Add --print=check-cfg

[Urgau]

Proposal

The --check-cfg flag allows rustc to lint on unexpected cfgs in source code, but source code is not the only place where cfgs appear and are used.

They also appear in:

• --cfg flags, which we ignore (for now) because RUSTFLAGS applies to all crates
• and in Cargo.toml with conditional keys

  [target.'cfg(windows)'.dependencies]
  libc = "0.2"

The way Cargo checks if the windows cfg is enabled is by using the --print=cfg flag, which prints all enabled cfgs, so Cargo only needs to check if the cfg is present in the output or not.

I therefore propose that we (unstably) add the corollary to --print=cfg by adding --print=check-cfg, so that Cargo can then lint over those unexpected cfgs.

This new print option would work similarly to --print=cfg (modulo check-cfg specifics):

• check_cfg syntax: output of --print
• cfg(windows): windows
• cfg(feature, values("foo", "bar")): feature="foo" and feature="bar"
• cfg(feature, values(none(), "")): feature and feature=""
• cfg(feature, values(any())): feature=any()
• cfg(any()): any()
• nothing: any()=any()

Aside from the potential Cargo use, regular users might also want a way to see the list of expected cfgs, either for debugging or verifying things or whatever.

Mentors or Reviewers

@Urgau (for the implementation)

Process

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• [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

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Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

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 <description of the concern> 

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cc @rust-lang/compiler @rust-lang/compiler-contributors

[petrochenkov]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted