Reference doc on `cfg` does not cover `cfg(feature = "...")`

[dhardy]

Rust appears to support #[cfg(feature = "foo")] to control optional features: Cargo doc.

The Rust reference on cfg does not mention feature: nightly doc.

[kennytm]

Rust support #[cfg(anything)] when you pass --cfg anything to rustc. Cargo is just passing --cfg feature=foo to the compiler.

The documentation could be improved to mention the --cfg flag though.

[dhardy]

Something's slightly odd with the doc though, because this says that the feature="foo" form is only allowed with compiler-defined configuration options, which means feature is compiler-defined.

[kennytm]

Probably outdated, or the reference is misinterpreting the meaning of RFC 194. The following works:

fn main() {
    println!("{}", cfg!(a="b"));
}

$ rustc --cfg 'a="b"' 1.rs && ./1
true

[steveklabnik]

The reference has its own repo now, I have migrated this issue over there: rust-lang/reference#21 Thanks!