error[E0767]: use of unreachable label `'a`
  --> $DIR/issue-73541-2.rs:7:26
   |
LL |     'a: loop {
   |     -- unreachable label defined here
...
LL |                 continue 'a
   |                          ^^ unreachable label `'a`
...
LL |                 b!();
   |                 ---- in this macro invocation
   |
   = note: labels are unreachable through functions, closures, async blocks and modules
   = note: this error originates in the macro `b` (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 E0767`.