diff --git a/src/build_system.ml b/src/build_system.ml
index adcd49c4..1c04b15e 100644
--- a/src/build_system.ml
+++ b/src/build_system.ml
@@ -473,12 +473,13 @@ end
 let create ~contexts ~file_tree ~rules =
   let all_source_files =
     File_tree.fold file_tree ~init:Pset.empty ~f:(fun dir acc ->
-        let path = File_tree.Dir.path dir in
-        Pset.union acc
-          (File_tree.Dir.files dir
-           |> String_set.elements
-           |> List.map ~f:(Path.relative path)
-           |> Pset.of_list))
+      let path = File_tree.Dir.path dir in
+      Cont
+        (Pset.union acc
+           (File_tree.Dir.files dir
+            |> String_set.elements
+            |> List.map ~f:(Path.relative path)
+            |> Pset.of_list)))
   in
   let all_copy_targets =
     List.fold_left contexts ~init:Pset.empty ~f:(fun acc (ctx : Context.t) ->
diff --git a/src/file_tree.ml b/src/file_tree.ml
index 296da89f..a9ef5a14 100644
--- a/src/file_tree.ml
+++ b/src/file_tree.ml
@@ -1,5 +1,9 @@
 open! Import
 
+type 'a fold_callback_result =
+  | Cont            of 'a
+  | Dont_recurse_in of String_set.t * 'a
+
 module Dir = struct
   type t =
     { path     : Path.t
@@ -12,9 +16,16 @@ module Dir = struct
   let sub_dirs t = t.sub_dirs
 
   let rec fold t ~init ~f =
-    let init = f t init in
-    String_map.fold t.sub_dirs ~init ~f:(fun ~key:_ ~data:t acc ->
+    match f t init with
+    | Cont init ->
+      String_map.fold t.sub_dirs ~init ~f:(fun ~key:_ ~data:t acc ->
         fold t ~init:acc ~f)
+    | Dont_recurse_in (forbidden, init) ->
+      String_map.fold t.sub_dirs ~init ~f:(fun ~key:sub_dir ~data:t acc ->
+        if String_set.mem sub_dir forbidden then
+          acc
+        else
+          fold t ~init:acc ~f)
 end
 
 type t =
@@ -54,14 +65,13 @@ let load path =
   let root = walk path in
   let dirs =
     Dir.fold root ~init:Path.Map.empty ~f:(fun dir acc ->
-      Path.Map.add acc ~key:dir.path ~data:dir)
+      Cont (Path.Map.add acc ~key:dir.path ~data:dir))
   in
   { root
   ; dirs
   }
 
-let fold t ~init ~f =
-  Path.Map.fold t.dirs ~init ~f:(fun ~key:_ ~data:dir acc -> f dir acc)
+let fold t ~init ~f = Dir.fold t.root ~init ~f
 
 let find_dir t path =
   Path.Map.find path t.dirs
@@ -81,5 +91,6 @@ let files_recursively_in t ?(prefix_with=Path.root) path =
   | Some dir ->
     Dir.fold dir ~init:Path.Set.empty ~f:(fun dir acc ->
       let path = Path.append prefix_with (Dir.path dir) in
-      String_set.fold (Dir.files dir) ~init:acc ~f:(fun fn acc ->
-        Path.Set.add (Path.relative path fn) acc))
+      Cont
+        (String_set.fold (Dir.files dir) ~init:acc ~f:(fun fn acc ->
+           Path.Set.add (Path.relative path fn) acc)))
diff --git a/src/file_tree.mli b/src/file_tree.mli
index 586a94d1..ad41e6ea 100644
--- a/src/file_tree.mli
+++ b/src/file_tree.mli
@@ -13,7 +13,11 @@ type t
 
 val load : Path.t -> t
 
-val fold : t -> init:'a -> f:(Dir.t -> 'a -> 'a) -> 'a
+type 'a fold_callback_result =
+  | Cont            of 'a
+  | Dont_recurse_in of String_set.t * 'a
+
+val fold : t -> init:'a -> f:(Dir.t -> 'a -> 'a fold_callback_result) -> 'a
 
 val root : t -> Dir.t
 
diff --git a/src/jbuild_load.ml b/src/jbuild_load.ml
index abe1bdb6..cc58b303 100644
--- a/src/jbuild_load.ml
+++ b/src/jbuild_load.ml
@@ -144,16 +144,18 @@ let load ~dir ~visible_packages ~closest_packages =
 
 let load () =
   let ftree = File_tree.load Path.root in
-  let packages =
-    File_tree.fold ftree ~init:[] ~f:(fun dir acc ->
+  let packages, ignored_subtrees =
+    File_tree.fold ftree ~init:([], Path.Set.empty) ~f:(fun dir (pkgs, ignored) ->
       let path = File_tree.Dir.path dir in
-      String_set.fold (File_tree.Dir.files dir) ~init:acc ~f:(fun fn acc ->
-        match Filename.split_ext fn with
-        | Some (pkg, ".opam") when pkg <> "" ->
-          let version_from_opam_file =
-            let lines = lines_of_file (Path.relative path fn |> Path.to_string) in
-            List.find_map lines ~f:(fun s ->
-              try
+      let files = File_tree.Dir.files dir in
+      let pkgs =
+        String_set.fold files ~init:pkgs ~f:(fun fn acc ->
+          match Filename.split_ext fn with
+          | Some (pkg, ".opam") when pkg <> "" ->
+            let version_from_opam_file =
+              let lines = lines_of_file (Path.relative path fn |> Path.to_string) in
+              List.find_map lines ~f:(fun s ->
+                try
                 Scanf.sscanf s "version: %S" (fun x -> Some x)
               with _ ->
                 None)
@@ -163,8 +165,23 @@ let load () =
            ; path
            ; version_from_opam_file
            }) :: acc
-        | _ -> acc))
-    |> String_map.of_alist_multi
+          | _ -> acc)
+      in
+      if String_set.mem "jbuild-ignore" files then
+        let ignore_set =
+          String_set.of_list
+            (lines_of_file (Path.to_string (Path.relative path "jbuild-ignore")))
+        in
+        Dont_recurse_in
+          (ignore_set,
+           (pkgs,
+            String_set.fold ignore_set ~init:ignored ~f:(fun fn acc ->
+              Path.Set.add (Path.relative path fn) acc)))
+      else
+        Cont (pkgs, ignored))
+  in
+  let packages =
+    String_map.of_alist_multi packages
     |> String_map.mapi ~f:(fun name pkgs ->
       match pkgs with
       | [pkg] -> pkg
@@ -201,12 +218,8 @@ let load () =
     in
     let sub_dirs =
       if String_set.mem "jbuild-ignore" files then
-        let ignore_set =
-          String_set.of_list
-            (lines_of_file (Path.to_string (Path.relative path "jbuild-ignore")))
-        in
-        String_map.filter sub_dirs ~f:(fun fn _ ->
-          not (String_set.mem fn ignore_set))
+        String_map.filter sub_dirs ~f:(fun _ dir ->
+          not (Path.Set.mem (File_tree.Dir.path dir) ignored_subtrees))
       else
         sub_dirs
     in
diff --git a/src/lib_db.ml b/src/lib_db.ml
index 6a71e8c0..b765a57e 100644
--- a/src/lib_db.ml
+++ b/src/lib_db.ml
@@ -54,7 +54,9 @@ module Local_closure = Top_closure.Make(String)(struct
     let key ((_, lib) : t) = lib.name
     let deps ((dir, lib) : Lib.Internal.t) graph =
       List.concat_map lib.buildable.libraries ~f:(fun dep ->
-        List.filter_map (Lib_dep.to_lib_names dep) ~f:(find_internal ~from:dir graph))
+        List.filter_map (Lib_dep.to_lib_names dep) ~f:(find_internal ~from:dir graph)) @
+      List.filter_map lib.ppx_runtime_libraries ~f:(fun dep ->
+        find_internal ~from:dir graph dep)
   end)
 
 let top_sort_internals t ~internal_libraries =
@@ -84,7 +86,9 @@ let compute_instalable_internal_libs t ~internal_libraries =
   List.fold_left (top_sort_internals t ~internal_libraries) ~init:t
     ~f:(fun t (dir, lib) ->
       if not lib.Library.optional ||
-         List.for_all (Library.all_lib_deps lib) ~f:(dep_is_installable t ~from:dir) then
+         (List.for_all (Library.all_lib_deps lib) ~f:(dep_is_installable t ~from:dir) &&
+          List.for_all lib.ppx_runtime_libraries  ~f:(lib_is_installable t ~from:dir))
+      then
         { t with
           instalable_internal_libs =
             String_map.add t.instalable_internal_libs