Set LIBC_CHECK_CFG=1 when building Rust code in bootstrap

[pietroalbini]

Downstream forks of the Rust compiler might want to use a custom libc to add support for targets that are not yet available upstream. Adding a patch to replace libc with a custom one would cause compilation errors though, because Cargo would interpret the custom libc as part of the workspace, and apply the check-cfg lints on it.

Since rust-lang/libc#3037, the libc build script emits check-cfg flags only when the LIBC_CHECK_CFG environment variable is set, so this PR allows the use of custom libcs.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Adding a patch to replace libc with a custom one would cause compilation errors though, because Cargo would interpret the custom libc as part of the workspace, and apply the check-cfg lints on it.

This is interesting to me - maybe a missing feature in Cargo? It seems common that you may want to patch a dependency (e.g., to switch to an as-yet unreleased version) without putting it into the workspace. Might be worth a bug report upstream.

@bors r+ for this in the meantime though.

[bors]

📌 Commit ead8b96 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[pietroalbini]

This is interesting to me - maybe a missing feature in Cargo? It seems common that you may want to patch a dependency (e.g., to switch to an as-yet unreleased version) without putting it into the workspace. Might be worth a bug report upstream.

Whoops, "part of the workspace" was the wrong way to say it. The problem is that it's not recognized as an external dependency, so --cap-lints is not applied anymore.

[Mark-Simulacrum]

Sure, but I guess my point is that I think it should be possible to patch/vendor sources without treating things as internal. For example, building from a source tarball with vendored sources should work without needing this. So I think there's still a disconnect of some kind here.

[petrochenkov]

I think it should be possible to patch/vendor sources without treating things as internal.

IIRC, overriding a dependency as git = 'my_path' instead of path = 'my_path' solves the issue.
(Maybe the path required file:// in the first case, but I don't remember exactly.)