Add a warning when generating source maps if there is no debugging information

Author: vouillon

compiler/bin-wasm_of_ocaml/compile.ml

@@ -262,7 +262,20 @@ let run
   if times () then Format.eprintf "  parsing js: %a@." Timer.print t1;
   if times () then Format.eprintf "Start parsing...@.";
   let need_debug = enable_source_maps || Config.Flag.debuginfo () in
+  let check_debug (one : Parse_bytecode.one) =
+    if (not runtime_only)
+       && enable_source_maps
+       && Parse_bytecode.Debug.is_empty one.debug
+       && not (Code.is_empty one.code)
+    then
+      warn
+        "Warning: '--source-map' is enabled but the bytecode program was compiled with \
+         no debugging information.\n\
+         Warning: Consider passing '-g' option to ocamlc.\n\
+         %!"
+  in
   let output (one : Parse_bytecode.one) ~unit_name ch =
+    check_debug one;
     let code = one.code in
     let standalone = Option.is_none unit_name in
     let live_vars, in_cps, p, debug =