error[E0426]: use of undeclared label `'x`
  --> $DIR/hygienic-label-1.rs:2:19
   |
LL |     () => { break 'x; }
   |                   ^^ undeclared label `'x`
...
LL |     'x: loop { 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

For more information about this error, try `rustc --explain E0426`.