Use absolute paths for ppx drivers in .merlin files (#201)

Fixes #196 and #199

CHANGES.md

@@ -8,6 +8,10 @@
   would do: `-I <path> file.cmxa`, now it does `-I <path>
   <path>/file.cmxa`. Fixes #118 and #177
 
+- Use an absolute path for ppx drivers in `.merlin` files. Merlin
+  <3.0.0 used to run ppx commands from the directory where the
+  `.merlin` was present but this is no longer the case
+
 - Allow to use `jbuilder install` in contexts other than opam; if
   `ocamlfind` is present in the `PATH` and the user didn't pass
   `--prefix` or `--libdir` explicitly, use the output of `ocamlfind

src/merlin.ml

@@ -11,12 +11,15 @@ type t =
   ; libname    : string option
   }
 
-let ppx_flags sctx ~dir ~src_dir { preprocess; libname; _ } =
+(* This must be forced after we change the cwd to the workspace root *)
+let root_absolute_name = lazy (Sys.getcwd ())
+
+let ppx_flags sctx ~dir ~src_dir:_ { preprocess; libname; _ } =
   match preprocess with
   | Pps { pps; flags } ->
     let exe = SC.PP.get_ppx_driver sctx pps ~dir ~dep_kind:Optional in
     let command =
-      List.map (Path.reach exe ~from:src_dir
+      List.map (Filename.concat (Lazy.force root_absolute_name) (Path.to_string exe)
                 :: "--as-ppx"
                 :: SC.PP.cookie_library_name libname
                 @ flags)