Check that no targets are generated outside of the current dir

doc/jbuild.rst

@@ -297,6 +297,45 @@ currently don't support patterns, such as a rule to produce ``%.y``
 from ``%.x`` for any given ``%``. This might be supported in the
 future.
 
+do (inferred rules)
+-------------------
+
+When using the action DSL (see `User actions`_), it is most of the
+time obvious what are the dependencies and targets.
+
+For instance:
+
+.. code:: scheme
+
+    (rule
+      ((targets (b)
+       (deps    (a)
+       (action  (copy ${<} ${@}))))))
+
+In this example it is obvious by inspecting the action what the
+dependencies and targets are. For this reason Jbuilder provides a
+simpler way to define rules where Jbuilder infers dependencies and
+targets for you. This is available through the ``do`` stanza:
+
+.. code:: scheme
+
+    (do <action>)
+
+For instance:
+
+.. code:: scheme
+
+    (do (copy a b))
+
+Note that in Jbuilder, targets must always be known
+statically. Especially, this mean that Jbuilder must be able to
+statically determine all targets. For instance, this ``(do ...)``
+stanza is rejected by Jbuilder:
+
+.. code:: scheme
+
+    (do (copy a b.${read:file}))
+
 ocamllex
 --------
 

src/super_context.ml

@@ -613,6 +613,14 @@ module Action = struct
         { deps; targets = Pset.union targets targets_written_by_user }
     in
     let targets = Pset.elements targets in
+    List.iter targets ~f:(fun target ->
+      if Path.parent target <> dir then
+        Loc.fail (Loc.in_file (Utils.jbuild_name_in ~dir))
+          "A rule in this jbuild has targets in a different directory \
+           than the current one, this is not allowed by Jbuilder at the moment:\n%s"
+          (List.map targets ~f:(fun target ->
+             sprintf "- %s" (Utils.describe_target target))
+           |> String.concat ~sep:"\n"));
     let build =
       Build.record_lib_deps_simple ~dir forms.lib_deps
       >>>