rust FnMut 闭包

fn consume_with_relish<F>(mut func: F) where F: FnMut() -> String
{
    // `func` consumes its captured variables, so it cannot be run more
    // than once
    println!("Consumed 1 ");
    func();
    println!("Consumed 2");
    func();

    println!("Delicious!");

    // Attempting to invoke `func()` again will throw a `use of moved
    // value` error for `func`
}
fn main() {
        let mut  x = String::from("x");
    let consume_and_return_x = move || {
                        x.push('*');
                        println!("x {}", x);
        };
    consume_with_relish(consume_and_return_x);

// `consume_and_return_x` can no longer be invoked at this point
}

cargo build
   Compiling hello_world v0.1.0 (/data2/rust/FnMut)
error[E0271]: type mismatch resolving `<[closure@src/main.rs:17:32: 20:3 x:_] as std::ops::FnOnce<()>>::Output == std::string::String`
  --> src/main.rs:21:5
   |
1  | fn consume_with_relish<F>(mut func: F) where F: FnMut() -> String
   |                                                            ------ required by this bound in `consume_with_relish`
...
21 |     consume_with_relish(consume_and_return_x);
   |     ^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.
error: could not compile `hello_world`.

To learn more, run the command again with --verbose.

原来 FnMut 没返回值

pub trait FnMut<Args>: FnOnce<Args> {
    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
}

[root@bogon FnMut]# cargo build
   Compiling hello_world v0.1.0 (/data2/rust/FnMut)
    Finished dev [unoptimized + debuginfo] target(s) in 0.43s
[root@bogon FnMut]# cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/hello_world`
Consumed 1 
x x*
Consumed 2
x x**
Delicious!
[root@bogon FnMut]# cat src/main.rs 
fn consume_with_relish<F>(mut func: F) where F: FnMut()
{
    // `func` consumes its captured variables, so it cannot be run more
    // than once
    println!("Consumed 1 ");
    func();
    println!("Consumed 2");
    func();

    println!("Delicious!");

    // Attempting to invoke `func()` again will throw a `use of moved
    // value` error for `func`
}
fn main() {
        let mut  x = String::from("x");
    let consume_and_return_x = move || {
                        x.push('*');
                        println!("x {}", x);
        };
    consume_with_relish(consume_and_return_x);

// `consume_and_return_x` can no longer be invoked at this point
}
[root@bogon FnMut]#