Allow +toolchain for rustup

[tbu-]

It was surprising to me that I couldn't use the +nightly parameter on rustup for adding the wasm32-unknown-unknown target to nightly:

rustup +nightly target add wasm32-unknown-unknown

(and other combinations didn't work either). Instead I had to find the --toolchain parameter of rustup target add. It would be nice if +nightly would work on rustup, too.

[tbu-]

CC #1489.

[vi]

In addition to +nightly all of theese should also work:

• rustup --toolchain=nightly target add wasm32-unknown-unknown
• rustup target --toolchain=nightly add wasm32-unknown-unknown

Also rustup target --help should mention --toolchain somewhere.

[pickfire]

I would like to do this. I noticed that cargo does not explicitly parse this but instead passes this to rustc to handle it. Should we check it on rustup side with internal function to list toolchains?

[BeniCheni]

Hi @pickfire, friendly note that from this comment from #1489, I started working on the 2nd PR for +foo variant in the same scope of #2031. Should be able to push an update later today for +foo variant.

[pickfire]

@BeniCheni Yeah, but that's just for which right? I want to help out on this too.

[BeniCheni]

@pickfire, from this comment from #1489:

and then have #1498 provide the +foo variant.

I'm interested in trying out. ...

From my comment there, I indicated my interest in working on #1498 here. But I'm happy to share, if you want to try out a PR for #1498 here.

Let me know if you are committed to this issue. If yes, I can check out other open issues. If no, I will continue to pick up this issue, since #2031 is in final shape.