The stable Rust compiler requires the nightly Rust compiler to build. This is no good for distros that want to bootstrap from their own toolchain and don't want to maintain a nightly toolchain just for building Rust. We'll teach the Rust makefiles to deal with bootstrapping off the stable compiler transparently (the stable compiler already contains nightly features, just inaccessible).
https://internals.rust-lang.org/t/perfecting-rust-packaging-the-plan/2767