Restore -vmthreads flag as an error (#2312)

This GPR restores -vmthread with an adapted version of the deprecation message as an error message and also keeps the use_vmthreads part of ppx contexts.

* Partially revert #2289
* Convert -vmthread to an error
* Neuter use_vmthreads in ppx context
* Remove Clflags.use_vmthreads

driver/main.ml

@@ -21,6 +21,12 @@ let usage = "Usage: ocamlc <options> <files>\nOptions are:"
 (* Error messages to standard error formatter *)
 let ppf = Format.err_formatter
 
+let vmthread_removed_message = "\
+The -vmthread argument of ocamlc is no longer supported\n\
+since OCaml 4.09.0.  Please switch to system threads, which have the\n\
+same API. Lightweight threads with VM-level scheduling are provided by\n\
+third-party libraries such as Lwt, but with a different API."
+
 module Options = Main_args.Make_bytecomp_options (struct
   let set r () = r := true
   let unset r () = r := false
@@ -102,6 +108,7 @@ module Options = Main_args.Make_bytecomp_options (struct
   let _strict_formats = set strict_formats
   let _no_strict_formats = unset strict_formats
   let _thread = set use_threads
+  let _vmthread = fun () -> fatal vmthread_removed_message
   let _unboxed_types = set unboxed_types
   let _no_unboxed_types = unset unboxed_types
   let _unsafe = set unsafe

driver/main_args.ml

@@ -562,6 +562,11 @@ let mk_no_version f =
   "-no-version", Arg.Unit f, " Do not print version at startup"
 ;;
 
+let mk_vmthread f =
+  "-vmthread", Arg.Unit f,
+  "  (no longer supported)"
+;;
+
 let mk_vnum f =
   "-vnum", Arg.Unit f, " Print version number and exit"
 ;;
@@ -958,6 +963,7 @@ module type Bytecomp_options = sig
   val _dllib : string -> unit
   val _dllpath : string -> unit
   val _make_runtime : unit -> unit
+  val _vmthread : unit -> unit
   val _use_runtime : string -> unit
 
   val _dinstr : unit -> unit
@@ -1060,6 +1066,7 @@ module type Ocamldoc_options = sig
   val _thread : unit -> unit
   val _v : unit -> unit
   val _verbose : unit -> unit
+  val _vmthread : unit -> unit
 end
 
 module type Arg_list = sig
@@ -1146,6 +1153,7 @@ struct
     mk_verbose F._verbose;
     mk_version F._version;
     mk__version F._version;
+    mk_vmthread F._vmthread;
     mk_vnum F._vnum;
     mk_w F._w;
     mk_warn_error F._warn_error;
@@ -1521,6 +1529,7 @@ struct
     mk_verbose F._verbose;
     mk_version F._version;
     mk__version F._version;
+    mk_vmthread F._vmthread;
     mk_vnum F._vnum;
     mk_w F._w;
     mk__ F.anonymous;

driver/main_args.mli

@@ -140,6 +140,7 @@ module type Bytecomp_options = sig
   val _dllib : string -> unit
   val _dllpath : string -> unit
   val _make_runtime : unit -> unit
+  val _vmthread : unit -> unit
   val _use_runtime : string -> unit
 
   val _dinstr : unit -> unit
@@ -241,6 +242,7 @@ module type Ocamldoc_options = sig
   val _thread : unit -> unit
   val _v : unit -> unit
   val _verbose : unit -> unit
+  val _vmthread : unit -> unit
 end;;
 
 module type Arg_list = sig

ocamldoc/odoc_args.ml

@@ -227,6 +227,7 @@ module Options = Main_args.Make_ocamldoc_options(struct
   let _strict_formats = set Clflags.strict_formats
   let _no_strict_formats = unset Clflags.strict_formats
   let _thread = set Clflags.use_threads
+  let _vmthread = ignore
   let _unboxed_types = set Clflags.unboxed_types
   let _no_unboxed_types = unset Clflags.unboxed_types
   let _unsafe () = assert false

parsing/ast_mapper.ml

@@ -803,6 +803,7 @@ module PpxContext = struct
         lid "for_package",  make_option make_string !Clflags.for_package;
         lid "debug",        make_bool !Clflags.debug;
         lid "use_threads",  make_bool !Clflags.use_threads;
+        lid "use_vmthreads", make_bool false;
         lid "recursive_types", make_bool !Clflags.recursive_types;
         lid "principal", make_bool !Clflags.principal;
         lid "transparent_modules", make_bool !Clflags.transparent_modules;
@@ -876,6 +877,9 @@ module PpxContext = struct
           Clflags.debug := get_bool payload
       | "use_threads" ->
           Clflags.use_threads := get_bool payload
+      | "use_vmthreads" ->
+          if get_bool payload then
+            raise_errorf "Internal error: vmthreads not supported after 4.09.0"
       | "recursive_types" ->
           Clflags.recursive_types := get_bool payload
       | "principal" ->

tools/ocamlcp.ml

@@ -94,6 +94,7 @@ module Options = Main_args.Make_bytecomp_options (struct
   let _strict_formats = ignore
   let _no_strict_formats = ignore
   let _thread = ignore
+  let _vmthread = ignore
   let _unboxed_types = ignore
   let _no_unboxed_types = ignore
   let _unsafe = ignore