这是我的代码，下面是编译器错误。

fn main() {
    let mut s = String::new();
    let mut push_if = |b,some_str| {
        if b {
            s.push_str(some_str);
        }
    };
    push_if(s.is_empty(),"Foo");
    println!("{}",s);
}

error[E0502]: cannot borrow `s` as immutable because it is also borrowed as mutable
 --> src/main.rs:8:13
  |
3 |     let mut push_if = |b,some_str| {
  |                       ------------- mutable borrow occurs here
4 |         if b {
5 |             s.push_str(some_str);
  |             - first borrow occurs due to use of `s` in closure
...
8 |     push_if(s.is_empty(),"Foo");
  |     ------- ^ immutable borrow occurs here
  |     |
  |     mutable borrow later used by call

为什么编译器抱怨s.is_empty()是不可变的借项？  我只是想退还布尔值，似乎我没有借任何东西。要成功编译程序，我需要进行哪些更改？