Fix/good error msg pkg enabled

doc/changes/fixed/12802.md

@@ -0,0 +1,3 @@
+- Provide a more informative error message when `(pkg enabled)` is put in
+  `dune-project` instead of `dune-workspace`. (#12802, fixes #12801,
+  @benodiwal)

src/dune_lang/dune_project.ml

@@ -953,6 +953,24 @@ let parse ~dir ~(lang : Lang.Instance.t) ~file =
          "warnings"
          ~default:Warning.Settings.empty
          (Syntax.since Stanza.syntax (3, 11) >>> Warning.Settings.decode)
+     and+ () =
+       field_o
+         "pkg"
+         (let+ loc = loc
+          and+ () = junk_everything in
+          loc)
+       >>| Option.iter ~f:(fun loc ->
+         User_error.raise
+           ~loc
+           ~hints:
+             [ Pp.text
+                 "Add this configuration to your dune-workspace file (create one in your \
+                  workspace root if you don't have one)."
+             ]
+           [ Pp.text
+               "The (pkg ...) configuration is only valid in dune-workspace, not a \
+                dune-project."
+           ])
      in
      fun (opam_packages : (Loc.t * Package.t Memo.t) Package.Name.Map.t) ->
        let opam_file_location =

test/blackbox-tests/test-cases/pkg/pkg-in-dune-project-error.t

@@ -0,0 +1,17 @@
+Test that putting (pkg enabled) in dune-project instead of dune-workspace
+gives a helpful error message.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.21)
+  > (pkg enabled)
+  > EOF
+
+  $ dune build
+  File "dune-project", line 2, characters 0-13:
+  2 | (pkg enabled)
+      ^^^^^^^^^^^^^
+  Error: The (pkg ...) configuration is only valid in dune-workspace, not a
+  dune-project.
+  Hint: Add this configuration to your dune-workspace file (create one in your
+  workspace root if you don't have one).
+  [1]