Heads up, with the release of rust-lang/cargo#13913 (in nightly-2024-05-19), there is no longer any need for the kind of workarounds employed in this PR. Cargo has now gain the ability to declare --check-cfg args directly inside the [lints] table with [lints.rust.unexpected_cfgs.check-cfg]¹:

Cargo.toml:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(foo)'] }

Note that the diagnostic output of the lint has been updated to suggest the [lints] approach first. You can use it to guide you through the --check-cfg arguments that may need to be added.