Add filename sets (ocaml#9040)

Signed-off-by: Andrey Mokhov <amokhov@janestreet.com>

otherlibs/stdune/src/filename_set.ml

@@ -0,0 +1,27 @@
+(* CR-someday amokhov: Switch from sets to "flat sets" backed by immutable arrays. *)
+type t =
+  { dir : Path.t
+  ; filenames : Filename.Set.t
+  }
+
+let equal t { dir; filenames } =
+  Path.equal t.dir dir && Filename.Set.equal filenames t.filenames
+;;
+
+let dir { dir; filenames = _ } = dir
+let filenames { dir = _; filenames } = filenames
+let empty ~dir = { dir; filenames = String.Set.empty }
+
+let create ?filter ~dir filenames =
+  match filter with
+  | None -> { dir; filenames }
+  | Some f ->
+    { dir
+    ; filenames =
+        Filename.Set.to_list filenames
+        |> List.filter ~f:(fun basename -> f ~basename)
+        |> Filename.Set.of_list
+    }
+;;
+
+let to_list { dir; filenames } = Filename.Set.to_list_map filenames ~f:(Path.relative dir)

otherlibs/stdune/src/filename_set.mli

@@ -0,0 +1,19 @@
+(** Like [Path.Set.t] but tailored for representing sets of file names in the same parent
+    directory. Compared to [Path.Set.t], [Filename_set.t] statically enforces an important
+    invariant, and can also be processed more efficiently. *)
+
+type t
+
+val equal : t -> t -> bool
+
+(** The directory of the filename set. *)
+val dir : t -> Path.t
+
+(** The set of file names, all relative to [dir]. *)
+val filenames : t -> Filename.Set.t
+
+val empty : dir:Path.t -> t
+
+(* CR-soon amokhov: Decouple [create] from [filter]. *)
+val create : ?filter:(basename:string -> bool) -> dir:Path.t -> Filename.Set.t -> t
+val to_list : t -> Path.t list