error: macro requires a cfg-pattern as an argument
  --> $DIR/cfg.rs:2:5
   |
LL |     cfg!();
   |     ^^^^^^^ cfg-pattern required

error: expected identifier, found `123`
  --> $DIR/cfg.rs:3:10
   |
LL |     cfg!(123);
   |          ^^^ expected identifier

error[E0565]: literal in `cfg` predicate value must be a string
  --> $DIR/cfg.rs:4:16
   |
LL |     cfg!(foo = 123);
   |                ^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0565`.