Allow to use installed libraries in jbuild plugins

doc/manual.org

@@ -189,11 +189,34 @@ everything Jbuilder needs to know about.
 *** OCaml syntax
 
 If a =jbuild= file starts with =(* -*- tuareg -*- *)=, then it is
-interpreted as an OCaml script that generated the =jbuild= file as
+interpreted as an OCaml script that generates the =jbuild= file as
 described in the rest of this section. The code in the script will
 have access to a [[../plugin/jbuild_plugin.mli][Jbuild_plugin]] module containing details about the
 build context it is executed in.
 
+The script can use the directive =#require= to access libraries:
+
+#+begin_src ocaml
+#require "base,re";;
+#+end_src
+
+Note that any library required by a =jbuild= file must be part of the
+installed world. This mean that your project won't be compilable in
+the same workspace as the required library, unless the library has
+been previously installed.
+
+If you don't like the S-expression syntax, then this method gives you
+a way to use whatever else you want. For instance you could have an
+API to describe your project in OCaml directly:
+
+#+begin_src ocaml
+(* -*- tuareg -*- *)
+#require "my_jbuild_api";;
+include My_jbuild_api.Make(Jbuild_plugin);;
+
+library "foo" ~modules:["plop"; "bidule"];;
+#+end_src
+
 *** Specification
 
 =jbuild= files are composed of stanzas. For instance a typical

src/context.ml

@@ -18,6 +18,7 @@ type t =
   ; for_host                : t option
   ; build_dir               : Path.t
   ; path                    : Path.t list
+  ; toplevel_path           : Path.t option
   ; ocaml_bin               : Path.t
   ; ocaml                   : Path.t
   ; ocamlc                  : Path.t
@@ -86,6 +87,21 @@ let opam_config_var ~env ~cache var =
       Hashtbl.add cache ~key:var ~data:s;
       Some s
 
+let get_env env var =
+  let prefix = var ^ "=" in
+  let rec loop i =
+    if i = Array.length env then
+      None
+    else
+      let entry = env.(i) in
+      if String.is_prefix entry ~prefix then
+        let len_p = String.length prefix in
+        Some (String.sub entry ~pos:len_p ~len:(String.length entry - len_p))
+      else
+        loop (i + 1)
+  in
+  loop 0
+
 let create ~(kind : Kind.t) ~path ~env ~name ~merlin =
   let opam_var_cache = Hashtbl.create 128 in
   (match kind with
@@ -183,6 +199,7 @@ let create ~(kind : Kind.t) ~path ~env ~name ~merlin =
     ; for_host = None
     ; build_dir
     ; path
+    ; toplevel_path = Option.map (get_env env "OCAML_TOPLEVEL_PATH") ~f:Path.of_string
 
     ; ocaml_bin  = dir
     ; ocaml      = Path.relative dir "ocaml"

src/context.mli

@@ -47,6 +47,9 @@ type t =
   ; (** [PATH] *)
     path : Path.t list
 
+  ; (** [OCAML_TOPLEVEL_PATH] *)
+    toplevel_path : Path.t option
+
   ; (** Ocaml bin directory with all ocaml tools *)
     ocaml_bin  : Path.t
   ; ocaml      : Path.t

src/jbuild_load.ml

@@ -34,6 +34,7 @@ module Jbuilds = struct
     let plugin_contents = read_file plugin in
     with_file_out (Path.to_string wrapper) ~f:(fun oc ->
       Printf.fprintf oc {|
+let () = Hashtbl.add Toploop.directive_table "require" (Toploop.Directive_string ignore)
 module Jbuild_plugin = struct
   module V1 = struct
     let context       = %S
@@ -75,12 +76,43 @@ end
         in
         let wrapper = Path.extend_basename generated_jbuild ~suffix:".ml" in
         ensure_parent_dir_exists generated_jbuild;
-        let _requires =
+        let requires =
           create_plugin_wrapper context ~exec_dir:dir ~plugin:file ~wrapper ~target:generated_jbuild
         in
+        let pkgs =
+          List.map requires ~f:(Findlib.find_exn context.findlib)
+          |> Findlib.closure
+        in
+        let includes =
+          List.fold_left pkgs ~init:Path.Set.empty ~f:(fun acc pkg ->
+            Path.Set.add pkg.Findlib.dir acc)
+          |> Path.Set.elements
+          |> List.concat_map ~f:(fun path ->
+              [ "-I"; Path.to_string path ])
+        in
+        let cmas =
+          List.concat_map pkgs ~f:(fun pkg -> pkg.archives.byte)
+        in
+        let args =
+          List.concat
+            [ [ "-I"; "+compiler-libs" ]
+            ; includes
+            ; cmas
+            ; [ Path.reach ~from:dir wrapper ]
+            ]
+        in
+        (* CR-someday jdimino: if we want to allow plugins to use findlib:
+           {[
+             let args =
+               match context.toplevel_path with
+               | None -> args
+               | Some path -> "-I" :: Path.reach ~from:dir path :: args
+             in
+           ]}
+        *)
         Future.run Strict ~dir:(Path.to_string dir) ~env:context.env
-          (Path.to_string context.Context.ocaml)
+          (Path.to_string context.ocaml)
-          [ Path.reach ~from:dir wrapper ]
+          args
         >>= fun () ->
         let sexps = Sexp_load.many (Path.to_string generated_jbuild) in
         return (dir, Stanzas.parse sexps ~dir ~visible_packages))