warning: label name `'b` shadows a label name that is already in scope
  --> $DIR/macro-lifetime-used-with-labels.rs:21:9
   |
LL |         'b: loop {
   |         ^^ lifetime 'b already in scope
...
LL |     'b: loop {
   |     -- first declared here
LL |         br2!('b);
   |         --------- in this macro invocation
   |
   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

warning: 1 warning emitted