Put back library variants in development mode (#2116)

* Put back library variants in development mode

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

* _

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

CHANGES.md

@@ -1,6 +1,13 @@
 unreleased
 ----------
 
+- Put back library variants in development mode. We discovered a
+  serious unexpected issue and we might need to adjust the design of
+  this feature before we are ready to commit to a final version. Users
+  will need to write `(using library_variants 0.1)` in their
+  `dune-project` file if they want to use it before the design is
+  finalized. (#2116, @diml)
+
 - Restricted the set of variables available for expansion in the destination filename
   of `install` stanza to simplify implementation and avoid dependency
   cycles. (#2073, @aalekseyev, @diml)

doc/variants.rst

@@ -79,6 +79,10 @@ implementation for every virtual library that we've used:
 Variants
 ========
 
+This feature is still under development and may change with new dune
+releases. You need to write ``(using library_variants 0.1)`` in your
+``dune-project`` file to unlock it.
+
 When building a binary, implementations can be selected using a set of variants
 rather than individually specifying implementations.
 
@@ -107,6 +111,8 @@ implementation would have the following configuration:
 Default implementation
 ======================
 
+This feature is also guarded by ``(using library_variants ...)``.
+
 A virtual library may select a default implementation, which is enabled after
 variant resolution, if no suitable implementation has been found.
 

src/dune_file.ml

@@ -50,9 +50,20 @@ let relative_file =
     else
       of_sexp_errorf loc "relative filename expected")
 
+
+let library_variants =
+  let syntax =
+    Syntax.create ~name:"library_variants"
+      ~desc:"the experimental library variants feature."
+      [ (0, 1) ]
+  in
+  Dune_project.Extension.register_simple ~experimental:true
+    syntax (Dune_lang.Decoder.return []);
+  syntax
+
 let variants_field =
   field_o "variants" (
-    let* () = Syntax.since Stanza.syntax (1, 9) in
+    let* () = Syntax.since library_variants (0, 1) in
     located (list Variant.decode >>| Variant.Set.of_list))
 
 (* Parse and resolve "package" fields *)
@@ -901,11 +912,11 @@ module Library = struct
            located Lib_name.decode)
        and+ variant =
          field_o "variant" (
-           Syntax.since Stanza.syntax (1, 9) >>>
+           Syntax.since library_variants (0, 1) >>>
            located Variant.decode)
        and+ default_implementation =
          field_o "default_implementation" (
-           Syntax.since Stanza.syntax (1, 9) >>>
+           Syntax.since library_variants (0, 1) >>>
            located Lib_name.decode)
        and+ private_modules =
          field_o "private_modules" (

test/blackbox-tests/test-cases/toplevel-stanza/variants/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants-multi-project/prj1/dune-project

@@ -1 +1,2 @@
 (lang dune 1.10)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants-multi-project/prj2/dune-project

@@ -1 +1,2 @@
 (lang dune 1.10)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/dune-package/dune-project

@@ -1,2 +1,3 @@
 (lang dune 1.9)
 (name a)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/external/exe/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/external/lib/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/multiple-implementations-for-variants/dune-project

@@ -1,2 +1,3 @@
 (lang dune 1.9)
 (using fmt 1.1)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/resolution-priority/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/same-lib-in-multiple-scopes/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/same-lib-in-multiple-scopes/test/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/variant-not-implementation/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/variant-with-concrete-impl/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/variants/variants-base/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/default-impl-not-virtual-lib/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/default-impl/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/dependency-cycle/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/dune-package/dune-project

@@ -1,2 +1,3 @@
 (lang dune 1.9)
 (name a)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/external/exe/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)

test/blackbox-tests/test-cases/vlib-default-impl/external/lib/dune-project

@@ -1 +1,2 @@
-(lang dune 1.9)
+(lang dune 1.9)
+(using library_variants 0.1)