rust提供了3种循环，loop、while 和 for循环
loop反复直到喊停
fn main() {
let mut _counter = 0;
let result = loop {
_counter += 1;
if _counter == 10 {
break _counter * 20;
}
};
println!("result = {}", result);
}
二、while 当循环，每次执行循环体之前都判断一次条件。
fn main() {
let mut _aaab = 0;
while _aaab < 10 {
_aaab += 1;
}
println!("_aaab = {}", _aaab);
}
倒计时
fn main() {
let mut _aaab = 10;
while _aaab != 0 {
println!("_aaab = {}", _aaab);
_aaab -= 1;
}
}
