ppx deriving on a type adversely affects merlin location

[ddickstein]

Given:

open Core
module M = struct
  type t =
    | Foo
    | Bar
end

Getting the type of t highlights just the type definition and gives type t = Foo | Bar as expected. However, given:

open Core
module M = struct
  type t =
    | Foo
    | Bar
  [@@deriving compare]
end

Getting the type of t selects the entire module and returns it as sig type t = Foo | Bar val compare : t -> t -> int end.

[voodoos]

Retrospectively, I discovered the merlin.hide and merlin.focus attributes which are specifically designed for ppxes. The better fix would be for ppx_compare to make use of merlin.focus to guide Merlin in the correct branch.

Doc: https://github.com/ocaml/merlin/blob/master/doc/dev/PROTOCOL.md#locations-in-ppx-rewriters