Promote into doesn't accept dune variables #12742
Description
Current behavior
Currently, Dune doesn't accept variables in (mode (promote (into ...))). When trying to use it, we get this error:
File "frontend/packages/admin-support/src/dune", line 61, characters 9-62:
61 | (into %{workspace_root}/public/app/assets/css/admin-support)))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Atom or quoted string expected
Had 1 error, waiting for filesystem changes...Desired Behavior
It would be nice if we could use variables like %{workspace_root} in the (mode (promote (into ...)))
Example
(rule
(target styles.css)
(alias fe.extract-styles)
(mode
(promote
(into %{workspace_root}/public/app/assets/css/admin-support)))
(deps
(glob_files_rec ./**.pp.ml))
(action
(progn
(echo "Generating styles.css")
(run styled-ppx.generate --output %{target} %{deps}))))