简单的示例：
enum Foo{ Zhangsan, Lisi, Wangwu(u8) } fn main() { let a = Foo::Zhangsan; let b = Foo::Lisi; let c = Foo::Wangwu(23); if let Foo::Zhangsan = a{ println!("a is Foo::zhangsan"); } if let Foo::Zhangsan = b{ println!("b is Foo::Zhangsan"); // 此行不会打印 } if let Foo::Wangwu(val) = c{ println!("c is Foo::Wangwu = {}",val) } println!("=================================="); let mut optional = Some(0); while let Some(i) = optional{ if i > 5{ println!("Greater than 5 ,quit!"); optional = None; }else { println!("i is {:?} Try again.", i); optional = Some(i+1) } } } // a is Foo::zhangsan // c is Foo::Wangwu = 23 // ================================== // i is 0 Try again. // i is 1 Try again. // i is 2 Try again. // i is 3 Try again. // i is 4 Try again. // i is 5 Try again. // Greater than 5 ,quit!