Add help message for missing toolchain

[jtr860830]

Closes#3573.

This patch adds a suggestion to run rustup install <toolchain> for missing toolchain.

[rami3l]

Thanks a lot for helping out! It'll be nice if you adjust our integration test accordingly as well :)

[jtr860830]

Thanks a lot for helping out! It'll be nice if you adjust our integration test accordingly as well :)

OK, I'll adjust it. Thx for replying!

[rami3l]

Thanks a lot for helping out! It'll be nice if you adjust our integration test accordingly as well :)

OK, I'll adjust it. Thx for replying!

@jtr860830 No worries! Just remember to squash everything into a single commit since it's a single unit of change: we use atomic commits across the repo.

The integration test you need to fix is suite::cli_exact::undefined_linked_toolchain, as indicated by the CI log.

[rami3l]

LGTM, and thanks again for helping out! :)

Concern: ignore non-official toolchain names

[rami3l]

@jtr860830 Sorry, but I've changed my mind regarding this change, since it doesn't make the distinction between a typo (like bogus) and an actual toolchain that can be installed with rustup toolchain install (like nightly-x86_64-unknown-linux-gnu). Can we print out the second line only in the second case?

Hint: Use PartialToolchainDesc::from_str().

[jtr860830]

@jtr860830 Sorry, but I've changed my mind regarding this change, since it doesn't make the distinction between a typo (like bogus) and an actual toolchain that can be installed with rustup toolchain install (like nightly-x86_64-unknown-linux-gnu). Can we print out the second line only in the second case?

Hint: Use PartialToolchainDesc::from_str().

I get it. I'll improve it!

[rami3l]

Sorry for the delay, and thanks again for your help!

[jtr860830]

Sorry for the delay, and thanks again for your help!

This PR has taught me a lot. Thx for your help!