error[E0658]: declaration of a nonstandard #[main] function may change over time, for now a top-level `fn main()` is required (see issue #29634)
  --> $DIR/feature-gate-main.rs:12:1
   |
LL | fn foo() {} //~ ERROR: declaration of a nonstandard #[main] function may change over time
   | ^^^^^^^^^^^
   |
   = help: add #![feature(main)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.