tests/ui/conditional-compilation/cfg-attr-unknown-attribute-macro-expansion.stderr

error: cannot find attribute `unknown` in this scope
  --> $DIR/cfg-attr-unknown-attribute-macro-expansion.rs:3:26
   |
LL |         #[cfg_attr(true, unknown)]
   |                          ^^^^^^^
...
LL | foo!();
   | ------ in this macro invocation
   |
   = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error