Update to most recent viable nightly

[jonhoo]

Previously, if the user had components installed on nightly which were not available for the latest nightly, rustup update would not update the user's nightly at all. With this patch, rustup will try progressively older nightlies until it finds a nightly that supports all the components the user has installed for their current nightly.

Fixes #1628.
Makes progress towards #1501.
Fixes the underlying issue in #1676.

[jonhoo]

As an anecdotal observation, the code for mocking a dist in tests/mock/clitools.rs should probably be refactor separately from this change. It's currently pretty messy, and would benefit from some cleanup, and probably some macros.

[jonhoo]

Cleanup in #1998

[norru]

This is ace. Thank you very much!

[kinnison]

Apart from my concern over missing nightlies and a possible bit of reformatting, this looks like a nice simple elegant solution to the problem.

[kinnison]

LGTM 👍 I'll merge once I have a confirm from CI.

[vi]

When will it be available for rustup self update?

[kinnison]

When will it be available for rustup self update?

@vi When we're ready to release 1.20.0 -- We have active tester requests out right now, and we may put further requests out on the way to 1.20.0. Once we're good, we'll release. I'd guess at a small (one or two) number of weeks.

[Arnavion]

@jonhoo @kinnison How would I restore the original behavior? Do I have to rustup component remove the components I don't mind losing before rustup updateing, then rustup component add them back? Or is there an easier way?

[najamelan]

@Arnavion one option is to check https://rust-lang-nursery.github.io/rust-toolstate/ manually and not update if not all your components are there...

[jonhoo]

@Arnavion I'm not sure I follow? The old behavior was to simply fail the update. If you mean to force the update, then you should be able to pass --force.

[Arnavion]

@jonhoo The new behavior, AFAICT from your change, is that rustup update will always succeed, but leave me with a nightly that is not the latest. The original behavior was that rustup update would fail, which my "update Rust" script would detect and handle by removing components that I don't need (clippy, rustfmt), re-running rustup update, then rustup component adding (rustfmt) or cargo installing (clippy, since it's usually fixed in git master even if the submodule in the rust repo is broken) the components back.

Yes, --force seems to be what I was asking for. Thanks.

[EliSnow]

@jonhoo, I was reminded of this great work today when I did a rustup update and saw the message skipping nightly which is missing installed component 'rls-preview'. Thank you!