refactor: move library path handling to single to one place (#7728)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

src/dune_rules/expander.ml

@@ -279,21 +279,23 @@ let expand_read_macro ~dir ~source s ~read ~pack =
 let file_of_lib { Artifacts.Public_libs.context; public_libs } ~loc ~lib ~file =
   let open Resolve.Memo.O in
   let+ lib = Lib.DB.resolve public_libs (loc, lib) in
-  if Lib.is_local lib then
-    let package, rest = Lib_name.split (Lib.name lib) in
-    let lib_install_dir =
-      let lib_install_dir =
+  let dir =
+    let info = Lib.info lib in
+    match Lib.is_local lib with
+    | false -> Lib_info.src_dir info
+    | true ->
+      let name = Lib.name lib in
+      let subdir =
+        Lib_info.Status.relative_to_package (Lib_info.status info) name
+        |> Option.value_exn
+      in
+      let pkg_root =
+        let package = Lib_name.package_name name in
         Local_install_path.lib_dir ~context:context.name ~package
       in
-      match rest with
-      | [] -> lib_install_dir
-      | _ -> Path.Build.relative lib_install_dir (String.concat rest ~sep:"/")
-    in
-    Path.build (Path.Build.relative lib_install_dir file)
-  else
-    let info = Lib.info lib in
-    let src_dir = Lib_info.src_dir info in
-    Path.relative src_dir file
+      Path.build (Path.Build.append_local pkg_root subdir)
+  in
+  Path.relative dir file
 
 let expand_lib_variable t source ~arg:s ~lib_exec ~lib_private =
   let loc = Dune_lang.Template.Pform.loc source in

src/dune_rules/install_rules.ml

@@ -527,18 +527,12 @@ end = struct
     in
     let lib_root lib =
       let subdir =
-        let name = Lib.name lib in
-        let _, subdir = Lib_name.split name in
-        match
-          let info = Lib.info lib in
-          Lib_info.status info
-        with
-        | Private (_, Some _) ->
-          Lib_name.Local.mangled_path_under_package (Lib_name.to_local_exn name)
-          @ subdir
-        | _ -> subdir
+        Lib_info.Status.relative_to_package
+          (Lib_info.status @@ Lib.info lib)
+          (Lib.name lib)
+        |> Option.value_exn
       in
-      Path.Build.L.relative pkg_root subdir
+      Path.Build.append_local pkg_root subdir
     in
     let* entries =
       Memo.parallel_map lib_entries ~f:(fun (stanza : Scope.DB.Lib_entry.t) ->

src/dune_rules/lib_info.ml

@@ -223,6 +223,18 @@ module Status = struct
   let project = function
     | Installed_private | Installed -> None
     | Private (project, _) | Public (project, _) -> Some project
+
+  let relative_to_package t name =
+    match t with
+    | Private (_, None) -> None
+    | _ ->
+      (let _, subdir = Lib_name.split name in
+       match t with
+       | Private (_, Some _) ->
+         Lib_name.Local.mangled_path_under_package (Lib_name.to_local_exn name)
+         @ subdir
+       | _ -> subdir)
+      |> String.concat ~sep:"/" |> Path.Local.of_string |> Option.some
 end
 
 module Source = struct

src/dune_rules/lib_info.mli

@@ -19,6 +19,10 @@ module Status : sig
 
   (** For local libraries, return the project they are part of *)
   val project : t -> Dune_project.t option
+
+  (** [relative_to_package t name] return the path of [name] relative to the
+      package determined by [t]. If there's no package, return [None] *)
+  val relative_to_package : t -> Lib_name.t -> Path.Local.t option
 end
 
 (** For values like modules that need to be evaluated to be fetched *)