Based on the discussion on bazelbuild/starlark#13, we're going to make argument ordering stricter. It also makes the semantics simpler to understand, and can simplify tooling.

All arguments should be in this order:

• First, positional arguments
• Then, keyword arguments
• Then, an optional *arg
• Finally, an optional **kwarg

e.g. f(a, b, c = 1, d = 2, *e, **f)

Migration: update buildifier to automatically fix the call sites.
It is also trivial to fix manually: change the order of the arguments.

- f(a, b, *c, d = 2)
+ f(a, b, d = 2, *c)

Expected timeline:

• released with Bazel 0.21 (December 2018), off by default
• enabled by default with Bazel 0.23 (February 2019)