways, in the case of `NonEmptyMasterContext`, the first returned builtins from `Merge_js.merge_lib_files` will be the same builtins in `NonEmptyMasterContext`.

Therefore, we can alwaus ignore the builtin from the first returned builtins from `Merge_js.merge_lib_files`. Let's just remove that return instead.

Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D70495943

fbshipit-source-id: 2d189e13c95da3e9bf1dddc7be0ab2714f230d8f

Author: SamChou19815

src/flow_dot_js.ml

@@ -83,7 +83,7 @@ let load_lib_files files =
       locs_to_dirtify = [];
     }
   in
-  let (_, _, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
+  let (_, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
   master_cx
 
 let stub_metadata ~root ~checked =

src/services/inference/init_js.ml

@@ -43,34 +43,31 @@ let load_lib_files ~ccx ~options ~reader files =
          (true, ErrorSet.empty, [])
   in
   let (builtin_exports, master_cx, cx_opt) =
-    if ok then
+    if ok then (
       let sig_opts = Type_sig_options.builtin_options options in
-      let (builtins, builtin_errors, master_cx) = Merge_js.merge_lib_files ~sig_opts ordered_asts in
-      let cx_opt =
-        match master_cx with
-        | Context.EmptyMasterContext -> None
-        | Context.NonEmptyMasterContext { builtin_leader_file_key; _ } ->
-          let metadata =
-            Context.(
-              let metadata = metadata_of_options options in
-              { metadata with checked = false }
-            )
-          in
-          let mk_builtins = Merge_js.mk_builtins metadata master_cx in
-          let cx =
-            Context.make
-              ccx
-              metadata
-              builtin_leader_file_key
-              (lazy (ALoc.empty_table builtin_leader_file_key))
-              (fun mref -> Context.MissingModule mref)
-              mk_builtins
-          in
-          Context.reset_errors cx builtin_errors;
-          Some cx
-      in
-      (Exports.of_builtins builtins, master_cx, cx_opt)
-    else
+      let (builtin_errors, master_cx) = Merge_js.merge_lib_files ~sig_opts ordered_asts in
+      match master_cx with
+      | Context.EmptyMasterContext -> (Exports.empty, Context.EmptyMasterContext, None)
+      | Context.NonEmptyMasterContext { builtin_leader_file_key; builtins; _ } ->
+        let metadata =
+          Context.(
+            let metadata = metadata_of_options options in
+            { metadata with checked = false }
+          )
+        in
+        let mk_builtins = Merge_js.mk_builtins metadata master_cx in
+        let cx =
+          Context.make
+            ccx
+            metadata
+            builtin_leader_file_key
+            (lazy (ALoc.empty_table builtin_leader_file_key))
+            (fun mref -> Context.MissingModule mref)
+            mk_builtins
+        in
+        Context.reset_errors cx builtin_errors;
+        (Exports.of_builtins builtins, master_cx, Some cx)
+    ) else
       (Exports.empty, Context.EmptyMasterContext, None)
   in
   Lwt.return (ok, master_cx, cx_opt, errors, builtin_exports)

src/typing/__tests__/type_hint_test.ml

@@ -181,7 +181,7 @@ end = struct
         locs_to_dirtify = [];
       }
     in
-    let (_, _, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
+    let (_, master_cx) = Merge_js.merge_lib_files ~sig_opts asts in
     master_cx
 
   let master_cx_ref = ref None

src/typing/merge_js.ml

@@ -901,11 +901,10 @@ let merge_lib_files ~sig_opts ordered_asts =
     |> Flow_error.ErrorSet.of_list
   in
   match ordered_asts with
-  | [] -> (builtins, builtin_errors, Context.EmptyMasterContext)
+  | [] -> (builtin_errors, Context.EmptyMasterContext)
   | fst_ast :: _ ->
     let builtin_leader_file_key = Base.Option.value_exn (fst_ast |> fst |> Loc.source) in
-    ( builtins,
-      builtin_errors,
+    ( builtin_errors,
       Context.NonEmptyMasterContext { builtin_leader_file_key; builtin_locs; builtins }
     )
 

src/typing/merge_js.mli

@@ -28,8 +28,6 @@ val copy_into :
 val merge_lib_files :
   sig_opts:Type_sig_options.t ->
   (Loc.t, Loc.t) Flow_ast.Program.t list ->
-  Type_sig_collections.Locs.index Packed_type_sig.Builtins.t
-  * Flow_error.ErrorSet.t
-  * Context.master_context
+  Flow_error.ErrorSet.t * Context.master_context
 
 val mk_builtins : Context.metadata -> Context.master_context -> Context.t -> Builtins.t