feat: Expost host and target sysroot to build scripts.

[c272]

What does this PR try to resolve?

Currently, when using -Zbuild-std, Cargo passes a --sysroot argument down to the relevant rustc invocations. However, the value of this argument is not exposed to build scripts, so if a build script wishes to mirror the rustc configuration of the host, they must somehow discover the sysroot themselves.

This patch exposes the host (RUSTC_HOST_SYSROOT) and target (RUSTC_TARGET_SYSROOT) sysroots to build scripts through additional environment variables.

Fixes #7501
Related to rust-lang/wg-cargo-std-aware#50

How should we test and review this PR?

An example build script displaying these environment variables is as follows:

use std::env;

fn main() {
    println!("host: {}", env::var("RUSTC_HOST_SYSROOT").unwrap());
    println!("target: {}", env::var("RUSTC_TARGET_SYSROOT").unwrap());
}

Additional information

N/A

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @weihanglo (or someone else) some time within the next two weeks.

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[weihanglo]

Sorry I don't quite follow. I read through the relevant issues but haven't found any design decision made. Do we have discussions in other places like Zulip I missed?

[weihanglo]

Asking because we would like to have design discussions in issue threads, and PRs is for discussing each implementation separately. That would make things a bit easier to follow :)

[adamgemmell]

Hi, sorry about that! I advised c272 to share the implementation first as the best way to move the discussion forwards. We should have followed the contributor's guide a bit more closely - hope it didn't seem too pushy.

Writing the patch fleshed this idea out a bit more in the process, so one of us will share that in a separate feature request issue.

[bors]

☔ The latest upstream changes (presumably #13409) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #13872) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

☔ The latest upstream changes (possibly 081d7ba) made this pull request unmergeable. Please resolve the merge conflicts.