首先，下面的代码是正确的：

fn main() {
    let a = &get_i32();
    println!("{}", a);
}
fn get_i32() -> i32 {
    return 100;
}

但是下面的代码出错：

fn main() {
    let a;
    a = &get_i32();
    println!("{}", a);
}

error[E0716]: temporary value dropped while borrowed
 --> src/bin/rust_course.rs:8:10
  |
8 |     a = &get_i32();
  |          ^^^^^^^^^- temporary value is freed at the end of this statement
  |          |
  |          creates a temporary value which is freed while still in use
9 |     println!("{}", a);
  |                    - borrow later used here
  |
help: consider using a `let` binding to create a longer lived value
  |
8 ~     let binding = get_i32();
9 ~     a = &binding;
  |

For more information about this error, try `rustc --explain E0716`.

这两段代码的本质区别是什么？我知道&get_i32()总是返回一个临时值，所以它应该总是报告一个错误。
类似的问题：

fn main() {
    let s1 = &String::from("hello world");
    println!("{}", s1);
    let s2 = String::from("hello world").as_str();
    println!("{}", s2);
}

error[E0716]: temporary value dropped while borrowed
 --> src/bin/rust_course.rs:6:14
  |
6 |     let s2 = String::from("hello world").as_str();
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^         - temporary value is freed at the end of this statement
  |              |
  |              creates a temporary value which is freed while still in use
7 |     println!("{}", s2);
  |                    -- borrow later used here
  |
help: consider using a `let` binding to create a longer lived value
  |
6 ~     let binding = String::from("hello world");
7 ~     let s2 = binding.as_str();
  |

For more information about this error, try `rustc --explain E0716`.

s1和s2有什么区别？
还有一个更类似的问题：

fn main() {
    print(String::from("hello world").as_str());
}

fn print(str: &str) {
    println!("{}", str);
}

上面的代码是正确的，但我不明白为什么String::from("hello world").as_str()可以传递给函数，但不能赋值给变量。