Expected Behavior

dune build should produce a byte target for https://github.com/disposedtrolley/hello_ocaml which is able to be debugged with ocamldebug. A breakpoint should be able to be set on line 2 of the helloworld module.

Actual Behavior

While dune build is able to build a .bc file, it seems to be missing debug symbols. For example, attempting to set a breakpoint on line 2 of helloworld.ml results in:

hello_ocaml · (main) ⟩ ocamldebug _build/default/helloworld.bc                         ~/p/o/hello_ocaml 2
        OCaml Debugger version 4.12.0

(ocd) break @helloworld 2
Loading program... done.
Can't find any event there.
(ocd)

When the project is built with ocamlc -g helloworld.ml, the resulting a.out file can be debugged with ocamldebug correctly.

Reproduction

• PR with a reproducing test:

1. Clone https://github.com/disposedtrolley/hello_ocaml
2. Run dune build
3. Run ocamldebug _build/default/helloworld.bc
4. Type break @helloworld 2
5. Observe that the debugger outputs "Can't find any event there."

Specifications

• Version of dune (output of dune --version): 2.8.4
• Version of ocaml (output of ocamlc --version) 4.12.0
• Operating system (distribution and version): Fedora 33 (x86), macOS 11.2 (arm64)

Additional information

I've tried this with OCaml 4.12 (on macOS with Apple Silicon) and 4.10.2 (on macOS with Apple Silicon and Fedora 33 x86).