Avoid generating useless .utop/.merlin files

src/gen_rules.ml

@@ -268,6 +268,7 @@ module Gen(P : Params) = struct
         ~libraries:lib.buildable.libraries
         ~preprocess:lib.buildable.preprocess
         ~virtual_deps:lib.virtual_deps
+        ~has_dot_merlin:lib.buildable.gen_dot_merlin
     in
 
     SC.Libs.setup_runtime_deps sctx ~dir ~dep_kind ~item:lib.name
@@ -520,6 +521,7 @@ module Gen(P : Params) = struct
         ~libraries:exes.buildable.libraries
         ~preprocess:exes.buildable.preprocess
         ~virtual_deps:[]
+        ~has_dot_merlin:exes.buildable.gen_dot_merlin
     in
 
     SC.Libs.add_select_rules sctx ~dir exes.buildable.libraries;
@@ -794,7 +796,6 @@ Add it to your jbuild file to remove this warning.
     Option.iter (Utop.exe_stanzas stanzas) ~f:(fun (exe, all_modules) ->
       let dir = Utop.utop_exe_dir ~dir:ctx_dir in
       let merlin = executables_rules exe ~dir ~all_modules ~scope in
-      Merlin.add_rules sctx ~dir merlin;
       Utop.add_module_rules sctx ~dir merlin.requires;
     )
 

src/jbuild.ml

@@ -442,6 +442,7 @@ module Buildable = struct
     ; ocamlc_flags             : Ordered_set_lang.Unexpanded.t
     ; ocamlopt_flags           : Ordered_set_lang.Unexpanded.t
     ; js_of_ocaml              : Js_of_ocaml.t
+    ; gen_dot_merlin           : bool
     }
 
   let v1 =
@@ -471,6 +472,7 @@ module Buildable = struct
       ; ocamlc_flags
       ; ocamlopt_flags
       ; js_of_ocaml
+      ; gen_dot_merlin = true
       }
 
   let single_preprocess t =

src/jbuild.mli

@@ -109,6 +109,9 @@ module Buildable : sig
     ; ocamlc_flags             : Ordered_set_lang.Unexpanded.t
     ; ocamlopt_flags           : Ordered_set_lang.Unexpanded.t
     ; js_of_ocaml              : Js_of_ocaml.t
+    ; (** [true] except for on-demand utops, to avoid generation
+          [.utop/.merlin] files everywhere. *)
+      gen_dot_merlin           : bool
     }
 
   (** Preprocessing specification used by all modules or [No_preprocessing] *)

src/super_context.ml

@@ -338,12 +338,13 @@ module Libs = struct
        Build.store_vfile vrequires);
     Build.vpath vrequires
 
-  let requires t ~dir ~dep_kind ~item ~libraries ~preprocess ~virtual_deps =
+  let requires t ~dir ~dep_kind ~item ~libraries ~preprocess ~virtual_deps
+        ~has_dot_merlin =
     let real_requires =
       real_requires t ~dir ~dep_kind ~item ~libraries ~preprocess ~virtual_deps
     in
     let requires =
-      if t.context.merlin then
+      if t.context.merlin && has_dot_merlin then
         (* We don't depend on the dot_merlin directly, otherwise everytime it changes we
            would have to rebuild everything.
 

src/super_context.mli

@@ -82,9 +82,10 @@ module Libs : sig
   (** Add rules for (select ...) forms *)
   val add_select_rules : t -> dir:Path.t -> Lib_deps.t -> unit
 
-  (** Returns the closed list of dependencies for a dependency list in a stanza. The
-      second arrow is the same as the first one but with an added dependency on the
-      .merlin. *)
+  (** Returns the closed list of dependencies for a dependency list in
+     a stanza. The second arrow is the same as the first one but with
+     an added dependency on the .merlin if [has_dot_merlin] is
+     [true]. *)
   val requires
     :  t
     -> dir:Path.t
@@ -93,6 +94,7 @@ module Libs : sig
     -> libraries:Lib_deps.t
     -> preprocess:Preprocess_map.t
     -> virtual_deps:string list
+    -> has_dot_merlin:bool
     -> (unit, Lib.t list) Build.t * (unit, Lib.t list) Build.t
 
   (** Setup the rules for ppx runtime dependencies *)

src/utop.ml

@@ -54,6 +54,7 @@ let utop_of_libs (libs : Library.t list) =
       ; ocamlc_flags = Ordered_set_lang.Unexpanded.standard
       ; ocamlopt_flags = Ordered_set_lang.Unexpanded.standard
       ; js_of_ocaml = Js_of_ocaml.default
+      ; gen_dot_merlin = false
       }
   }