Rust empty explicit generic argument for Option<impl Fn(i64)>

Question

I have Option<impl Fn(i64) -> i64> as an argument to a function apply() and I want to pass None value. apply(None). How do I write the apply(None) function call correctly so it compiles?

fn main(){
    let a = apply(Some(|i| i + 1));
    
    let b = apply(None);
    let b2 = apply(None::<dyn Fn(i64) -> i64>);
    
    println!("a: {0}; b: {1}", a, b)
}

fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
    let i = 0_i64;
    match f {
        Some(f) => f(i),
        None => i,
    }
}

compile error:

Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
 --> src/main.rs:4:16
  |
4 |     let b = apply(None);
  |                   ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
  |
help: consider specifying the generic argument
  |
4 |     let b = apply(None::<T>);
  |                       +++++

error[E0283]: type annotations needed
  --> src/main.rs:4:16
   |
4  |     let b = apply(None);
   |             ----- ^^^^ cannot infer type of the type parameter `T` declared on the enum `Option`
   |             |
   |             required by a bound introduced by this call
   |
   = note: multiple `impl`s satisfying `_: Fn<(i64,)>` found in the following crates: `alloc`, `core`:
           - impl<A, F> Fn<A> for &F
             where A: Tuple, F: Fn<A>, F: ?Sized;
           - impl<Args, F, A> Fn<Args> for Box<F, A>
             where Args: Tuple, F: Fn<Args>, A: Allocator, F: ?Sized;
note: required by a bound in `apply`
  --> src/main.rs:10:25
   |
10 | fn apply(f: Option<impl Fn(i64) -> i64>) -> i64 {
   |                         ^^^^^^^^^^^^^^ required by this bound in `apply`
help: consider specifying the generic argument
   |
4  |     let b = apply(None::<T>);
   |                       +++++

Some errors have detailed explanations: E0282, E0283.
For more information about an error, try `rustc --explain E0282`.

I tried let b2 = apply(None::<dyn Fn(i64) -> i64>); but it still doesn't compile the errors are

 Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `dyn Fn(i64) -> i64` cannot be known at compilation time
 --> src/main.rs:5:20
  |
5 |     let b2 = apply(None::<dyn Fn(i64) -> i64>);
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `dyn Fn(i64) -> i64`
note: required by a bound in `None`
 --> /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/option.rs:567:5

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to previous error

Answer 1

The type dyn Fn(i64) -> i64 is unsized, meaning you can't store it in an enum like Option.

But a trait object reference is sized, so you can use &dyn instead:

apply(None::<&dyn Fn(i64) -> i64>);

However, it's probably better to use a function pointer type instead:

apply(None::<fn(i64) -> i64>);

playground