Enable cached builds with Hadrian
As Neil is working on turning Shake into Cloud Shake , Shake has got a new feature of "locally cached builds", making it possible to reuse build artefacts from previous builds on the same machine. This means, in particular, that a clean build could be finished very quickly if you've built GHC before from the same/similar sources. Hopefully, this could also help us speed up GHC's CI some day.
To enable cached builds, it is important to find and fix all instances of untracked dependencies in Hadrian, or at least record them using the
produces feature of Shake , because otherwise Shake has no idea that these files are actually needed and should therefore be stored in the cache. Here are a couple of tickets related to untracked dependencies:
I am currently working this and hope to produce a patch soon. I marked this is a "bug", not a "feature request", since untracked dependencies are bugs, and they prevent us from using Shake's shiny new feature.
 See the Build Systems à la Carte paper: https://dl.acm.org/ft_gateway.cfm?id=3236774