I don't know why function type cannot be used as a type parameter

function_table.rs

use std::collections::HashMap;
use std::any::Any;

fn main() {
    let funcs: Option<HashMap<isize, &Any>> = Some(HashMap::new());

    // expected expression, found keyword `fn`
    let func = funcs.map(|v| (*v.get(0)).downcast_ref<fn(n: isize)>());
}