[eieio]

The tool (iron) isn't open source, but there are a bunch of public talks and blogs about how it works, many of which are linked from the github repo[1].

It used to be "open source" in that some of the code was available, but afaik it wasn't ever possible to actually run it externally because of how tightly it integrated with other internal systems.

[1] https://github.com/janestreet/iron