warning: `auto` traits are unstable
  --> $DIR/pre-cfg.rs:4:1
   |
LL | auto trait Foo {}
   | ^^^^
   |
   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
   = help: add `#![feature(auto_traits)]` to the crate attributes to enable
   = warning: unstable syntax can change at any point in the future, causing a hard error!
   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>

warning: 1 warning emitted