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.