Default to `stable` when no active toolchain is present

[soloturn]

Problem

having cargo and rustup installed and call it first time it errors out, the only action i did not expect when building something. if you can print "run a command to choose stable" then really the software can choose a reasonable default by itself please. " you did not specify, so i chose ... for you".

❯ cargo build -p daemon --bin kanidmd --release
error: rustup could not choose a version of cargo to run, because one wasn't specified explicitly, and no default is configured.
help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain.

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

[epage]

@rustbot transfer rustup

[rami3l]

@soloturn Sorry, but I'm a bit confused. How did you install the rustup binary without getting a recent version of cargo? Maybe you have installed it through a third party distribution?

Anyway, this looks like a problem of distribution of responsibility among different subcommands. As far as rustup is concerned, it used to be very smart in that it will install the active toolchain for you before running any proxy command such as cargo, which has caused some problems to our downstream. So a rule that we are enforcing now (see #3635 for more details) is that calling a proxy should not lead to any toolchain installation (which is subsumed by rustup default), and I don't see a point in only setting the default toolchain without installing it, so we might only be able to go as far as giving a hint.

[soloturn]

reading #3635 it makes perfectly sense. but the conclusion drawn might be insufficient. giving an error message fulfills "not silent", do something and print it as well is not silent.