[Urgau]

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What does this PR try to resolve?

Since #13571, we now lint by default on unexpected cfgs in Rust code, but one area that is missing are cfg expressions in Cargo.toml and .cargo/config.toml. This PR address this missing piece by (unstably) introducing -Zcheck-target-cfgs based on rustc --print=check-cfg.

The feature voluntarily does not always follow RUSTFLAGS (to avoid mistakes with one-off and for consistency) but instead follows Rust unexpected_cfgs lint configuration:

[target.'cfg(foo)'.dependencies] # will not get linted about
cfg-if = "1.0"

[lints.rust.unexpected_cfgs]
level = "warn"
check-cfg = ['cfg(foo)'] # because it's mark as expected here

In terms of implementation, one global --print=check-cfg is retrieved and fetched, packages that have their own check-cfg inputs have each their own --print=check-cfg, otherwise the expected cfgs will get mixed up. Each --print=check-cfg is cached and only done if necessary.

How should we test and review this PR?

As asked I tried putting everything into small commits, they may not reflect the de-coupling possible as they follow the implementation but they are still IMO very useful. However I strongly recommand doing a first-pass of the complete diff first.

In terms of tests, I added some combinations that I though would be useful, I can add more if desired.

Additional information

Documentation for --print=check-cfg. It was one of motivation of the compiler MCP rust-lang/compiler-team#743.

r? @epage