This crate offers the ability to build a rustc sysroot from source. You can think of it as a very lightweight version of xargo (which was a useful source for information on how to do this), or a version of cargo -Zbuild-std that builds a sysroot rather than building the standard library for the current crate.
Building the sysroot from source is useful for tools like Miri and cargo-careful that need the standard library to be built with different flags. Building a sysroot from different sources is not a goal of this crate.
The crate builds on stable Rust, but actually building a sysroot requires a recent nightly version. Nightly versions from the last 3 months are supported.