pkg: Display error message when no dependencies exist in dune-project

src/dune_pkg/local_package.ml

@@ -145,7 +145,26 @@ let of_package (t : Dune_lang.Package.t) =
   let name = Package.name t in
   match Package.original_opam_file t with
   | None ->
-    let dependencies = t |> Package.depends |> Dependency_formula.of_dependencies in
+    let depends =
+      match Package.depends t, Package.has_opam_file t with
+      | [], Exists true ->
+        let name = t |> Package.name |> Package_name.to_string in
+        let hints =
+          [ Pp.textf "Add a `depends` field in the `package` stanza of package %S or" name
+          ; Pp.textf "Delete the package %S from `dune-project`" name
+          ]
+        in
+        User_warning.emit
+          ~hints
+          [ Pp.textf
+              "Package %S loaded from `dune-project` but does not declare any \
+               dependencies"
+              name
+          ];
+        []
+      | otherwise, _ -> otherwise
+    in
+    let dependencies = Dependency_formula.of_dependencies depends in
     { name
     ; version
     ; dependencies

test/blackbox-tests/test-cases/pkg/pin-stanza/mixed-opam-dune.t

@@ -29,18 +29,34 @@ should favor the dune metadata in such a case.
   > EOF
 
   $ dune pkg lock
+  Warning: Package "bar" loaded from `dune-project` but does not declare any
+  dependencies
+  Hint: Add a `depends` field in the `package` stanza of package "bar" or
+  Hint: Delete the package "bar" from `dune-project`
+  Warning: Package "bar" loaded from `dune-project` but does not declare any
+  dependencies
+  Hint: Add a `depends` field in the `package` stanza of package "bar" or
+  Hint: Delete the package "bar" from `dune-project`
+  Warning: Package "bar" loaded from `dune-project` but does not declare any
+  dependencies
+  Hint: Add a `depends` field in the `package` stanza of package "bar" or
+  Hint: Delete the package "bar" from `dune-project`
+  Warning: Package "bar" loaded from `dune-project` but does not declare any
+  dependencies
+  Hint: Add a `depends` field in the `package` stanza of package "bar" or
+  Hint: Delete the package "bar" from `dune-project`
   Solution for dune.lock:
   - bar.dev
   - foo.dev
 
-  $  cat dune.lock/bar.pkg | sed "/source/,//d"
+  $ cat dune.lock/bar.pkg | sed "/source/,//d"
   (version dev)
 
   (dune)
 
 
   (dev)
-  $  cat dune.lock/foo.pkg | sed "/source/,//d"
+  $ cat dune.lock/foo.pkg | sed "/source/,//d"
   (version dev)
 
   (dune)