error: functions cannot be both `const` and `async`
  --> $DIR/const-async-fn-in-main.rs:5:5
   |
LL |     const async fn a() {}
   |     ^^^^^-^^^^^----------
   |     |     |
   |     |     `async` because of this
   |     `const` because of this

error: aborting due to 1 previous error