Add support for (empty_module_interface_if_absent)

CHANGES.md

@@ -201,6 +201,9 @@ Unreleased
 - Dune no longer reads installed META files for libraries distributed with the
   compiler, instead using its own internal database. (#4946, @nojb)
 
+- Add support for `(empty_module_interface_if_absent)` in executable and library
+  stanzas. (#4955, @nojb)
+
 2.9.1 (07/09/2021)
 ------------------
 

doc/dune-files.rst

@@ -626,6 +626,11 @@ to use the :ref:`include_subdirs` stanza.
   is useful whenever a library is shadowed by a local module. The library may
   then still be accessible via this root module
 
+- ``(empty_module_interface_if_absent)`` causes the generation of empty
+  interfaces for every module that does not have an interface file already.
+  Useful when modules are used solely for their side-effects. This field is
+  available since the 3.0 version of the dune language.
+
 Note that when binding C libraries, dune doesn't provide special support for
 tools such as ``pkg-config``, however it integrates easily with
 :ref:`configurator` by
@@ -825,6 +830,11 @@ files for executables. See `executables_implicit_empty_intf`_.
   flag if some of the libraries listed here are not referenced from any of the
   plugin modules.
 
+- ``(empty_module_interface_if_absent)`` causes the generation of empty
+  interfaces for every module that does not have an interface file already.
+  Useful when modules are used solely for their side-effects. This field is
+  available since the 3.0 version of the dune language.
+
 Linking modes
 ~~~~~~~~~~~~~
 

src/dune_rules/dune_file.ml

@@ -152,6 +152,7 @@ module Buildable = struct
     { loc : Loc.t
     ; modules : Ordered_set_lang.t
     ; modules_without_implementation : Ordered_set_lang.t
+    ; empty_module_interface_if_absent : bool
     ; libraries : Lib_dep.t list
     ; foreign_archives : (Loc.t * Foreign.Archive.t) list
     ; foreign_stubs : Foreign.Stubs.t list
@@ -263,6 +264,9 @@ module Buildable = struct
     and+ root_module =
       field_o "root_module"
         (Dune_lang.Syntax.since Stanza.syntax (2, 8) >>> Module_name.decode_loc)
+    and+ empty_module_interface_if_absent =
+      field_b "empty_module_interface_if_absent"
+        ~check:(Dune_lang.Syntax.since Stanza.syntax (3, 0))
     in
     let preprocess =
       let init =
@@ -312,6 +316,7 @@ module Buildable = struct
     ; lint
     ; modules
     ; modules_without_implementation
+    ; empty_module_interface_if_absent
     ; foreign_stubs
     ; foreign_archives
     ; libraries

src/dune_rules/dune_file.mli

@@ -42,6 +42,7 @@ module Buildable : sig
     { loc : Loc.t
     ; modules : Ordered_set_lang.t
     ; modules_without_implementation : Ordered_set_lang.t
+    ; empty_module_interface_if_absent : bool
     ; libraries : Lib_dep.t list
     ; foreign_archives : (Loc.t * Foreign.Archive.t) list
     ; foreign_stubs : Foreign.Stubs.t list

src/dune_rules/exe_rules.ml

@@ -91,20 +91,6 @@ let o_files sctx ~dir ~expander ~(exes : Executables.t) ~linkages ~dir_contents
     in
     List.map o_files ~f:Path.build
 
-let with_empty_intf ~sctx ~dir module_ =
-  let name =
-    Module.file module_ ~ml_kind:Impl
-    |> Option.value_exn
-    |> Path.set_extension ~ext:".mli"
-  in
-  let rule =
-    Action_builder.write_file
-      (Path.as_in_build_dir_exn name)
-      "(* Auto-generated by Dune *)"
-  in
-  let+ () = Super_context.add_rule sctx ~dir rule in
-  Module.add_file module_ Ml_kind.Intf (Module.File.make Dialect.ocaml name)
-
 let executables_rules ~sctx ~dir ~expander ~dir_contents ~scope ~compile_info
     ~embed_in_plugin_libraries (exes : Dune_file.Executables.t) =
   (* Use "eobjs" rather than "objs" to avoid a potential conflict with a library
@@ -138,17 +124,20 @@ let executables_rules ~sctx ~dir ~expander ~dir_contents ~scope ~compile_info
     let executable_names =
       List.map exes.names ~f:Module_name.of_string_allow_invalid
     in
+    let add_empty_intf = exes.buildable.empty_module_interface_if_absent in
     Modules.map_user_written modules ~f:(fun m ->
         let name = Module.name m in
         let* m = Pp_spec.pp_module_as pp name m in
         let add_empty_intf =
+          (add_empty_intf
+          ||
           let project = Scope.project scope in
           Dune_project.executables_implicit_empty_intf project
-          && List.mem executable_names name ~equal:Module_name.equal
+          && List.mem executable_names name ~equal:Module_name.equal)
           && not (Module.has m ~ml_kind:Intf)
         in
         if add_empty_intf then
-          with_empty_intf ~sctx ~dir m
+          Module_compilation.with_empty_intf ~sctx ~dir m
         else
           Memo.Build.return m)
   in

src/dune_rules/lib_rules.ml

@@ -375,7 +375,13 @@ let cctx (lib : Library.t) ~sctx ~source_modules ~dir ~expander ~scope
       ~lib_name:(Some (snd lib.name))
   in
   let+ modules =
-    Modules.map_user_written source_modules ~f:(Pp_spec.pp_module pp)
+    let add_empty_intf = lib.buildable.empty_module_interface_if_absent in
+    Modules.map_user_written source_modules ~f:(fun m ->
+        let* m = Pp_spec.pp_module pp m in
+        if add_empty_intf && not (Module.has m ~ml_kind:Intf) then
+          Module_compilation.with_empty_intf ~sctx ~dir m
+        else
+          Memo.Build.return m)
   in
   let modules = Vimpl.impl_modules vimpl modules in
   let requires_compile = Lib.Compile.direct_requires compile_info in

src/dune_rules/module_compilation.ml

@@ -387,3 +387,18 @@ let build_all cctx ~dep_graphs =
             cctx
         in
         build_module cctx ~dep_graphs m)
+
+let with_empty_intf ~sctx ~dir module_ =
+  let name =
+    Module.file module_ ~ml_kind:Impl
+    |> Option.value_exn
+    |> Path.set_extension ~ext:".mli"
+  in
+  let rule =
+    Action_builder.write_file
+      (Path.as_in_build_dir_exn name)
+      "(* Auto-generated by Dune *)"
+  in
+  let open Memo.Build.O in
+  let+ () = Super_context.add_rule sctx ~dir rule in
+  Module.add_file module_ Ml_kind.Intf (Module.File.make Dialect.ocaml name)

src/dune_rules/module_compilation.mli

@@ -21,3 +21,6 @@ val ocamlc_i :
 
 val build_all :
   Compilation_context.t -> dep_graphs:Dep_graph.Ml_kind.t -> unit Memo.Build.t
+
+val with_empty_intf :
+  sctx:Super_context.t -> dir:Path.Build.t -> Module.t -> Module.t Memo.Build.t

test/blackbox-tests/test-cases/buildable-empty-intf-if-absent.t/run.t

@@ -0,0 +1,70 @@
+Generate empty interfaces when using (empty_module_interface_if_absent)
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+Executables
+
+  $ cat >main.ml <<EOF
+  > module D = Dep
+  > EOF
+
+  $ touch dep.ml
+
+  $ cat >dune <<EOF
+  > (executable (name main) (empty_module_interface_if_absent))
+  > EOF
+
+  $ dune build ./main.exe
+
+  $ cat _build/default/dep.mli
+  (* Auto-generated by Dune *)
+
+Libraries
+
+  $ cat >dune <<EOF
+  > (library (name l) (modules a b) (empty_module_interface_if_absent))
+  > EOF
+
+  $ touch a.ml b.ml
+
+  $ dune build
+
+  $ cat _build/default/a.mli
+  (* Auto-generated by Dune *)
+  $ cat _build/default/b.mli
+  (* Auto-generated by Dune *)
+
+Version check
+
+  $ cat >dune-project <<EOF
+  > (lang dune 2.9)
+  > EOF
+
+  $ dune build
+  File "dune", line 1, characters 32-66:
+  1 | (library (name l) (modules a b) (empty_module_interface_if_absent))
+                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: 'empty_module_interface_if_absent' is only available since version 3.0
+  of the dune language. Please update your dune-project file to have (lang dune
+  3.0).
+  [1]
+
+Check that interfaces are *not* generated if field is not used
+
+  $ cat >dune <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library (name l) (modules a b))
+  > EOF
+
+  $ touch a.mli
+
+  $ dune build
+
+  $ test -f _build/default/a.mli
+
+  $ test ! -f _build/default/b.mli