Include_subdirs qualified + menhir produces unbound module

Learning
,
1

TL;DR

I am working with dune and have the option (include_subdirs qualified).
I’m trying to move the files related to parsing (Syntax.mly, Ast.ml, Tokens.mll) to a subdirectory Parsing. Unfortunately Ast.ml specifically is not found when building and I can’t figure out why.

Details

My original project structure was

--- lib/
     |--- dune
     |--- Ast.ml
     |--- Syntax.mly
     |--- Tokens.mll
     '--- ...

with

;; lib/dune
...
(include_subdirs qualified)
(ocamllex Tokens)
(menhir (modules Syntax))

(* lib/Syntax.mly *)
%{ open Ast }%
...

This works but is unsatisfactory.

My current project structure is

--- lib/
     |--- dune
     |--- Ast.ml
     |--- Parsing/
     |     |--- dune
     |     |--- Syntax.mly
     |     '--- Tokens.mll
     '--- ...

with

;; lib/dune
...
(include_subdirs qualified)

;; lib/Parsing/dune
(ocamllex Tokens)
(menhir (modules Syntax))

(* lib/Parsing/Syntax.mly *)
%{ open Ast }%
...

This still works.

My desired project structure is

--- lib/
     |--- dune
     |--- Parsing/
     |     |--- dune
     |     |--- Ast.ml
     |     |--- Syntax.mly
     |     '--- Tokens.mll
     '--- ...

with

;; lib/dune
...
(include_subdirs qualified)

;; lib/Parsing/dune
(ocamllex Tokens)
(menhir (modules Syntax))

(* lib/Parsing/Syntax.mly *)
%{ open ??? }%
...

but I don’t know what to put in place of ??? in lib/Parsing/Syntax.mly to make this build.
If I open Ast then I get

File "lib/Parsing/Syntax.mly", line 2, characters 9-12:
Error: Unbound module Ast

while if I try open Parsing.Ast then I get instead

Error: Dependency cycle between:
   _build/default/lib/Parsing/Syntax__mock.mli.inferred
-> _build/default/lib/Parsing/Syntax.mli
-> _build/default/lib/.demo.objs/parsing__Syntax.intf.d
-> _build/default/lib/.demo.objs/parsing__Syntax.intf.all-deps
-> _build/default/lib/.demo.objs/parsing__Tokens.impl.all-deps
-> _build/default/lib/.demo.objs/Syntax__mock.impl.all-deps
-> _build/default/lib/Parsing/Syntax__mock.mli.inferred
-> required by alias lib/Parsing/all
-> required by alias default

Is there a way to refer to files in the current subdirectory in the .mly ?

Related (?) resources

Threads that seem related but actually don’t address this specific issue: