Add NixOS example for following rust-toolchain.toml to docs

[ZeeQyu]

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Objective

To make it easier for projects using nix to work with changing channels.

Solution

Adding a section to the documentation to show how this can be done with a modified nix file.

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[ZeeQyu]

A friend of mine contributed a nix file to a project of mine, and added this neat little snippet. I thought more people would have use of it, so I suggested he contribute it. He did not have time so he gave the go-ahead for me to contribute it.

I don't read much nix, but I tested it at least. I do recommend someone else testing it on their nix system as well.

Hope this is helpful!

[alice-i-cecile]

Seconding James' stance here :)

[Austreelis]

I'm a bit confused: Why run rustup toolchain install when entering the shell, when rustup tools (including cargo) already install and use the toolchain specified by rust-toolchain.toml automatically ? As far as I can tell, the only difference between this file and the example already documented is bringing in rustup and installing a toolchain.

If using rustup to handle rust toolchains (instead of say rust-overlay), this shouldn't need to be handled by nix. Just having rustup in the nativeBuildInputs should be enough. I don't really understand why this is nix-specific ?

I feel like maybe I don't understand the issue in the first place ?

[ZeeQyu]

Asked my friend, he said that wasn't his experience, and that he needs to try again. I'll update this pull when he's had time to check.

[MinerSebas]

I'm a bit confused: Why run rustup toolchain install when entering the shell, when rustup tools (including cargo) already install

This is the confusion, on nix installing rustup doesn't automatically add a toolchain (unlike the official rust installer that adds one by default), which means no cargo that reads the rust-toolchain.toml, to add the correct toolchain.

The classical chicken and egg problem. You need cargo to use rust-toolchain.toml but don't have one as nix itself wasn't told to add one.

The used solution solves this by installing a toolchain "manually".

[Austreelis]

@rustbicycle (on discord) and me tested it, and the rustup-provided cargo (installed with the rustup package) does follow and automatically installs the right toolchain following appropriate rustup overrides as on other distros (be it the default toolchain, the per-directory manual overrides, the rust-toolchain.toml file or toolchain override arguments like cargo +beta). I can still reproduce it: nix-shell -p rustup -c cargo build (without a shell.nix) will build with the toolchain specified in a rust-toolchain.toml file.

Are you sure you don't have the nixpkgs-distributed cargo in your $PATH ? The cargo program from the nixpkgs cargo package (unlike cargo from the rustup package) doesn't allow changing toolchain, which is intended to ensure build reproducibility (the way I understand it).

TL;DR: I don't think this is an issue people should encounter assuming they use either rustup or nixpkgs' cargo with the rust overlay, but not both. I believe using both is discouraged, partly because of the issue I suspect you're encountering.

[MinerSebas]

When I tested it many months ago it didn't work, but perhaps it was since fixed (or I did something wrong).

[mockersf]

This seems more "how to use Rust with NixOS" than "how to use Bevy with NixOS". The current doc says https://github.com/bevyengine/bevy/blob/2fd432c463b3f02ee39c3ab3a47b8fa8c56c0689/docs/linux_dependencies.md?plain=1#L144 which seems good enough for me