fix: root module should have no dependencies

The root module is generated and only depends on the entry modules of
the libraries field.

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg

doc/changes/12227.md

@@ -0,0 +1,2 @@
+- Don't run ocamldep to compute false dependencies on the `root_module`
+  (#12227, @rgrinberg)

src/dune_rules/dep_rules.ml

@@ -137,15 +137,15 @@ let rec deps_of
           ~ml_kind
           (m : Modules.Sourced_module.t)
   =
-  let is_alias =
+  let is_alias_or_root =
     match m with
     | Impl_of_virtual_module _ -> false
     | Imported_from_vlib m | Normal m ->
       (match Module.kind m with
-       | Alias _ -> true
+       | Root | Alias _ -> true
        | _ -> false)
   in
-  if is_alias
+  if is_alias_or_root
   then Memo.return (Action_builder.return [])
   else (
     let skip_if_source_absent f sourced_module =
@@ -176,7 +176,7 @@ let has_single_file modules = Option.is_some @@ Modules.With_vlib.as_singleton m
 
 let immediate_deps_of unit modules ~obj_dir ~ml_kind =
   match Module.kind unit with
-  | Alias _ -> Action_builder.return []
+  | Root | Alias _ -> Action_builder.return []
   | Wrapped_compat ->
     let interface_module =
       match Modules.With_vlib.lib_interface modules with

src/dune_rules/obj_dir.ml

@@ -618,7 +618,7 @@ module Module = struct
     match (dep : Dep.t) with
     | Immediate (m, _) | Transitive (m, _) ->
       (match Module.kind m with
-       | Module.Kind.Alias _ -> None
+       | Module.Kind.Alias _ | Root -> None
        | _ ->
          let dir = obj_dir t in
          let name = Dep.basename dep in

src/dune_rules/ocamldep.ml

@@ -109,7 +109,7 @@ let parse_deps_exn =
 let transitive_deps =
   let transive_dep obj_dir m =
     (match Module.kind m with
-     | Alias _ -> None
+     | Root | Alias _ -> None
      | _ -> if Module.has m ~ml_kind:Intf then Some Ml_kind.Intf else Some Impl)
     |> Option.map ~f:(fun ml_kind ->
       Obj_dir.Module.dep obj_dir (Transitive (m, ml_kind))

test/blackbox-tests/test-cases/root-module/root-module-ocamldep.t

@@ -26,8 +26,6 @@ compilation unit.
   _build/default/.foo.objs/byte/foo__Root.cmi
   _build/default/.foo.objs/byte/foo__Root.cmo
   _build/default/.foo.objs/byte/foo__Root.cmt
-  _build/default/.foo.objs/foo__Root.impl.all-deps
-  _build/default/.foo.objs/foo__Root.impl.d
 
 Not only is running ocamldep wasteful, but it can also lead to cycles:
 
@@ -36,10 +34,3 @@ Not only is running ocamldep wasteful, but it can also lead to cycles:
   > EOF
 
   $ dune build foo.cma
-  Error: Dependency cycle between:
-     _build/default/.foo.objs/foo__Bar.impl.all-deps
-  -> _build/default/.foo.objs/foo__Root.impl.all-deps
-  -> _build/default/.foo.objs/foo__Bar.impl.all-deps
-  -> required by _build/default/.foo.objs/byte/foo__Bar.cmo
-  -> required by _build/default/foo.cma
-  [1]