Name the intput file for action preprocessors

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

doc/dune-files.rst

@@ -1038,11 +1038,11 @@ the ``-pp`` or ``-ppx`` of the various OCaml tools.
 Preprocessing with actions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-``<action>`` uses the same DSL as described in the `User actions`_ section, and
-for the same reason given in that section, it will be executed from the root of
-the current build context. It is expected to be an action that reads the file
-given as only dependency and outputs the preprocessed file on its standard
-output.
+``<action>`` uses the same DSL as described in the `User actions`_
+section, and for the same reason given in that section, it will be
+executed from the root of the current build context. It is expected to
+be an action that reads the file given as only dependency named
+``input-file`` and outputs the preprocessed file on its standard output.
 
 More precisely, ``(preprocess (action <action>))`` acts as if
 you had setup a rule for every file of the form:
@@ -1055,7 +1055,7 @@ you had setup a rule for every file of the form:
         (action  (with-stdout-to %{@} (chdir %{root} <action>))))
 
 The equivalent of a ``-pp <command>`` option passed to the OCaml compiler is
-``(system "<command> %{<}")``.
+``(system "<command> %{input-file}")``.
 
 Preprocessing with ppx rewriters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/preprocessing.ml

@@ -456,7 +456,8 @@ let lint_module sctx ~dir ~dep_kind ~lint ~lib_name ~scope ~dir_kind =
                let src_path = Path.relative dir src.name in
                add_alias src.name
                  (Build.path src_path
-                  >>^ (fun _ -> Jbuild.Bindings.singleton src_path)
+                  >>^ (fun _ ->
+                    [Jbuild.Bindings.Named ("input-file", [src_path])])
                   >>> SC.Action.run sctx
                         action
                         ~loc
@@ -532,7 +533,7 @@ let make sctx ~dir ~dep_kind ~lint ~preprocess
                (preprocessor_deps
                 >>>
                 Build.path src
-                >>^ (fun _ -> Jbuild.Bindings.singleton src)
+                >>^ (fun _ -> [Jbuild.Bindings.Named ("input-file", [src])])
                 >>>
                 SC.Action.run sctx
                   (Redirect

test/blackbox-tests/test-cases/preprocess-with-action/dune/dune

@@ -1,3 +1,3 @@
 (test
  (name test)
- (preprocess (action (run pp/pp.exe %{first-dep}))))
+ (preprocess (action (run pp/pp.exe %{input-file}))))

test/blackbox-tests/test-cases/preprocess-with-action/run.t

@@ -1,4 +1 @@
   $ dune runtest
-  File "dune/dune", line 3, characters 38-48:
-  Error: unknown variable "first-dep"
-  [1]