do not set -json-compilation-database when Yojson is not present

Makefile.generating

@@ -61,6 +61,7 @@ $(CONFIG_FILE): $(CONFIG_FILE).in VERSION share/Makefile.config Makefile.generat
           -e "s|@COMPILATION_UNITS@|$(COMPILATION_UNITS)|" \
           -e "s|@LIBRARY_NAMES@|$(foreach p,$(ALL_LIBRARY_NAMES),\"$p\";)|" \
 	  -e "s|@OPTDOT@|$(OPTDOT)|" \
+          -e "s|@HAS_YOJSON@|$(if $(filter yes,$(HAS_YOJSON)),true,false)|" \
           -e "s|@EXE@|$(EXE)|" \
           $< > $@.tmp
 	@touch -r $@.tmp $<

src/kernel_internals/runtime/config.ml.in

@@ -78,4 +78,6 @@ let preprocessor_keep_comments =
 let compilation_unit_names = [@COMPILATION_UNITS@]
 let library_names = [@LIBRARY_NAMES@]
 
+let has_yojson = @HAS_YOJSON@
+
 let dot = @OPTDOT@

src/kernel_internals/runtime/config.mli

@@ -104,6 +104,11 @@ val dot: string option
     @return [None] if `dot' is not installed.
     @since Carbon-20101201 *)
 
+val has_yojson: bool
+(** [true] if Frama-C has been compiled with yojson support.
+    @since Frama-C+dev
+*)
+
 (*
   Local Variables:
   compile-command: "make -C ../../.."

src/kernel_services/ast_queries/json_compilation_database.ko.ml

@@ -21,9 +21,14 @@
 (**************************************************************************)
 
 let get_flags _ =
-  if Kernel.JsonCompilationDatabase.get () <> "" then begin
-    Kernel.warning ~once:true "ignoring option %s: Frama-C was not compiled \
-                               with the required libraries (yojson)"
-      Kernel.JsonCompilationDatabase.option_name;
+  let db = Kernel.JsonCompilationDatabase.get () in
+  if db <> "" then begin
+    (* If Yojson is disabled, JsonCompilationDatabase prevents any attempt
+       to set the option to a non-empty string. If someone bypasses the
+       mechanism with an unsafe_set, they deserve a fatal error.
+    *)
+    Kernel.fatal
+      "Attempt to parse json compilation database %s \
+       with a Frama-C compiled without Yojson support." db
   end;
   ""

src/kernel_services/plugin_entry_points/kernel.ml

@@ -864,14 +864,27 @@ module JsonCompilationDatabase =
       let option_name = "-json-compilation-database"
       let default = ""
       let arg_name = "path"
-      let help = "when set, preprocessing of each file will include \
-                  corresponding flags (e.g. -I, -D) from the JSON compilation \
-                  database specified by <path>. If <path> is a directory, \
-                  use '<path>/compile_commands.json'. Disabled by default. \
-                  NOTE: this requires Frama-C to have been compiled with \
-                  yojson support."
+      let help =
+        if Fc_config.has_yojson then
+          "when set, preprocessing of each file will include corresponding \
+           flags (e.g. -I, -D) from the JSON compilation database \
+           specified by <path>. If <path> is a directory, use \
+           '<path>/compile_commands.json'. Disabled by default. \
+           NOTE: this requires Frama-C to be compiled with yojson support."
+        else
+          "Unsupported: recompile Frama-C with Yojson library to enable it"
     end)
 
+let () =
+  if not Fc_config.has_yojson then begin
+    JsonCompilationDatabase.add_set_hook
+      (fun _ _ ->
+         warning ~once:true
+           "trying to set -json-compilation-database even though Yojson \
+            is not available. Ignoring argument.";
+         JsonCompilationDatabase.clear ())
+  end
+
 (* ************************************************************************* *)
 (** {2 Customizing Normalization} *)
 (* ************************************************************************* *)