#![feature(rust_2018_preview)] is an error in the 2018 edition

[alexcrichton]

This makes for relatively bad ergonomics, we should make this code a warning only on the 2018 edition (at most):

#![feature(rust_2018_preview)]

fn main() {
}

compiled with:

$ rustc +nightly foo.rs --edition 2018
error[E0557]: feature has been removed
 --> foo.rs:1:12
  |
1 | #![feature(rust_2018_preview)]
  |            ^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0557`.