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Conditional compilation makes it sound like #[cfg(foo)] will evaluate to true for --cfg=foo="bar" but it doesn't #2004
Description
From the Reference:
A configuration option. The predicate is true if the option is set, and false if it is unset.
And from the Reference
Configuration options are either names or key-value pairs, and are either set or unset.
but rustc's behavior for #[cfg(foo)] is if its set and a name.
A minimal reproduction:
#!/usr/bin/env -S cargo +nightly -Zscript
---
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(foo, values(any()))'] }
---
fn main() {
println!("Hello from the control grup!");
#[cfg(foo)]
println!("Hello from `foo`");
#[cfg(foo = "value")]
println!("Hello, from `foo=value`");
}$ RUSTFLAGS='' ./check-cfg-exists.rs # unset
Hello from the control grup!
$ RUSTFLAGS='--cfg=foo' ./check-cfg-exists.rs # set with name
Hello from the control grup!
Hello from `foo`
$ RUSTFLAGS='--cfg=foo="value"' ./check-cfg-exists.rs # set with key-value pair
Hello from the control grup!
Hello, from `foo=value`
Activity
epage commentedon Sep 17, 2025
A proposed re-wording
I've not put up a PR for this yet because I feel like this might need some further investigation first.