Add tests for all permutations of virtual modules field

All permutations of:

* presence of implementation
* presence in virtual_modules
* presence in modules_without_implementation

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

test/blackbox-tests/test-cases/variants/module-fields/test.ml

@@ -0,0 +1,60 @@
+module List = ListLabels
+open Printf
+
+let write_file ~file ~contents =
+  let ch = open_out file in
+  output_string ch contents;
+  close_out_noerr ch
+
+let lib_stanza ~modules_without_implementation ~virtual_modules =
+  sprintf "(library\n (name foo)%s%s)"
+    (if modules_without_implementation then
+       "\n (modules_without_implementation m)"
+     else
+       "")
+    (if virtual_modules then
+       "\n (virtual_modules m)"
+     else
+       "")
+
+let chdir dir ~f =
+  let old_dir = Sys.getcwd () in
+  begin try
+    Sys.chdir dir;
+    f ()
+  with e ->
+    Sys.chdir old_dir;
+    raise e
+  end;
+  Sys.chdir old_dir
+
+let gen_test ~impl ~modules_without_implementation ~virtual_modules =
+  printf "impl: %b. modules_without_implementation: %b. virtual_modules: %b\n%!"
+    impl modules_without_implementation virtual_modules;
+  let dir =
+    sprintf "%b-%b-%b" impl modules_without_implementation virtual_modules
+  in
+  let _ = Sys.command (sprintf "mkdir -p %s" dir) in
+  chdir dir ~f:(fun () ->
+    write_file ~file:"dune-project"
+      ~contents:"(lang dune 1.2)\n\
+                 (using in_development_do_not_use_variants 0.1)";
+    write_file ~file:"m.mli" ~contents:"";
+    if impl then
+      write_file ~file:"m.ml" ~contents:"";
+    write_file ~file:"dune" ~contents:(
+      lib_stanza ~modules_without_implementation ~virtual_modules);
+    ignore (Sys.command "dune build");
+    print_endline "-------------------------"
+  )
+
+let bools = [true; false]
+
+let () =
+  List.iter bools ~f:(fun impl ->
+    List.iter bools ~f:(fun modules_without_implementation ->
+      List.iter bools ~f:(fun virtual_modules ->
+        gen_test ~impl ~modules_without_implementation ~virtual_modules
+      )
+    )
+  )

test/blackbox-tests/test-cases/variants/run.t

@@ -15,3 +15,48 @@ virtual libraries may not implement their virtual modules
                         ^^^
   Error: Module Foo has an implementation, it cannot be listed here
   [1]
+
+  $ cd module-fields && ocaml test.ml
+  impl: true. modules_without_implementation: true. virtual_modules: true
+  File "dune", line 3, characters 33-34:
+  3 |  (modules_without_implementation m)
+                                       ^
+  Error: Module M has an implementation, it cannot be listed here
+  -------------------------
+  impl: true. modules_without_implementation: true. virtual_modules: false
+  File "dune", line 3, characters 33-34:
+  3 |  (modules_without_implementation m))
+                                       ^
+  Error: Module M has an implementation, it cannot be listed here
+  -------------------------
+  impl: true. modules_without_implementation: false. virtual_modules: true
+  File "dune", line 3, characters 18-19:
+  3 |  (virtual_modules m))
+                        ^
+  Error: Module M has an implementation, it cannot be listed here
+  -------------------------
+  impl: true. modules_without_implementation: false. virtual_modules: false
+  -------------------------
+  impl: false. modules_without_implementation: true. virtual_modules: true
+  File "dune", line 4, characters 18-19:
+  4 |  (virtual_modules m))
+                        ^
+  Error: These modules appear in the virtual_libraries and modules_without_implementation fields: 
+  - m
+  This is not possible.
+  -------------------------
+  impl: false. modules_without_implementation: true. virtual_modules: false
+  -------------------------
+  impl: false. modules_without_implementation: false. virtual_modules: true
+  -------------------------
+  impl: false. modules_without_implementation: false. virtual_modules: false
+  File "dune", line 1, characters 0-21:
+  1 | (library
+  2 |  (name foo))
+  Warning: Some modules don't have an implementation.
+  You need to add the following field to this stanza:
+  
+    (modules_without_implementation m)
+  
+  This will become an error in the future.
+  -------------------------