Fix handling of root module dependencies

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

@@ -0,0 +1,36 @@
+Demonstrate if we're running ocamldep for the root module. There should be no
+need to do so since this module cannot depend on any other module in the same
+compilation unit.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.20)
+  > EOF
+
+  $ mkdir lib/
+  $ cat >lib/dune <<EOF
+  > (library
+  >  (name bar))
+  > EOF
+  $ touch lib/bar.ml
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name foo)
+  >  (libraries bar)
+  >  (root_module root))
+  > EOF
+
+  $ dune build foo.cma
+
+  $ find _build/default/.foo.objs | grep -i root | sort -u
+  _build/default/.foo.objs/byte/foo__Root.cmi
+  _build/default/.foo.objs/byte/foo__Root.cmo
+  _build/default/.foo.objs/byte/foo__Root.cmt
+
+Not only is running ocamldep wasteful, but it can also lead to cycles:
+
+  $ cat >bar.ml <<EOF
+  > module X = Root.Bar
+  > EOF
+
+  $ dune build foo.cma