Substitute watermarks for jbuilder itself

bin/main.ml

@@ -646,7 +646,7 @@ let subst =
            what topkg does in the default configuration.|}
     ; `P {|If you use topkg to handle the releases of your project, then you
            should add this line to the $(b,build:) instructions in your opam file:|}
-    ; `Pre {|  ["jbuilder" "subst" name]|}
+    ; `Pre {|  ["jbuilder" "subst" name] {pinned}|}
     ; `Blocks help_secs
     ]
   in

jbuilder.opam

@@ -7,6 +7,7 @@ dev-repo: "git+https://github.com/janestreet/jbuilder.git"
 license: "Apache-2.0"
 build: [
   ["ocaml" "bootstrap.ml"]
+  ["./boot.exe" "--subst"] {pinned}
   ["./boot.exe" "-j" jobs]
 ]
 depends: [

src/main.ml

@@ -159,11 +159,15 @@ let bootstrap () =
   let pkg = "jbuilder" in
   let main () =
     let anon s = raise (Arg.Bad (Printf.sprintf "don't know what to do with %s\n" s)) in
+    let subst () =
+      Future.Scheduler.go (Watermarks.subst ~package:"jbuilder")
+    in
     Arg.parse
       [ "-j"           , Set_int Clflags.concurrency, "JOBS concurrency"
       ; "--dev"        , Set Clflags.dev_mode       , " set development mode"
       ; "--debug-rules", Set Clflags.debug_rules    , " print out rules"
       ; "--verbose"    , Set Clflags.verbose        , " print detailed information about commands being run"
+      ; "--subst"      , Unit subst                 , " substitute watermarks in source files"
       ]
       anon "Usage: boot.exe [-j JOBS] [--dev]\nOptions are:";
     let log = Log.create () in