variants: tweak doc and error message

Signed-off-by: Lucas Pluvinage <[email protected]>

doc/variants.rst

@@ -110,11 +110,11 @@ implementation would have the following configuration:
     (implements time)
     (variant js))
 
-Since dune 1.11.0, the list of available variants is computed while building
-the virtual library. This means only variant implementations that are part of
-the same project are implicitely taken into account. It's possible to declare
-an external implementation by using the `external_variant` stanza in the
-virtual library scope.
+The list of available variants is computed while building the virtual library.
+This means only variant implementations that are part of the same project are
+implicitely taken into account. It's possible to declare an external
+implementation by using the `external_variant` stanza in the virtual library
+scope.
 
 .. code:: scheme
 

src/lib.ml

@@ -831,16 +831,20 @@ let rec instantiate db name (info : Lib_info.t) ~stack ~hidden =
             Ok vlib
           else
             Errors.fail loc
-              "Library implementation %a for variant %a implements a \
-               library @ outside the project.@ Instead of using \
+              "Library implementation %a with variant %a implements@ a \
+               library outside the project.@ Instead of using \
                (variant %a) here,@ you need to reference it in the \
-               virtual library definition, using @ \
-               (external_implementations (%a %a))."
-              Lib_name.pp (snd name)
+               virtual library project,@ using the external_variant stanza:@ \
+               (external_variant@\n\
+               \  (virtual_library %a)@\n\
+               \  (variant %a)@\n\
+               \  (implementation %a))"
+              Lib_name.pp info.name
               Variant.pp variant
               Variant.pp variant
+              Lib_name.pp vlib.name
               Variant.pp variant
-              Lib_name.pp vlib.name)
+              Lib_name.pp info.name)
   in
   let default_implementation =
     Option.map info.default_implementation ~f:(fun l -> lazy (resolve l)) in

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

@@ -5,8 +5,12 @@ variant.
   File "prj2/dune", line 4, characters 13-20:
   4 |  (implements vlibfoo)
                    ^^^^^^^
-  Error: Library implementation vlibfoo for variant "somevariant" implements a library 
-  outside the project. Instead of using (variant "somevariant") here,
-  you need to reference it in the virtual library definition, using 
-  (external_implementations ("somevariant" vlibfoo)).
+  Error: Library implementation impl with variant "somevariant" implements
+  a library outside the project. Instead of using (variant "somevariant") here,
+  you need to reference it in the virtual library project,
+  using the external_variant stanza:
+  (external_variant
+    (virtual_library vlibfoo)
+    (variant "somevariant")
+    (implementation impl))
   [1]