error[E0425]: cannot find value `abcdef` in this scope
  --> $DIR/hygiene.rs:5:10
   |
LL |         ${concat($lhs, $rhs)}
   |          ^^^^^^^^^^^^^^^^^^^^ not found in this scope
...
LL |     let _another = join!(abc, def);
   |                    --------------- in this macro invocation
   |
   = note: this error originates in the macro `join` (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 E0425`.