buck2

A change that we've been keeping in our prelude fork, not sure if you want to adopt it into upstream as-is, let me know if you have suggestions for changes or a better implementation! This change implements support for running and targets under exposed as a sub-target. Under the hood this makes two changes: 1. we add the sub-target in that will rust call instead of with the explicit miri sysroot given to the toolchain as described here. This is the easy part. 2. we also introduce a new that will be propagated through the existing dependency resolution and compilation logic and be superficially treated as a regular "link" type compilation but allows us to switch out the compiler to a wrapper script (similar to how the clippy wrapper works). This will ensure the rlibs have the right structure and will be understood when we pass them to the "interpretation phase" of miri. Note that as a QOL feature when injecting the sub-target in we switch on the presence of to - instead of a provider - inject an so that users can just run instead of .