Bootstrapping 1.12.1 from 1.12.0

We just released 1.12.1, the first point release since 1.0. This brought up a sticky issue with bootstrapping.

Rust bootstraps from a single previous release, usually the previous stable release. So 1.12.0 bootstraps from 1.11.0. Rust 1.12.1 also bootstraps from 1.11.0.

Some folks might want to bootstrap 1.12.1 from 1.12.0 though. An example is distributions that have their own bootstrap chains. In the future we would like this to simply work, but for this point release I have prepared a patch that lets rustc 1.12.1 bootstrap from 1.12.0. It can be applied like:

patch -p1 < rust-boot-1.12.1-from-1.12.0.diff

cc @cuviper

Thanks! I will try to get this going in Fedora Rawhide soon. (update: looks great!)

I hadn't shipped 1.12.0 to stable Fedora yet, because the opt+debug issues were biting us. So those can probably now just jump right from 1.11.0 to 1.12.1.

I think between your relaxed bootstrap key and my tweak to local-rebuild detection, we should be all set for future point releases. But it could be a while before the next trial-by-fire. If anyone sees other possible gotchas, let's figure it out now!