feat: introduce the @pkg-install alias

src/dune_rules/alias0.ml

@@ -25,4 +25,5 @@ let check = standard "check"
 let install = standard "install"
 let ocaml_index = standard "ocaml-index"
 let runtest = standard "runtest"
+let pkg_install = standard "pkg-install"
 let all = standard "all"

src/dune_rules/alias0.mli

@@ -14,6 +14,7 @@ val check : Name.t
 val ocaml_index : Name.t
 val install : Name.t
 val runtest : Name.t
+val pkg_install : Name.t
 val all : Name.t
 val is_standard : Name.t -> bool
 val register_as_standard : Name.t -> unit

src/dune_rules/gen_rules.ml

@@ -338,6 +338,19 @@ let gen_rules_group_part_or_root sctx dir_contents cctxs ~source_dir ~dir
   contexts
 ;;
 
+let gen_project_rule_for_package_alias sctx project =
+  let ctx = Super_context.context sctx in
+  let ctx_name = Context.name ctx in
+  let* is_lock_dir_active = Lock_dir.lock_dir_active ctx_name in
+  if is_lock_dir_active
+  then (
+    let dir =
+      Path.Build.append_source (Context.build_dir ctx) @@ Dune_project.root project
+    in
+    Pkg_rules.gen_rule_alias_from_package_universe ~dir ctx_name)
+  else Memo.return ()
+;;
+
 (* Warn whenever [(name <name>)]) is missing from the [dune-project] file *)
 let missing_project_name =
   Warning.make
@@ -355,6 +368,7 @@ let gen_project_rules =
     and+ () = Odoc.gen_project_rules sctx project
     and+ () = Odoc_new.gen_project_rules sctx project
     and+ () = Ocaml_index.project_rule sctx project
+    and+ () = gen_project_rule_for_package_alias sctx project
     and+ () =
       let version = 2, 8 in
       match Dune_project.allow_approximate_merlin project with

src/dune_rules/pkg_rules.ml

@@ -1937,6 +1937,25 @@ let build_rule context_name ~source_deps (pkg : Pkg.t) =
        ~directory_targets:[ pkg.write_paths.target_dir ]
 ;;
 
+let gen_rule_alias_from_package_universe ~dir ctx_name =
+  let target_path_of_pkg pkg =
+    let pkg_name = Dune_lang.Package_name.to_string pkg in
+    Path.Build.L.relative
+      Private_context.t.build_dir
+      [ Context_name.to_string ctx_name; ".pkg"; pkg_name; "target" ]
+    |> Path.build
+  in
+  let* packages =
+    Package_universe.lock_dir (Project_dependencies ctx_name)
+    >>| (fun lock_dir -> lock_dir.packages)
+    >>| Dune_lang.Package_name.Map.keys
+  in
+  let alias = Alias.make Alias0.pkg_install ~dir in
+  List.map ~f:target_path_of_pkg packages
+  |> Action_builder.paths
+  |> Rules.Produce.Alias.add_deps alias
+;;
+
 let gen_rules context_name (pkg : Pkg.t) =
   let* source_deps, copy_rules = source_rules pkg in
   let* () = copy_rules

src/dune_rules/pkg_rules.mli

@@ -7,7 +7,10 @@ open Import
     - A rule for fetching the source to produce .pkg/$package/source
 
     - A rule to build the package and produce the artifacts in
-      .pkg/$package/target *)
+      .pkg/$package/target.
+
+    It setups an alias rules to trigger the fetch and build of the
+    package universe. *)
 
 val setup_rules
   :  components:string list
@@ -22,3 +25,4 @@ val which : Context_name.t -> (Filename.t -> Path.t option Memo.t) Staged.t
 val exported_env : Context_name.t -> Env.t Memo.t
 val ocamlpath : Context_name.t -> Path.t list Memo.t
 val find_package : Context_name.t -> Package.Name.t -> unit Action_builder.t option Memo.t
+val gen_rule_alias_from_package_universe : dir:Path.Build.t -> Context_name.t -> unit Memo.t

test/blackbox-tests/test-cases/pkg/build-pkg-deps.t

@@ -0,0 +1,74 @@
+This test verifies the @pkg-deps alias fetch and build the project dependencies
+without building the project itself.
+
+  $ . ./helpers.sh
+
+Create a fake library to install in the target project as a dependency:
+  $ mkdir foo
+  $ cd foo
+  $ cat > dune-project <<EOF
+  > (lang dune 3.16)
+  > (package (name foo))
+  > EOF
+  $ cat > foo.ml <<EOF
+  > let foo = "Hello, World!"
+  > EOF
+  $ cat > dune <<EOF
+  > (library
+  >  (public_name foo))
+  > EOF
+  $ cd ..
+  $ tar cf foo.tar foo
+  $ rm -rf foo
+
+Make an opam package for the library:
+  $ mkpkg foo <<EOF
+  > build: [
+  >   [
+  >     "dune"
+  >     "build"
+  >     "-p"
+  >     name
+  >     "@install"
+  >   ]
+  > ]
+  > url {
+  >  src: "$PWD/foo.tar"
+  > }
+  > EOF
+
+Create a project using the fake library as a dependency:
+  $ cat > dune-project << EOF
+  > (lang dune 3.16)
+  > (package
+  >  (name bar)
+  >  (allow_empty)
+  >  (depends foo))
+  > EOF
+  $ add_mock_repo_if_needed
+  $ dune pkg lock
+  Solution for dune.lock:
+  - foo.0.0.1
+
+Create an executable which uses the fake dependency. It ensures we build the
+dependency package but not the package:
+  $ cat > dune << EOF
+  > (executable
+  >  (name bar)
+  >  (libraries foo))
+  > EOF
+  $ cat > bar.ml << EOF
+  > let () = print_endline "source"
+  > EOF
+
+The alias call builds the `foo` dependency but not the project itself. We
+verify we have the `foo` package but not the `bar.exe` in the build directory:
+  $ dune build @pkg-deps
+  $ ls _build/_private/default/.pkg/
+  foo
+  $ ls _build/default/
+
+Verify that building the `bar.exe` file is working:
+  $ dune build ./bar.exe
+  $ ls _build/default/bar.exe
+  _build/default/bar.exe