Unhardcode _build dir everywhere

bin/main.ml

@@ -728,7 +728,7 @@ let clean =
       set_common common ~targets:[];
       Build_system.files_in_source_tree_to_delete ()
       |> List.iter ~f:Path.unlink_no_err;
-      Path.(rm_rf (append root (of_string "_build")))
+      Path.rm_rf Path.build_dir
     end
   in
   ( Term.(const go $ common)

src/action.ml

@@ -647,10 +647,10 @@ module Promotion = struct
       Io.copy_file ~src ~dst
   end
 
-  let db_file = Path.of_string "_build/.to-promote"
+  let db_file = Path.relative_build_dir ".to-promote"
 
   let dump_db db =
-    if Sys.file_exists "_build" then begin
+    if Path.build_dir_exists () then begin
       match db with
       | [] -> if Path.exists db_file then Path.unlink_no_err db_file
       | l ->

src/build_system.ml

@@ -14,7 +14,7 @@ let misc_dir = Path.(relative build_dir) ".misc"
 module Promoted_to_delete = struct
   let db = ref []
 
-  let fn = Path.of_string "_build/.to-delete-in-source-tree"
+  let fn = Path.relative_build_dir ".to-delete-in-source-tree"
 
   let add p = db := p :: !db
 
@@ -27,7 +27,7 @@ module Promoted_to_delete = struct
 
   let dump () =
     let db = Pset.union (Pset.of_list !db) (Pset.of_list (load ())) in
-    if Sys.file_exists "_build" then
+    if Path.build_dir_exists () then
       Io.write_file fn
         (String.concat ~sep:""
            (List.map (Pset.to_list db) ~f:(fun p ->
@@ -1106,7 +1106,7 @@ let stamp_file_for_files_of t ~dir ~ext =
 module Trace = struct
   type t = (Path.t, Digest.t) Hashtbl.t
 
-  let file = Path.of_string "_build/.db"
+  let file = Path.relative_build_dir ".db"
 
   let dump (trace : t) =
     let sexp =
@@ -1118,7 +1118,7 @@ module Trace = struct
                Sexp.List [ Path.sexp_of_t path;
                            Atom (Sexp.Atom.of_digest hash) ]))
     in
-    if Sys.file_exists "_build" then
+    if Path.build_dir_exists () then
       Io.write_file file (Sexp.to_string sexp)
 
   let load () =
@@ -1451,7 +1451,7 @@ let get_collector t ~dir =
       (if Path.is_in_source_tree dir then
          "Build_system.get_collector called on source directory"
        else if dir = Path.build_dir then
-         "Build_system.get_collector called on _build"
+         "Build_system.get_collector called on build_dir"
        else if not (Path.is_local dir) then
          "Build_system.get_collector called on external directory"
        else

src/context.mli

@@ -8,7 +8,7 @@
 
     - opam switch contexts, where one opam switch correspond to one context
 
-    each context is built into a sub-directory of "_build":
+    each context is built into a sub-directory of Path.build_dir (usually _build):
 
     - _build/default for the default context
     - _build/<switch> for other contexts

src/log.ml

@@ -12,9 +12,8 @@ type t = real option
 let no_log = None
 
 let create ?(display=Config.default.display) () =
-  if not (Sys.file_exists "_build") then
-    Unix.mkdir "_build" 0o777;
-  let oc = Io.open_out (Path.of_string "_build/log") in
+  Path.ensure_build_dir_exists ();
+  let oc = Io.open_out (Path.relative_build_dir "log") in
   Printf.fprintf oc "# %s\n# OCAMLPARAM: %s\n%!"
     (String.concat (List.map (Array.to_list Sys.argv) ~f:quote_for_shell) ~sep:" ")
     (match Env.get Env.initial "OCAMLPARAM" with

src/stdune/path.ml

@@ -453,6 +453,12 @@ let unlink t =
   unlink_operation (to_string t)
 let unlink_no_err t = try unlink t with _ -> ()
 
+let build_dir_exists () = is_directory build_dir
+
+let ensure_build_dir_exists () = Local.mkdir_p build_dir
+
+let relative_build_dir = relative build_dir
+
 let extend_basename t ~suffix = t ^ suffix
 
 let insert_after_build_dir_exn =

src/stdune/path.mli

@@ -146,3 +146,9 @@ val extension : t -> string
 val drop_prefix : t -> prefix:t -> string option
 
 val pp : Format.formatter -> t -> unit
+
+val build_dir_exists : unit -> bool
+
+val ensure_build_dir_exists : unit -> unit
+
+val relative_build_dir : string -> t

src/utils.ml

@@ -183,7 +183,7 @@ module Cached_digest = struct
 
   let remove fn = Hashtbl.remove cache fn
 
-  let db_file = Path.of_string "_build/.digest-db"
+  let db_file = Path.relative_build_dir ".digest-db"
 
   let dump () =
     let module Pmap = Path.Map in
@@ -199,7 +199,7 @@ module Cached_digest = struct
                               (Int64.bits_of_float file.timestamp))
                     ]))
     in
-    if Sys.file_exists "_build" then
+    if Path.build_dir_exists () then
       Io.write_file db_file (Sexp.to_string sexp)
 
   let load () =