feat(melange): support enabled_if in melange.emit (#7848)

Signed-off-by: Antonio Nuno Monteiro <anmonteiro@gmail.com>

src/dune_rules/gen_rules.ml

@@ -144,16 +144,19 @@ end = struct
       | true ->
         let+ () = Mdx.gen_rules ~sctx ~dir ~scope ~expander mdx in
         empty_none)
-    | Melange_stanzas.Emit.T mel ->
-      let+ cctx, merlin =
-        Melange_rules.setup_emit_cmj_rules ~dir_contents ~dir ~scope ~sctx
-          ~expander mel
-      in
-      { merlin = Some merlin
-      ; cctx = Some (mel.loc, cctx)
-      ; js = None
-      ; source_dirs = None
-      }
+    | Melange_stanzas.Emit.T mel -> (
+      Expander.eval_blang expander mel.enabled_if >>= function
+      | false -> Memo.return empty_none
+      | true ->
+        let+ cctx, merlin =
+          Melange_rules.setup_emit_cmj_rules ~dir_contents ~dir ~scope ~sctx
+            ~expander mel
+        in
+        { merlin = Some merlin
+        ; cctx = Some (mel.loc, cctx)
+        ; js = None
+        ; source_dirs = None
+        })
     | _ -> Memo.return empty_none
 
   let of_stanzas stanzas ~cctxs ~sctx ~src_dir ~ctx_dir ~scope ~dir_contents

src/dune_rules/melange/melange_stanzas.ml

@@ -17,6 +17,7 @@ module Emit = struct
     ; promote : Rule.Promote.t option
     ; compile_flags : Ordered_set_lang.Unexpanded.t
     ; allow_overlapping_dependencies : bool
+    ; enabled_if : Blang.t
     }
 
   type Stanza.t += T of t
@@ -108,7 +109,12 @@ module Emit = struct
        and+ allow_overlapping_dependencies =
          field_b "allow_overlapping_dependencies"
        and+ emit_stdlib = field "emit_stdlib" bool ~default:true
-       and+ modules = Stanza_common.Modules_settings.decode in
+       and+ modules = Stanza_common.Modules_settings.decode
+       and+ enabled_if =
+         let open Enabled_if in
+         let allowed_vars = Any in
+         decode ~allowed_vars ~since:None ()
+       in
        let preprocess =
          let init =
            let f libname = Preprocess.With_instrumentation.Ordinary libname in
@@ -133,6 +139,7 @@ module Emit = struct
        ; promote
        ; compile_flags
        ; allow_overlapping_dependencies
+       ; enabled_if
        })
 
   let target_dir (emit : t) ~dir = Path.Build.relative dir emit.target

src/dune_rules/melange/melange_stanzas.mli

@@ -17,6 +17,7 @@ module Emit : sig
     ; promote : Rule.Promote.t option
     ; compile_flags : Ordered_set_lang.Unexpanded.t
     ; allow_overlapping_dependencies : bool
+    ; enabled_if : Blang.t
     }
 
   type Stanza.t += T of t

test/blackbox-tests/test-cases/melange/enabled_if.t

@@ -0,0 +1,40 @@
+`enabled_if` in `melange.emit`
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (using melange 0.1)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target out)
+  >  (emit_stdlib false)
+  >  (enabled_if %{bin-available:melc}))
+  > EOF
+  $ cat > x.ml <<EOF
+  > let () = Js.log "hello"
+  > EOF
+
+  $ dune rules @melange | grep '\.cmj'
+     (File (In_build_dir _build/default/.out.mobjs/melange/melange__X.cmj))))
+      .out.mobjs/melange/melange__X.cmj))))
+  $ dune build @melange --display short
+          melc .out.mobjs/melange/melange__X.{cmi,cmj,cmt}
+          melc out/x.js
+
+  $ dune clean
+
+`(enabled_if false)` shouldn't build any JS
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target out)
+  >  (emit_stdlib false)
+  >  (enabled_if false))
+  > EOF
+
+  $ dune build @melange --display short
+
+No rules attached to the alias
+
+  $ dune rules @melange