The master branch of the Rust repo is built every night and distributed. That's a nightly build. Most people are on the stable release, which is updated every six weeks.

A minority use the nightly build for various reasons: a feature that hasn't reached stable yet, or because they want to help test the nightly releases and prevent bugs from reaching stable.

It's a bit unfortunate wording but it basically requires any nightly toolchain version. It uses `std::intrinsics::breakpoint()` which is a compiler intrinstic. This has been available for a long time, but afaik will never be exposed on a stable toolchain.

Per https://dev-doc.rust-lang.org/nightly/unstable-book/library-...

>This feature is internal to the Rust compiler and is not intended for general use.

You can pin versions with the rust-toolchain.toml file you need to be using Rustup afaik. Nightly is just the daily builds.