src/test/ui/cfg-attr-unknown-attribute-macro-expansion.stderr

   1 error[E0658]: The attribute `unknown` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
   2   --> $DIR/cfg-attr-unknown-attribute-macro-expansion.rs:13:27
   3    |
   4 LL |         #[cfg_attr(all(), unknown)] //~ ERROR `unknown` is currently unknown
   5    |                           ^^^^^^^
   6 ...
   7 LL | foo!();
   8    | ------- in this macro invocation
   9    |
  10    = help: add #![feature(custom_attribute)] to the crate attributes to enable
  11
  12 error: aborting due to previous error
  13
  14 For more information about this error, try `rustc --explain E0658`.