rustc --version should not trigger/initiate toolchain updates

[rami3l]

Transferred from rust-lang/rust#127649.

I expect rustc --version to just output version information, like how --version would behave in any other application.

In my humble opinion, rustc --version should not be responsible for upgrading toolchains. The user should do that either explicitly or maybe when running rustc with any other flags for which it makes more sense for silent updates to be performed.

The --version argument should really just be a 'request' for version information and not trigger any other actions.

To be clear... I think it's ok to trigger silent updates for different calls to rustc with different arguments, but I believe --version should really only output the version (and maybe a hint that the current version is a mismatch by what is specified in rust-toolchain.toml).

Performing silent updates, and maybe even more actions, may even pose security risks to users that do not expect these actions to happen silently when running a command like rustc --version.

Originally posted by @Doineann in #3635 (comment)

[panekj]

When invoked via rustup proxy, it should not fail, it should show the resolved active rustc version (without downloading the toolchain), whichever that may be at the moment considering all override methods. Given that rustup already knows which toolchain version to pull, it shouldn't be impossible to present that info.

[rami3l]

When invoked via rustup proxy, it should not fail, it should show the resolved active rustc version (without downloading the toolchain), whichever that may be at the moment considering all override methods. Given that rustup already knows which toolchain version to pull, it shouldn't be impossible to present that info.

@panekj I don't believe it can in general, as we have to take into account custom toolchains that are not distributed by (but linked against) rustup, as well as "moving targets" such as stable and 1.79. This action should in no way access the internet.

[rami3l]

A note for myself:

rustup/src/cli/proxy_mode.rs

Line 35 in 4207838

let cmd = cfg.local_toolchain(toolchain).await?.command(arg0)?;

... calls:

rustup/src/config.rs

Line 732 in 4207838

None => self.find_or_install_active_toolchain().await?.0,

So modulo the tests this should be a simple change.