pkg: sandbox format action when using devtool

Signed-off-by: Stephen Sherratt <[email protected]>

src/dune_rules/format_rules.ml

@@ -97,7 +97,9 @@ module Ocamlformat = struct
     let open Action_builder.With_targets.O in
     (* Depend on [extra_deps] so if the ocamlformat config file
        changes then ocamlformat will run again. *)
-    extra_deps dir >>> action
+    extra_deps dir
+    >>> action
+    |> With_targets.map ~f:(Action.Full.add_sandbox Sandbox_config.needs_sandboxing)
   ;;
 
   let action_when_ocamlformat_isn't_locked ~input kind =

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

@@ -18,18 +18,19 @@ 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]
 
-After formatting the fake ocamlformat has added a suffix:
+No formatting occured because input file wasn't copied to the sandbox.
   $ cat foo.ml
-  let () = print_endline "Hello, world"
   (* formatted with fake ocamlformat *)
 
 Update the file:
@@ -44,7 +45,6 @@ Update the file:
   Promoting _build/default/.formatted/foo.ml to foo.ml.
   [1]
 
-After formatting a second time, the recent change to the file was ignored:
+No formatting occured because input file wasn't copied to the sandbox.
   $ cat foo.ml
-  let () = print_endline "Hello, world"
   (* formatted with fake ocamlformat *)