Deprecate -vmthread

Changes

@@ -112,6 +112,9 @@ Working version
   (Christopher Zimmermann, review by Xavier Leroy, Damien Doligez, David
    Allsopp, David Sheets)
 
+- GPR#2038: Deprecate vm threads
+  (Jérémie Dimino)
+
 ### Compiler user-interface and warnings:
 
 - PR#6416, GPR#1120: unique printed names for identifiers

driver/main.ml

@@ -142,6 +142,12 @@ module Options = Main_args.Make_bytecomp_options (struct
   let anonymous = anonymous
 end)
 
+let vmthread_deprecated_message = "\
+The -vmthread argument of ocamlc is deprecated\n\
+and will soon be abandoned. For VM-level scheduling, consider using a\n\
+third-party library such as Lwt. Otherwise, please switch to system\n\
+threads."
+
 let main () =
   Clflags.add_arguments __LOC__ Options.list;
   Clflags.add_arguments __LOC__
@@ -151,6 +157,8 @@ let main () =
     readenv ppf Before_args;
     Clflags.parse_arguments anonymous usage;
     Compmisc.read_color_env ();
+    if !Clflags.use_vmthreads then
+      Location.deprecated Location.none vmthread_deprecated_message;
     begin try
       Compenv.process_deferred_actions
         (ppf,

driver/main_args.ml

@@ -553,8 +553,8 @@ let mk_no_version f =
 
 let mk_vmthread f =
   "-vmthread", Arg.Unit f,
-  " Generate code that supports the threads library with VM-level\n\
-  \     scheduling"
+  " (deprecated) Generate code that supports the threads library\n\
+  \     with VM-level scheduling"
 ;;
 
 let mk_vnum f =