build: pin rust toolchain to specified version in ci and locally

[chris-oo]

Rather than specifying stable as the toolchain to use in CI and locally, instead pin to a specific version of stable. This allows developers to manually fixup lints and issues with new toolchain updates and rev the appropriate versions across the code.

Additionally, add a minimum supported rust version to the main svsm crate, to give a more helpful compiler error when your toolchain does not match.

[chris-oo]

I have a separate meta comment about rust-toolchain.toml, do we feel like we still need it? It stops folks from easily building with newer versions without doing some overrides.

In other projects, I've generally relied on the minimum rust version and pinning CI to a specific toolchain version. And, if you declare a given commit as a release branch, then we'd add the corresponding rust-toolchain.toml to pin the whole branch to a specific version. Otherwise, on main folks are free to run whatever toolchain version they'd like as long as it's greater than the minimum specified in Cargo.toml. Thoughts on that change?

[00xc]

We should probably discuss this in the next SVSM call

[AdamCDunlap]

Something like this would be helpful for us as well.

You'll also want to update the Makefile where it specifies "cargo +nightly" in the bin/test-kernel.elf rule (even better would be if it is possible to have this specified in a Cargo.toml file instead of the makefile?)

You should also consider updating scripts/container/opensuse-rust.docker to install the correct version in the container, but this is not technically necessary since rustup will install the correct version for you at build time.

[joergroedel]

Hmm, this requires to specify the stable Rust version at three places. I guess there is no way to specify it in a single place for local and CI builds?

[joergroedel]

Hey @chris-oo, can you please update this PR to make the CI pass? This is currently blocking the merge.

[chris-oo]

Sorry about the delay - I'll fix it up shortly.

[chris-oo]

Hmm, this requires to specify the stable Rust version at three places. I guess there is no way to specify it in a single place for local and CI builds?

So the rust-version in Cargo.toml is the minimum supported rust version for the crate. Generally, I would keep it in lockstep with whatever CI is set to. I think though, that we should consider removing rust-toolchain.toml and just have the Cargo.toml enforce the minimum version, and CI can run against that version/be manually updated as appropriate, but I won't remove that in this PR.