Don't call the archiver/librarian for empty .cmxa

Changes

@@ -648,8 +648,10 @@ OCaml 4.11
   avoiding a problem with x87 excess precision in Float.round.
   (Xavier Leroy, review by Sébastien Hinderer)
 
-- #9011: Allow linking .cmxa files with no units on MSVC by not requiring the
-  .lib file to be present.
+* #9011: Do not create .a/.lib files when creating a .cmxa with no modules.
+  macOS ar doesn't support creating empty .a files (#1094) and MSVC doesn't
+  permit .lib files to contain no objects. When linking with a .cmxa containing
+  no modules, it is now not an error for there to be no .a/.lib file.
   (David Allsopp, report by Dimitry Bely, review by Xavier Leroy)
 
 - #9064: Relax the level handling when unifying row fields

asmcomp/asmlink.ml

@@ -212,13 +212,12 @@ let scan_file obj_name (tolink, objfiles) = match read_file obj_name with
                reqd)
           infos.lib_units tolink
       and objfiles =
-        if Config.ccomp_type = "msvc"
-        && infos.lib_units = []
+        if infos.lib_units = []
         && not (Sys.file_exists (object_file_name obj_name)) then
-          (* MSVC doesn't support empty .lib files, so there shouldn't be one
-             if the .cmxa contains no units. The file_exists check is added to
-             be ultra-defensive for the case where a user has manually added
-             things to the .lib file *)
+          (* MSVC doesn't support empty .lib files, and macOS struggles to make
+             them (#6550), so there shouldn't be one if the .cmxa contains no
+             units. The file_exists check is added to be ultra-defensive for the
+             case where a user has manually added things to the .a/.lib file *)
           objfiles
         else
           obj_name :: objfiles

utils/ccomp.ml

@@ -128,36 +128,18 @@ let compile_file ?output ?(opt="") ?stable_name name =
   then display_msvc_output file name;
   exit
 
-let macos_create_empty_archive ~quoted_archive =
-  let result =
-    command (Printf.sprintf "%s rc %s /dev/null" Config.ar quoted_archive)
-  in
-  if result <> 0 then result
-  else
-    let result =
-      command (Printf.sprintf "%s %s 2> /dev/null" Config.ranlib quoted_archive)
-    in
-    if result <> 0 then result
-    else
-      command (Printf.sprintf "%s d %s /dev/null" Config.ar quoted_archive)
-
 let create_archive archive file_list =
   Misc.remove_file archive;
   let quoted_archive = Filename.quote archive in
-  match Config.ccomp_type with
-    "msvc" ->
-      command(Printf.sprintf "link /lib /nologo /out:%s %s"
-                             quoted_archive (quote_files file_list))
-  | _ ->
-      assert(String.length Config.ar > 0);
-      let is_macosx =
-        match Config.system with
-        | "macosx" -> true
-        | _ -> false
-      in
-      if is_macosx && file_list = [] then  (* PR#6550 *)
-        macos_create_empty_archive ~quoted_archive
-      else
+  if file_list = [] then
+    0 (* Don't call the archiver: #6550/#1094/#9011 *)
+  else
+    match Config.ccomp_type with
+      "msvc" ->
+        command(Printf.sprintf "link /lib /nologo /out:%s %s"
+                               quoted_archive (quote_files file_list))
+    | _ ->
+        assert(String.length Config.ar > 0);
         let r1 =
           command(Printf.sprintf "%s rc %s %s"
                   Config.ar quoted_archive (quote_files file_list)) in