Reduce the size of the clone for people who just build rustc from source

[dns2utf8]

This speeds up the download process for the user.

Tested on armhf and amd64.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[steveklabnik]

It does, but it also has downsides as well. You can't do all the stuff you can do with a 'real' checkout.

[tomjakubowski]

It does, but it also has downsides as well. You can't do all the stuff you can do with a 'real' checkout.

At one point shallow clones were pretty broken compared to 'real' clones (you couldn't push/pull from/to shallow clones, for example), but the situation is much better as of git 2.0. See git/git@82fba2b and https://blog.oio.de/2014/02/06/better-support-for-shallow-clones-in-git-1-9/

Of course, with a shallow clone one's options for digging through history are limited. But for someone who just wants to build, that may not be a big deal.

My personal preference is to leave --depth out, though. Not everyone is on git 2.0 and the Rust README hardly seems like the right place for git education ;-)

[MagaTailor]

This change is almost useless because the heaviest submodules are still going to be cloned later even when they're not needed. Because of that it's completely harmless though.

Here's the main issue to solve:
#30107

[brson]

I also have reservations about this. Knowledgeable developers will know this is an option. And as @petevine says you're still going to end up cloning LLVM.

[dns2utf8]

I agree.

I tried a reduced clone of llvm and the submodules, but it did not work anymore.

So close?

[alexcrichton]

Ah yeah I'm going to close this for the aforementioned reasons. Thanks for the PR regardless though!