Merge pull request #1222 from rgrinberg/experimental-extensions

Add support for experimental extensions

src/dune_file.ml

@@ -861,7 +861,8 @@ module Library = struct
           ~desc:"the experimental variants feature"
           [ (0, 1) ]
       in
-      Dune_project.Extension.register syntax (Dsexp.Of_sexp.return []);
+      Dune_project.Extension.register ~experimental:true
+        syntax (Dsexp.Of_sexp.return []);
       syntax
   end
 

src/dune_project.ml

@@ -202,8 +202,9 @@ let append_to_project_file t str =
 
 module Extension = struct
   type t =
-    { syntax  : Syntax.t
-    ; stanzas : Stanza.Parser.t list Dsexp.Of_sexp.t
+    { syntax       : Syntax.t
+    ; stanzas      : Stanza.Parser.t list Dsexp.Of_sexp.t
+    ; experimental : bool
     }
 
   type instance =
@@ -215,12 +216,12 @@ module Extension = struct
 
   let extensions = Hashtbl.create 32
 
-  let register syntax stanzas =
+  let register ?(experimental=false) syntax stanzas =
     let name = Syntax.name syntax in
     if Hashtbl.mem extensions name then
       Exn.code_error "Dune_project.Extension.register: already registered"
         [ "name", Sexp.To_sexp.string name ];
-    Hashtbl.add extensions name { syntax; stanzas }
+    Hashtbl.add extensions name { syntax; stanzas ; experimental }
 
   let instantiate ~loc ~parse_args (name_loc, name) (ver_loc, ver) =
     match Hashtbl.find extensions name with
@@ -241,7 +242,12 @@ module Extension = struct
   let automatic ~project_file ~f =
     Hashtbl.foldi extensions ~init:[] ~f:(fun name ext acc ->
       if f name then
-        let version = Syntax.greatest_supported_version ext.syntax in
+        let version =
+          if ext.experimental then
+            (0, 0)
+          else
+            Syntax.greatest_supported_version ext.syntax
+        in
         let parse_args p =
           let open Dsexp.Of_sexp in
           let dune_project_edited = ref false in

src/dune_project.mli

@@ -63,7 +63,11 @@ module Extension : sig
 
       in their [dune-project] file. [parser] is used to describe
       what [<args>] might be. *)
-  val register : Syntax.t -> Stanza.Parser.t list Dsexp.Of_sexp.t -> unit
+  val register
+    :  ?experimental:bool
+    -> Syntax.t
+    -> Stanza.Parser.t list Dsexp.Of_sexp.t
+    -> unit
 end
 
 (** Load a project description from the following directory. [files]

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

@@ -1,7 +1,11 @@
 Variant feature is auto enabled when virtual_modules is used
 
   $ dune build --root variants-without-using
-  Entering directory 'variants-without-using'
+  File "dune", line 3, characters 1-25:
+  3 |  (virtual_modules foobar))
+       ^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: 'virtual_modules' is only available since version 0.1 of the experimental variants feature
+  [1]
 
   $ dune build --root variants-using
   Entering directory 'variants-using'