Construct domain executor only on main domain

lib/Domain_compat.mli

@@ -0,0 +1 @@
+val is_main_domain : unit -> bool

lib/Domain_compat_v4.ml

@@ -0,0 +1 @@
+let is_main_domain () = true

lib/Domain_compat_v5.ml

@@ -0,0 +1 @@
+let is_main_domain = Domain.is_main_domain

lib/Rust_async.ml

@@ -21,6 +21,10 @@ module Runtime = struct
   ;;
 
   let current () =
+    if not (Domain_compat.is_main_domain ())
+    then
+      failwith
+        "Initializing Rust_async executor from non-main domain is not going to work well";
     match !current with
     | Some executor -> executor
     | None ->
@@ -48,5 +52,7 @@ let () =
       fut
       (fun value -> Stubs.Future.resolve wrapper value)
       (fun exn -> Stubs.Future.reject wrapper (Printexc.to_string exn));
-    wrapper)
+    wrapper);
+  (* Ensure we initialize the executor during module init *)
+  Runtime.current () |> ignore
 ;;

lib/dune

@@ -2,6 +2,10 @@
  (name rust_async)
  (public_name rust-async)
  (libraries lwt.unix ocaml-rs-smartptr rust-staticlib-virtual)
+ (modules
+  :standard
+  \
+  ("Domain_compat_v4" "Domain_compat_v5"))
  (flags (-linkall))
  (preprocess
   (pps lwt_ppx)))
@@ -10,6 +14,18 @@
  (action
   (copy %{workspace_root}/stubs-gen/Ocaml_lwt_interop.ml Stubs.ml.new)))
 
+(rule
+ (action
+  (copy Domain_compat_v4.ml Domain_compat.ml))
+ (enabled_if
+  (< %{ocaml_version} 5.00)))
+
+(rule
+ (action
+  (copy Domain_compat_v5.ml Domain_compat.ml))
+ (enabled_if
+  (>= %{ocaml_version} 5.00)))
+
 (rule
  (alias runtest)
  (action