Rust 使用函数指针时出现Expected fn item, found a different fn item

我有以下代码(Playground):

// Two dummy functions, both with the signature `fn(u32) -> bool`
fn foo(x: u32) -> bool {
    x % 2 == 0
}
fn bar(x: u32) -> bool {
    x == 27
}


fn call_both<F>(a: F, b: F)
where
    F: Fn(u32) -> bool,
{
    a(5);
    b(5);
}

fn main() {
    call_both(foo, bar);  // <-- error
}

在我看来，这似乎应该编译为foo和bar有相同的签名:fn(u32) -> bool.然而，我得到了以下错误:

error[E0308]: mismatched types
  --> src/main.rs:20:20
   |
20 |     call_both(foo, bar);
   |                    ^^^ expected fn item, found a different fn item
   |
   = note: expected type `fn(u32) -> bool {foo}`
              found type `fn(u32) -> bool {bar}`

此代码可能会触发相同的错误:

let mut x = foo;
x = bar;  // <-- error

我还try 再次将bar转换为函数指针类型:

let mut x = foo;
x = bar as fn(u32) -> bool;  // <-- error

这导致了一个稍有不同的错误:

error[E0308]: mismatched types
  --> src/main.rs:20:9
   |
20 |     x = bar as fn(u32) -> bool;
   |         ^^^^^^^^^^^^^^^^^^^^^^ expected fn item, found fn pointer
   |
   = note: expected type `fn(u32) -> bool {foo}`
              found type `fn(u32) -> bool`

我完全不理解这些错误.What are fn items vs. fn pointers and why are 100 and 101 different fn items?