Bring back advanced logic for finding runtime path

Author: cknitt

compiler/ext/config.ml

@@ -1,15 +1,34 @@
 (* This resolves the location of the standard library starting from the location of bsc.exe
-   (@rescript/{platform}/bin/bsc.exe), assuming the standard npm node_modules layout.
-
-   For pnpm and other special node_modules layouts, the correct path shall be passed
-   via the `-runtime` flag. *)
+   (@rescript/{platform}/bin/bsc.exe), handling different supported package layouts. *)
 let standard_library =
-  let exe_path = Sys.executable_name in
-  let bin_dir = Filename.dirname exe_path in
-  let platform_dir = Filename.dirname bin_dir in
-  let rescript_dir = Filename.dirname platform_dir in
-  let ( // ) = Filename.concat in
-  rescript_dir // "runtime" // "lib" // "ocaml"
+  let build_path rest path =
+    String.concat Filename.dir_sep (List.rev_append rest path)
+  in
+  match
+    Sys.executable_name |> Filename.dirname
+    |> String.split_on_char Filename.dir_sep.[0]
+    |> List.rev
+  with
+  (* 1. Packages installed via pnpm
+     - bin:    node_modules/.pnpm/@[email protected]/node_modules/@rescript/darwin-arm64/bin
+     - stdlib: node_modules/@rescript/runtime/lib/ocaml (symlink)
+  *)
+  | "bin" :: _platform :: "@rescript" :: "node_modules" :: _package :: ".pnpm"
+    :: "node_modules" :: rest ->
+    build_path rest ["node_modules"; "@rescript"; "runtime"; "lib"; "ocaml"]
+  (* 2. Packages installed via npm
+     - bin:    node_modules/@rescript/{platform}/bin
+     - stdlib: node_modules/@rescript/runtime/lib/ocaml
+  *)
+  | "bin" :: _platform :: "@rescript" :: "node_modules" :: rest ->
+    build_path rest ["node_modules"; "@rescript"; "runtime"; "lib"; "ocaml"]
+  (* 3. Several other cases that can occur in local development, e.g.
+     - bin:    <repo>/packages/@rescript/{platform}/bin, <repo>/_build/install/default/bin
+     - stdlib: <repo>/packages/@rescript/runtime/lib/ocaml
+  *)
+  | _ :: _ :: _ :: _ :: rest ->
+    build_path rest ["packages"; "@rescript"; "runtime"; "lib"; "ocaml"]
+  | _ -> ""
 
 let cmi_magic_number = "Caml1999I022"