so the rustup toolchain will be updated whenever `x.py build` or `x.py test` are run for
that toolchain/stage.
+**Note:** the toolchain we've built does not include `cargo`. In this case, `rustup` will
+fall back to using `cargo` from the installed `nightly`, `beta`, or `stable` toolchain
+(in that order). If you need to use unstable `cargo` flags, be sure to run
+`rustup install nightly` if you haven't already. See the
+[rustup documentation on custom toolchains](https://rust-lang.github.io/rustup/concepts/toolchains.html#custom-toolchains).
+
## Other `x.py` commands
Here are a few other useful `x.py` commands. We'll cover some of them in detail