Allow setting ENV vars in findlib.conf per toolchain

This is useful for configuring PKG CONFIG for cross compilation

src/context.ml

@@ -315,6 +315,10 @@ let create ~(kind : Kind.t) ~path ~env ~name ~merlin ~targets () =
         | Some host ->
           Env.get host.env "PATH"
       )
+      |> Env.extend_env (
+        Option.value ~default:Env.empty
+          (Option.map findlib_config ~f:Findlib.Config.env)
+      )
     in
     let stdlib_dir = Path.of_string (Ocaml_config.standard_library ocfg) in
     let natdynlink_supported = Ocaml_config.natdynlink_supported ocfg in

src/env.ml

@@ -22,6 +22,8 @@ let make vars =
   ; unix = None
   }
 
+let empty = make Map.empty
+
 let get t k = Map.find t.vars k
 
 let to_unix t =
@@ -73,3 +75,6 @@ let diff x y =
 
 let update t ~var ~f =
   make (Map.update t.vars var ~f)
+
+let of_string_map m =
+  make (String.Map.foldi ~init:Map.empty ~f:(fun k v acc -> Map.add acc k v) m)

src/env.mli

@@ -9,6 +9,8 @@ type t
 
 module Map : Map.S with type key = Var.t
 
+val empty : t
+
 (** The environment when the process started *)
 val initial : t
 
@@ -28,3 +30,4 @@ val update : t -> var:string -> f:(string option -> string option) -> t
 
 val sexp_of_t : t -> Sexp.t
 
+val of_string_map : string String.Map.t -> t

src/findlib.ml

@@ -45,22 +45,24 @@ module Rules = struct
     ; add_rules : Rule.t list
     }
 
-  let interpret t ~preds =
+  let interpret' t ~preds =
     let rec find_set_rule = function
-      | [] -> ""
+      | [] -> None
       | rule :: rules ->
         if Rule.matches rule ~preds then
-          rule.value
+          Some rule.value
         else
           find_set_rule rules
     in
     let v = find_set_rule t.set_rules in
     List.fold_left t.add_rules ~init:v ~f:(fun v rule ->
       if Rule.matches rule ~preds then
-        v ^ " " ^ rule.value
+        Some ((Option.value ~default:"" v) ^ " " ^ rule.value)
       else
         v)
 
+  let interpret t ~preds = Option.value ~default:"" (interpret' t ~preds)
+
   let of_meta_rules (rules : Meta.Simplified.Rules.t) =
     let add_rules = List.map rules.add_rules ~f:Rule.make in
     let set_rules =
@@ -106,6 +108,13 @@ module Config = struct
 
   let get { vars; preds } var =
     Vars.get vars var preds
+
+  let env t =
+    let preds = Ps.add t.preds (P.make "env") in
+    String.Map.filter_map ~f:(fun rules ->
+      Rules.interpret' rules ~preds
+    ) t.vars
+    |> Env.of_string_map
 end
 
 module Package = struct

src/findlib.mli

@@ -62,4 +62,6 @@ module Config : sig
   type t
   val load : Path.t -> toolchain:string -> context:string -> t
   val get : t -> string -> string option
+
+  val env : t -> Env.t
 end