Error in documentation for foreign build sandboxing

[samoht]

https://dune.readthedocs.io/en/latest/foreign-code.html#foreign-sandboxing is recommending to write this dune file:

(data_only_dirs libfoo)

(rule
 (deps (source_tree libfoo))
 (targets libfoo.a dllfoo.so)
 (action (progn
          (chdir libfoo (run make))
          (copy libfoo/libfoo.a libfoo.a)
          (copy libfoo/libfoo.so dllfoo.so)))

(library
 (name bar)
 (foreign_archives foo))

Otherwise this doesn't work as the copy stanzas will fail because the files do not exist before the build ir run, and dune complains that it doens't know how to build these.

[ghost]

Indeed. We need to add a mode to disable targets/deps inference. Directory targets (#3316) would also help here.