Add option to check for updates without updating

[clarfonthey]

It'd be nice to have a command to run rustup update and check if anything would be downloaded without actually downloading anything. For example, I have a MOTD that shows me if I need to update any packages, and it'd be nice to also show a message if a new stable version of Rust is available to download.

[ordovicia]

I'd like to try.
What name is good for the subcommand?

[clarfonthey]

Personally I'd add it to the update subcommand with a --no-download option

[ordovicia]

I see. Thank you for your comment.
I'd prefer to add an individual subcommand with name check or check-update.
I'd like more suggestions widely.

[Zteve]

I have added my 2¢ to #1271. I hope this feature comes soon.

[matthiaskrgr]

rustup check has been implemented, I think this can be closed...?

[kinnison]

Yep, well spotted Matthias.