error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642) --> $DIR/custom_attribute.rs:3:3 | LL | #[foo] //~ ERROR The attribute `foo` | ^^^ | = help: add #![feature(custom_attribute)] to the crate attributes to enable error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642) --> $DIR/custom_attribute.rs:5:7 | LL | #[foo] //~ ERROR The attribute `foo` | ^^^ | = help: add #![feature(custom_attribute)] to the crate attributes to enable error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642) --> $DIR/custom_attribute.rs:7:7 | LL | #[foo] //~ ERROR The attribute `foo` | ^^^ | = help: add #![feature(custom_attribute)] to the crate attributes to enable error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0658`.