add `rust-toolchain.toml`

[TheAlgorythm]

I'm not sure if it works with this setup, but it should make the setup process easier.
The setup of Documentation/rust/quick-start.rst includes the following commands:

rustup override set $(scripts/min-tool-version.sh rustc)
rustup component add rust-src

An equivalent rust-toolchain.toml should be:

[toolchain]
channel = "1.66.0"
components = [ "rust-src" ]

This should setup the environment automatically and therefore reduce the steps for building the kernel.
The documentation for this can be found here

(I am not a kernel developer (yet) and therefore don't know if it works with its infrastructure)

[ojeda]

The directory override will take priority over that file, so users giving an override (in the root folder) would still be able to use their own toolchain, right? e.g. @nbdd0121 often uses a newer toolchain, IIRC.

[bjorn3]

Indeed. rust-toolchain.toml is the last thing checked before falling back to the global default. Directory overrides happen before that. See https://rust-lang.github.io/rustup/overrides.html

[ojeda]

Thanks @bjorn3! I wanted to make sure I read the docs correctly. In that case, I think we can do this.

Would you like to submit the patch @TheAlgorythm or do you prefer somebody else does it (with e.g. a Suggested-by from you)?

If you want to send it yourself, please see https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin for details (the kernel requires using the real name and signing your work under the DCO).

I will mark this as a good first issue too.

[nbdd0121]

I wanted to do this earlier, but then it feels ugly having to name it rust-toolchain.toml and place it in the git root directory. I would probably send the patch long while ago if the file could be named .rust-toolchain.toml.

[ojeda]

Ah, yeah, if it does not allow to be hidden, that is indeed quite painful. Then we should request that to rustup first.

[ojeda]

Done! rust-lang/rustup#3182

[TheAlgorythm]

Would you like to submit the patch @TheAlgorythm or do you prefer somebody else does it?

I can do that, but probably we should wait for rustup to add the support as I can understand, that such file shouldn't clutter the root. If they will change it I can create a PR.
In the meantime I will try if it's possible to place the file in another directory than the root.

[ojeda]

For reference, @aliceinwire sent a PR about this before you answered but after I asked you #954. Maybe you could please talk to each other about how to handle this when the time comes to send it upstream :)

[TheAlgorythm]

Ah ok. I haven't seen the PR before. I'm fine with it :)