fixes

src/build.ml

@@ -104,7 +104,10 @@ let dyn_paths t = Dyn_paths t
 let contents p = Contents p
 let lines_of p = Lines_of p
 
-let fail x = Fail x
+let fail ?targets x =
+  match targets with
+  | None -> Fail x
+  | Some l -> Targets l >>> Fail x
 
 let files_recursively_in ~dir =
   let ctx_dir, src_dir =

src/build.mli

@@ -47,7 +47,7 @@ val lines_of : Path.t -> ('a, string list) t
 
 (** Always fail when executed. We pass a function rather than an exception to get a proper
     backtrace *)
-val fail : fail -> (_, _) t
+val fail : ?targets:Path.t list -> fail -> (_, _) t
 
 module Prog_spec : sig
   type 'a t =

src/gen_rules.ml

@@ -643,7 +643,7 @@ module Gen(P : Params) = struct
           >>>
           Build.action t ~dir ~targets
         | exception e ->
-          Build.fail { fail = fun () -> raise e }
+          Build.fail ~targets { fail = fun () -> raise e }
       in
       let build =
         Build.record_lib_deps ~dir ~kind:dep_kind
@@ -799,7 +799,8 @@ module Gen(P : Params) = struct
            else [])
       ]
 
-  let target = String_with_vars.of_string "${@}"
+  let target_var = String_with_vars.of_string "${@}"
+  let root_var   = String_with_vars.of_string "${ROOT}"
 
   (* Generate rules to build the .pp files and return a new module map where all filenames
      point to the .pp files *)
@@ -816,8 +817,11 @@ module Gen(P : Params) = struct
              Build.path src
              >>>
              Action_interpret.run
-               (With_stdout_to (target, action))
-               ~dir:ctx.build_dir
+               (With_stdout_to
+                  (target_var,
+                   Chdir (root_var,
+                          action)))
+               ~dir
                ~dep_kind
                ~targets:[dst]
                ~deps:[Some src]))