Add support for cargo-args=["-Zbuild-std"]

[jyn514]

• Update to a version of rustwide that supports fetch_build_std_targets

• Don't try to download rust-std for tier 3 targets

• Add a local test

• Don't try to make API calls to crates.io for local crates

  This shaves 16 seconds off the build, and also avoids some unnecessary network calls.

Note that this calls cargo fetch -Zbuild-std unconditionally just because it was easier; I doubt it'll cause any trouble but I can change it if you like.

Fixes #1561. Blocked on rust-lang/rustwide#66.

[syphar]

I don't know enough about the build-process itself to do a proper review, perhaps @Nemo157 can help here?

From the code, I only had two small comments.

[jyn514]

This is blocked on frewsxcv/rust-crates-index#102 (unless we can switch rustwide to gix: https://rust-lang.zulipchat.com/#narrow/stream/356853-t-docs-rs/topic/updating.20git.20version/near/355335923)

[jyn514]

This is unblocked now :)

[syphar]

I'll approve, the change makes sense to me looking at the code.

I have two caveats:

• is there a build speed impact? ( see comment )
• I don't know much about the build specifics and can't know possible side-effects, perhaps @Nemo157 knows more?

[Nemo157]

I don't think there should be any bad side effects from this, we don't change the build step at all it's up to the user to pass -Zbuild-std still, we just install an extra rustup component and fetch some extra crates (I'm surprised these aren't part of rust-src actually, but if that's how it's designed now I guess we gotta work with it 🤷).

[jyn514]

I don't think there should be any bad side effects from this, we don't change the build step at all it's up to the user to pass -Zbuild-std still

ah hmm, I guess an alternative approach would be to pass -Zbuild-std automatically if rustup target add fails. That seems a little too magic, though, I think especially since build-std is still unstable it makes sense to have people opt-in explicitly.

I'm surprised these aren't part of rust-src actually, but if that's how it's designed now I guess we gotta work with it 🤷

these are dependencies of the standard library - although I suppose infra could vendor them in the rust-src tarball 🤔
anyway, not something we need to fix here.