error[E0658]: The attribute `unknown` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
  --> $DIR/cfg-attr-unknown-attribute-macro-expansion.rs:13:27
   |
LL |         #[cfg_attr(all(), unknown)] //~ ERROR `unknown` is currently unknown
   |                           ^^^^^^^^
...
LL | foo!();
   | ------- in this macro invocation
   |
   = help: add #![feature(custom_attribute)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.