Some projects like the OCaml compiler need to use different C-stubs implementations depending on wheter the target is byte or native. This can be done by passing different C-flags (like preprocessor flags) depending on the mode or by using different source files.

Dune already allows multiple foreign_stubs definitions in the same executable or library stanza:

(executable
 (name stubs_exe)
  (foreign_stubs
    (language c)
    (names c_stubs))      ; c_stubs.c
  (foreign_stubs
    (language c)
    (names c_stubs_2)))   ; c_stubs_2.c

which is equivalent to using (names c_stubs c_stubs_2) if no other field (such as the flags field) differ.

However, as expected, using twice the same source file is forbidden:

(executable
 (name stubs_exe)
  (foreign_stubs
    (language c)
    (names c_stubs))
  (foreign_stubs
    (language c)
    (names c_stubs)))

dune exec ./stubs_exe.exe
File "dune", line 5, characters 11-19:
5 |     (names c_stubs))
               ^^^^^^^^
Error: Multiple sources map to the same object name "c_stubs":
- c_stubs.c
- c_stubs.c

We propose to add a field mode to the foreign_stubs field that would act as a filter defaulting to all modes.

• This optional field would accept a single value: byte or native
• A foreign_stubs field with a mode specified will be ignored ("filtered-out") if the current build mode does not match the one specified in its mode field. Thus two different foreign_stubs fields for the same source can coexist if they specify different modes.
• Absence of the field means both byte and native. That makes this proposal backward compatible. That also means that a foreign_stubs field without a mode field will never be silently filtered-out in favor of another foreign_stubs with an explicit mode field.

For example, one could feed different flags to the compiler depending on the mode:

(executable
 (name stubs_exe)
  (foreign_stubs
    (language c)
    (mode byte)
    (names c_stubs))
  (foreign_stubs
    (language c)
    (mode native)
    (flags :standard -DNATIVE_CODE)
    (names c_stubs)))

Or use different sources depending on the mode:

(executable
 (name stubs_exe)
  (foreign_stubs
    (language c)
    (mode byte)
    (names c_stubs_byte))
  (foreign_stubs
    (language c)
    (mode native)
    (names c_stubs_native)))

Part of this proposal has already been implemented in a fork of Dune used to compile the flambda backend. Our plan is too carefully upstream and complete these changes.