Make the (interface) field of a (dialect) optional

Signed-off-by: Guillaume Petiot <guillaume@tarides.com>

src/dune_rules/dialect.ml

@@ -12,6 +12,13 @@ module File_kind = struct
     ; format : (Loc.t * Action.t * string list) option
     }
 
+  let dummy_intf =
+    { kind = Ml_kind.Intf
+    ; extension = "__DUMMY__"
+    ; preprocess = None
+    ; format = None
+    }
+
   let encode { kind; extension; preprocess; format } =
     let open Dune_lang.Encoder in
     let kind =
@@ -88,7 +95,8 @@ let decode =
   fields
     (let+ name = field "name" string
      and+ impl = field "implementation" (fields (kind Ml_kind.Impl))
-     and+ intf = field "interface" (fields (kind Ml_kind.Intf)) in
+     and+ intf = field_o "interface" (fields (kind Ml_kind.Intf)) in
+     let intf = Option.value intf ~default:File_kind.dummy_intf in
      { name; file_kinds = Ml_kind.Dict.make ~intf ~impl })
 
 let extension { file_kinds; _ } ml_kind =

test/blackbox-tests/test-cases/dialects/no_intf.t/dune

@@ -0,0 +1,9 @@
+(executable
+ (name main)
+ (public_name main)
+ (modules main))
+
+(executable
+ (name fmt)
+ (public_name fmt)
+ (modules fmt))

test/blackbox-tests/test-cases/dialects/no_intf.t/dune-project

@@ -0,0 +1,8 @@
+(lang dune 1.11)
+(using fmt 1.2 (enabled_for mlfi))
+
+(dialect
+ (name mlfi)
+ (implementation
+  (extension mf)
+  (format (run fmt %{input-file}))))

test/blackbox-tests/test-cases/dialects/no_intf.t/fmt.ml

@@ -0,0 +1 @@
+prerr_endline ("Formatting " ^ Sys.argv.(1))

test/blackbox-tests/test-cases/dialects/no_intf.t/run.t

@@ -0,0 +1,6 @@
+Test the (dialect ...) stanza inside the dune-project file.
+
+  $ dune exec ./main.exe
+
+  $ dune build @fmt
+  Formatting main.mf