Additions

• Port to Jbuilder (#374, Andrew Ray).
• Lwt_io.establish_server_with_client_address (#346, Rudi Grinberg).
• Lwt_unix.getcwd (#403, Raphaël Proust).

Planned to break in 4.0.0

• Delete lwt.simple-top (#371).
• Delete Lwt_chan (#441).

Fixes

• Make Lwt_log functions tail-recursive (#348, Jan Doms).
• Make more of Lwt_list tail-recursive (#347, Jan Doms).
• Improve string messages in exceptions (#368, #382, Jan Doms, Raphaël Proust).
• Don't call Unix.set_nonblock or Unix.clear_nonblock unnecessarily on some fds (#356, David Sheets).
• Lwt_unix.sleep and Lwt_unix.timeout returning too early when using libev (#433, Stijn Devriendt).
• Lwt_sequence.fold_r iterating the wrong way in some cases (#405, Stijn Devriendt).
• Build conflicts in some cases due to duplicate cst_to_constr function (#362, Jérémie Dimino).
• Don't use deprecated readdir_r system call (#430, Raphaël Proust).

Miscellaneous

• The Lwt core, lwt.ml, has been thoroughly refactored and commented (#354, reviewed Gabriel Radanne, Edwin Török, Raphaël Proust, Jan Doms, Fabian Hemmer, Sebastien Mondet, Simon Cruanes, Anil Madhavapeddy, Pierre Chambart, and many others).
• Lots of tests for most of the Lwt core (#339, #389, #392, #440, #448, #450, Joseph Thomas, Ryan Slade).
• Documentation fixes (including by Joseph Thomas, Raphaël Proust, Richard Degenne, Stavros Polymenis).
• Contributing documentation (#379).
• Massively adjust whitespace for legibility (#400, #409, #416, Richard Degenne).
• Improvements to CI (Etienne Millon, Raphael Rafatpanah, Zack Coker, Yotam Barnoy).
• The additional packages lwt_ssl, lwt_react, lwt_glib get new minor releases, the change being new Jbuilder build systems (#374, Andrew Ray).