[WIP] Add subcommand to check for available updates without updating

[ordovicia]

This PR adds subcommand to check for available updates for installed toolchains and rustup itself without actually updating.

This PR is WIP because no consensus is built about what name is proper for the subcommand or option.
Currently I have chosen check subcommand.
I guess there are many candidates:

• New subcommand
  • check
  • check-update
• Adding an option to update subcommand
  • --no-download
  • --dry
  • --dry-run
• and any other suggestions

Fixes #1249.

[Zteve]

I would love this feature; staging updates requires knowing when they are available.

My suggestion for a command structure is a subcommand of its own (like check but could be called query) which is able to be placed before another subcommand and which announces what that would do, without actually doing it. Then the call would be rustup query update, or rustup query self update, or whatever.

This allows the possibility of extending this to other subcommands in future, giving it its own special syntax in the first instance. Initially, rustup check would be an acceptable shorthand for rustup check update.

Options, like --no-download (too specific) and --dry-run (parochial) offer the opportunity for conflict with other options and aren't obvious enough. Special decorators (like the current +beta shorthand) should be avoided, since they, too, are non-obvious.

[bors]

☔ The latest upstream changes (presumably #1337) made this pull request unmergeable. Please resolve the merge conflicts.

[matthiaskrgr]

Any updates on this? I would also really like to have this :)

[ordovicia]

Sorry, I have done nothing recently.
This PR has many conflicts, so it may be somewhat hard to rebase on master.
Feel free to make a progress on this 😄

I now personally think that we should consider whether we really need this feature.

[Diggsey]

Closing due to inactivity - please feel feel free to open a new PR if you wish to continue working on this feature!