Clerk: list targets on clerk build without target (#765)

build_system/clerk_driver.ml

@@ -1023,17 +1023,31 @@ let build_cmd =
   let run ninja_init (targets : string list) (ninja_flags : string list) =
     ninja_init ~extra:Seq.empty ~test_flags:[]
     @@ fun _build_dir fix_path nin_file ->
-    let targets =
-      List.map
-        (fun f ->
-          if String.exists (function '/' | '.' -> true | _ -> false) f then
-            fix_path f
-          else f)
-        targets
-    in
-    let ninja_cmd = ninja_cmdline ninja_flags nin_file targets in
-    Message.debug "executing '%s'..." (String.concat " " ninja_cmd);
-    raise (Catala_utils.Cli.Exit_with (run_ninja ~clean_up_env:false ninja_cmd))
+    if targets <> [] then
+      let targets =
+        List.map
+          (fun f ->
+             if String.exists (function '/' | '.' -> true | _ -> false) f then
+               fix_path f
+             else f)
+          targets
+      in
+      let ninja_cmd = ninja_cmdline ninja_flags nin_file targets in
+      Message.debug "executing '%s'..." (String.concat " " ninja_cmd);
+      raise (Catala_utils.Cli.Exit_with (run_ninja ~clean_up_env:false ninja_cmd))
+    else (* List targets command *)
+      let ninja_cmd = ninja_cmdline ninja_flags nin_file ["-t"; "targets"] in
+      let result = File.process_out (List.hd ninja_cmd) (List.tl ninja_cmd) in
+      let targets =
+        String.split_on_char '\n' result |>
+        List.filter_map (fun line ->
+            match String.split_on_char ':' line with
+            | [] | "always" :: _ -> None
+            | target :: _ -> Some target)
+      in
+      Format.eprintf "Available targets:%!";
+      List.iter print_endline targets;
+      0
   in
   let doc =
     "Low-level build command: can be used to forward build targets or options \