Restrict (executables_implicit_empty_intf) to Dune lang 2.9

Signed-off-by: Craig Ferguson <me@craigfe.io>

Author: craigfe

doc/dune-files.rst

@@ -102,7 +102,7 @@ By default, executables defined via ``(executables(s) ...)`` or ``(test(s)
 common to give them empty interface files to strengthen the compiler's ability
 to detect unused values in these modules.
 
-Starting from dune 2.8, an option is available to automatically generate empty
+Starting from dune 2.9, an option is available to automatically generate empty
 interface files for executables and tests that don't already have them:
 
 .. code:: scheme
@@ -651,7 +651,7 @@ binary at the same place as where ``ocamlc`` was found.
 Executables can also be linked as object or shared object files. See
 `linking modes`_ for more information.
 
-Starting from dune 2.8, it's possible to automatically generate empty interface
+Starting from dune 2.9, it's possible to automatically generate empty interface
 files for executables. See `executables_implicit_empty_intf`_.
 
 ``<optional-fields>`` are:
@@ -1365,7 +1365,7 @@ running dune runtest you can use the following stanza:
     (libraries alcotest mylib)
     (action (run %{test} -e)))
 
-Starting from dune 2.8, it's possible to automatically generate empty interface
+Starting from dune 2.9, it's possible to automatically generate empty interface
 files for test executables. See `executables_implicit_empty_intf`_.
 
 test

src/dune_engine/dune_project.ml

@@ -707,7 +707,7 @@ let parse ~dir ~lang ~opam_packages ~file ~dir_status =
            loc lang.syntax (2, 8) ~what:"This field"
      and+ executables_implicit_empty_intf =
        field_o_b "executables_implicit_empty_intf"
-         ~check:(Dune_lang.Syntax.since Stanza.syntax (2, 8))
+         ~check:(Dune_lang.Syntax.since Stanza.syntax (2, 9))
      and+ () = Dune_lang.Versioned_file.no_more_lang
      and+ generate_opam_files =
        field_o_b "generate_opam_files"

test/blackbox-tests/test-cases/executables-implicit-empty-intf.t/run.t

@@ -2,7 +2,7 @@ Executables with no corresponding `.mli` file will have one generated for them
 by Dune:
 
   $ cat >dune-project <<EOF
-  > (lang dune 2.8)
+  > (lang dune 2.9)
   > (executables_implicit_empty_intf true)
   > EOF
 
@@ -39,7 +39,7 @@ If an executable already has an interface, it is preserved:
 Generation of empty `.mli` files is disabled by default prior to lang 3.0:
 
   $ dune clean
-  $ echo >dune-project "(lang dune 2.8)"
+  $ echo >dune-project "(lang dune 2.9)"
   $ dune build ./bin/executable.exe
   $ dune runtest
   $ test ! -f _build/default/bin/executable.mli