pkg: fix dev-tool bug where dune fmt would revert file

The rule for running ocamlformat on source files when dev-tools are in
use did not depend on the input file, so changes to the input file
wouldn't cause ocamlformat to be re-run on the updated file. The
consequence of this is that `dune fmt` would promote stale versions of
files.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

src/dune_rules/format_rules.ml

@@ -90,7 +90,10 @@ module Ocamlformat = struct
         (let open Action_builder.O in
          (* This ensures that at is installed as a dev tool before
             running it. *)
-         let+ () = Action_builder.path path in
+         let+ () = Action_builder.path path
+         (* Declare the dependency on the input file so changes to the input
+            file trigger ocamlformat to run again on the updated file. *)
+         and+ () = Action_builder.path (Path.build input) in
          let args = [ flag_of_kind kind; Path.Build.basename input ] in
          Action.chdir (Path.build dir) @@ Action.run (Ok path) args |> Action.Full.make)
     in

test/blackbox-tests/test-cases/pkg/ocamlformat/gh10991.t

@@ -18,19 +18,18 @@ Initial file:
   $ cat foo.ml
   let () = print_endline "Hello, world"
 
-Formatting failed because the input file was not copied into the sandbox.
   $ DUNE_CONFIG__LOCK_DEV_TOOL=enabled dune fmt
   Solution for dev-tools.locks/ocamlformat:
   - ocamlformat.0.0.1
-  cat: foo.ml: No such file or directory
   File "foo.ml", line 1, characters 0-0:
   Error: Files _build/default/foo.ml and _build/default/.formatted/foo.ml
   differ.
   Promoting _build/default/.formatted/foo.ml to foo.ml.
   [1]
 
-No formatting occured because input file wasn't copied to the sandbox.
+After formatting the fake ocamlformat has added a suffix:
   $ cat foo.ml
+  let () = print_endline "Hello, world"
   (* formatted with fake ocamlformat *)
 
 Update the file:
@@ -45,6 +44,7 @@ Update the file:
   Promoting _build/default/.formatted/foo.ml to foo.ml.
   [1]
 
-No formatting occured because input file wasn't copied to the sandbox.
+The update to the file persists after formatting it a second time:
   $ cat foo.ml
+  let () = print_endline "Hello, ocaml!"
   (* formatted with fake ocamlformat *)