error[E0379]: functions in trait impls cannot be declared const
  --> $DIR/const-fn-mismatch.rs:11:5
   |
LL |     const fn f() -> u32 {
   |     ^^^^^-
   |     |
   |     functions in trait impls cannot be const
   |     help: remove the `const`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0379`.