error: macro requires a cfg-pattern as an argument
  --> $DIR/cfg.rs:2:5
   |
LL |     cfg!();
   |     ^^^^^^ cfg-pattern required
   |
   = note: this error originates in the macro `cfg` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0565]: literal in `cfg` predicate value must be a boolean
  --> $DIR/cfg.rs:3:10
   |
LL |     cfg!(123);
   |          ^^^

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

error: expected 1 cfg-pattern
  --> $DIR/cfg.rs:5:5
   |
LL |     cfg!(foo, bar);
   |     ^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `cfg` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 4 previous errors

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