Why does passing a closure to function which accepts a function pointer not work?

In the second edition of The Rust Programming Language (emphasis mine):


Function pointers implement all three of the closure traits (Fn, FnMut, and FnOnce), so you can always pass a function pointer as an argument for a function that expects a closure. It’s best to write functions using a generic type and one of the closure traits so your functions can accept either functions or closures.

Passing a closure to a function which accepts a function pointer as an argument doesn't compile:

fn main() {
    let a = String::from("abc");

    let x = || println!("{}", a);

    fn wrap(c: fn() -> ()) -> () {
        c()
    }

    wrap(x);
}

error[E0308]: mismatched types
--> src/main.rs:10:10
|
10 | wrap(x);
| ^ expected fn pointer, found closure
|
= note: expected type fn()
found type [closure@src/main.rs:4:13: 4:33 a:_]


Why does this not work?

A closure isn't a function.

You can pass a function to a function expecting a closure, but there's no reason for the reverse to be true.

If you want to be able to pass both closures and functions as argument, just prepare it for closures.

For example:

let a = String::from("abc");

let x = || println!("x {}", a);

fn y() {
    println!("y")
}

fn wrap(c: impl Fn()) {
    c()
}

wrap(x); // pass a closure
wrap(y); // pass a function

Stack Overflow Q#52696907, score: 43