Design: Skylark build configuration

[Greg Estren]

This design proposes a Skylark API for custom command line flags, multiarch binaries, and other configuration controls as a component of Bazel's Configurability Roadmap. The intention is to implement this by mid-summer.

https://docs.google.com/document/d/1vc8v-kXjvgZOdQdnxPTaV0rrLxtP2XwnD2tAZlYJOqw/edit?usp=sharing

If you want to write Skylark rules that can do neat multiplatform things, this is the design doc for you.

Some already known discussions that will have to happen in review (either in the doc or on this thread):

• How to encourage most transitions to be platform transitions while still allowing the possibility of non-platform transitions when the need makes sense
• How to roll out increased Skylark powers without compromising build speed, cacheability, and correctness
• How "feature flags", which are an evolving interface for user-defined configuration, fit in

Cheers and happy reviewing!

[Greg Estren]

After lots of great feedback, this design has been updated into a V2 proposal. 

It looks pretty different from the first version. So you may want to read through it again as interested.

We're still interested in actively implementing this as soon as we have critical mass approval.

[Julie Xia]

Update: Adding an additional proposal to this design. This proposal drills down into the API of parameterized skylark-defined transitions. 

[Marissa Staib]

Here are some thoughts about merging config_feature_flag with Starlark build configuration, to reduce redundancy:

https://docs.google.com/document/d/1kfZ2fQPY32fbjoCpzF2bFMoidhjVtkTmgW-6Zhxwhqk/edit?usp=sharing