Planned to break in 5.0.0

For general discussion of breakage in Lwt 5.0.0, see #584. See #293 about how Lwt announces and does breaking changes.

• The signature of Lwt.async will change from (unit -> 'a Lwt.t) -> unit to (unit -> unit Lwt.t) -> unit (#603, prompted @cfcs).
• Lwt_unix.send_msg and Lwt_unix.recv_msg will be changed to take Lwt_unix.IO_vectors.t instead of Lwt_unix.io_vectors (#702, prompted Marcello Seri).
• Nesting calls to Lwt_main.run will be forbidden, and will raise an Failure. Most programs don't do this (#609, prompted François-René Rideau).
• configure.ml will be removed in favor of an improved discover.ml This affects only users who are configuring Lwt as part of a manual installation (i.e., not users of opam or esy). See discover.ml for usage (#700).
• Lwt_unix.async_method will have no effect. In practice, it already does nothing, and has almost no users (#572).

The following planned breaking changes have already been announced:

• Lwt.pick will raise Invalid_argument on the empty list, instead of returning a forever-pending promise. Also applies to Lwt.choose, Lwt.npick, Lwt.nchoose, and Lwt.nchoose_split (#562, Tim Reinke, prompted Hezekiah Carty).
• Remove translation of [%lwt ...] to Lwt.catch from the PPX (#527).
• Remove -no-debug option from the PPX (#528).
• Remove Lwt_log support from the PPX (#520).

Additions

• Lwt_process: allow setting working directory for new processes (#694, Thomas Leonard).
• PPX: use OCaml 4.08 ASTs to support latest features (#697).
• Lwt_io.NumberIO: use compiler intrinsics for better performance (#178, requested Mauricio Fernandez).

Bugs fixed

• Race condition in Lwt_react.S.limit (4e592eb).
• Use fallback rule during configuration (#693, Hongchang Wu).
• Fix typos (#688, #692, @Fourchaux).