Simplify a bit expand_step2

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

src/jbuild.ml

@@ -251,14 +251,6 @@ module Bindings = struct
       | Unnamed _ -> None
       | Named (k', x) -> Option.some_if (k = k') x)
 
-  let first t =
-    let rec loop acc = function
-      | [] -> acc
-      | Unnamed x :: xs -> loop (Result.Ok x) xs
-      | Named (_, _) :: _ -> Result.Error `Named_exists
-    in
-    loop (Result.Error `Empty) t
-
   let empty = []
 
   let singleton x = [Unnamed x]

src/jbuild.mli

@@ -98,8 +98,6 @@ module Bindings : sig
 
   val singleton : 'a -> 'a t
 
-  val first : 'a t -> ('a, [`Empty | `Named_exists]) Result.t
-
   val sexp_of_t : ('a -> Usexp.t) -> 'a t -> Usexp.t
 end
 

src/super_context.ml

@@ -792,14 +792,17 @@ module Action = struct
             |> Value.L.paths
             |> Option.some
           | First_dep ->
-            begin match Jbuild.Bindings.first deps_written_by_user with
-            | Error `Named_exists ->
-              Loc.fail loc "%%{first-dep} is not allowed with named dependencies"
-            | Error `Empty ->
+            begin match deps_written_by_user with
+            | Named _ :: _ ->
+              (* This case is not possible: ${<} only exist in jbuild
+                 files and named dependencies are not available in
+                 jbuild files *)
+              assert false
+            | Unnamed v :: _ -> Some [Path v]
+            | [] ->
               Loc.warn loc "Variable '%s' used with no explicit \
                             dependencies@." key;
               Some [Value.String ""]
-            | Ok v  -> Some [Path v]
             end
           | _ ->
             Exn.code_error "Unexpected variable in step2"