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