error[E0282]: type annotations needed for `fn() -> A`
  --> $DIR/fn-needing-specified-return-type-param.rs:3:13
   |
LL |     let _ = f;
   |         -   ^ cannot infer type for type parameter `A` declared on the function `f`
   |         |
   |         consider giving this pattern the explicit type `fn() -> A`, where the type parameter `A` is specified

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.