Iron is Jane Street’s internal code review tool.
For more on the design of iron, see the following blog posts:
A key design decision was to review diffs rather than patches:
We no longer release the code of iron, not because the code is proprietary but because one couldn’t run the code elsewhere without first reproducing many trivial features of our internal environment. We’re still happy to share or discuss the code, though: just email firstname.lastname@example.org.