https://sn.1w6.org/file/clacke-20180214T032615-d3pv4y4.html

I may have some Rust coding ahead of me, so I looked up the options for doing the equivalent of rustup, but within #nix. Looks like others had the same need:

https://www.mail-archive.com/nix-dev@lists.science.uu.nl/msg33296.html

> I created an overlay which mimic rustup behaviour, and allow you to
spawn a nix-shell with the latest nightly, beta, or stable version of
rustc & cargo.

> To use this overlay, checkout nixpkgs-mozilla [1], and create a
symbolic link to the file rust-overlay.nix [2] in the
~/.config/nixpkgs/overlays

Ok, so whenever you want to use a newer rust, you just git pull this thing and ...

> This overlay should auto-update it-self as if you were running rustup
each time you go through the rustChannels attributes. It works by
using the fetchurl builtin function to pull the same file as rustup do
through https.

Oh! Awesome.

[1] https://github.com/mozilla/nixpkgs-mozilla
[2] https://github.com/mozilla/nixpkgs-mozilla/blob/master/rust-overlay.nix