Filtering of component list by available, required, and installed

[das-sein]

Allows filtering of rustup component list command.

Example output:

canis@latrans$ rustup component add clippycanis@latrans$ rustup component list --filter installedcargo-x86_64-unknown-linux-gnu (default)clippy-x86_64-unknown-linux-gnu (installed)rust-docs-x86_64-unknown-linux-gnu (default)rust-std-x86_64-unknown-linux-gnu (default)rustc-x86_64-unknown-linux-gnu (default)canis@latrans$ rustup component list --filter requiredcargo-x86_64-unknown-linux-gnu (default)rust-docs-x86_64-unknown-linux-gnu (default)rust-std-x86_64-unknown-linux-gnu (default)rustc-x86_64-unknown-linux-gnu (default)canis@latrans$ rustup component list --filter availablellvm-tools-preview-x86_64-unknown-linux-gnurls-x86_64-unknown-linux-gnurust-analysis-x86_64-unknown-linux-gnurust-src...

[kinnison]

Hi @lazorgator,

Thanks for your contribution to Rustup. Could you please confirm if this is just a wishlist feature from you, or is there an issue already filed which you think you're addressing with this commit? If there's the latter then I'd really like to see the commit mention it.

Also, thank you for including a test for the feature, but it'd also be helpful if you could write a small section for README.md which details the filters, and also what it means for a component to be required or not.

If you aren't comfortable writing that then let me know, I wouldn't block the feature purely on that, but it'd be super-nice to have.

[das-sein]

Hi there @kinnison ,

I actually noticed this wasn't implemented from hacking on rustup for the past couple days and implemented it as a feature off-hand. If that's not okay due to concerns about feature creep, it's fine if it's not merged. I can work on the README entry for it, most certainly.

[kinnison]

Slight wording change to point the user at --toolchain and I think this will be golden.

[das-sein]

@kinnison Added mention of --toolchain flag for rustup component add and removed unrelated whitespace hunk.

[kinnison]

LGTM.

r? @dwijnand

[bors]

☔ The latest upstream changes (presumably #1643) made this pull request unmergeable. Please resolve the merge conflicts.

[das-sein]

Messed up the PR. README changes overwritten. I'll close this now in favor of a new PR on top of the 2018 Edition changes :<