Make Dolmen the default frontend

The current plan is for Dolmen to be the default frontend in the next
release. Making the switch as early as possible in the development
branch will help us use it and test it more.

src/bin/common/parse_command.ml

@@ -884,25 +884,19 @@ let parse_output_opt =
 
 
     let frontend =
-      let doc = "Select the parsing and typing frontend." in
+      let doc =
+        "Select the parsing and typing frontend. Support for non-default \
+         frontends is deprecated and will be removed in the next release."
+      in
       let docv = "FTD" in
-      Arg.(value & opt (some string) None &
-           info ["frontend"] ~docv ~docs:s_execution ~doc)
-    in
-
-    (* Use the dolmen frontend by default with --produce-models *)
-    let mk_frontend frontend produce_models =
-      match frontend with
-      | None ->
-        if produce_models then
-          "dolmen"
-        else
-          get_frontend ()
-      | Some frontend -> frontend
+      let deprecated =
+        "this option is deprecated and will be ignored in the \
+         next version"
+      in
+      Arg.(value & opt string "dolmen" &
+           info ["frontend"] ~docv ~docs:s_execution ~doc ~deprecated)
     in
 
-    let frontend = Term.(const mk_frontend $ frontend $ produce_models) in
-
     let dump_models =
       let doc =
         "Display a model each time the result is unknown (implies \

src/lib/util/options.ml

@@ -259,7 +259,7 @@ let output_with_colors = ref false
 let output_with_headers = ref true
 let output_with_formatting = ref true
 let output_with_forced_flush = ref true
-let frontend = ref "legacy"
+let frontend = ref "dolmen"
 let input_format = ref Native
 let infer_input_format = ref true
 let parse_only = ref false