if let语法使您能够将if和let组合成一种不太简短的形式来解决与一种模式匹配的值，而疏忽其余模式。 例如：该程序与Option <u8>值匹配，但仅想在值为3时执行代码：
fn main() { let some_u8_value = Some(0u8); match some_u8_value { Some(3) => println!("three"), _ => (println!("啥也不是")), }}咱们想对Some(3)匹配做点什么，但对其余Some <u8>值或None值什么都不做。 为了满足match表达式，咱们只须要解决一个变体就增加_ =>()，然而须要增加的许多样板代码。
相同，咱们能够应用if let以更短的形式编写此代码并实现完全一致的匹配成果：
fn main() { let some_u8_value = Some(8u8); if let Some(3) = some_u8_value { println!("three"); } if let Some(8) = some_u8_value { println!("匹配到了"); }}D:\learn\cargo_learn>cargo run Compiling cargo_learn v0.1.0 (D:\learn\cargo_learn) Finished dev [unoptimized + debuginfo] target(s) in 0.27s Running `target\debug\cargo_learn.exe`匹配到了相应的如果咱们想要失去一个确切的后果，比方如果没有匹配到咱们须要的那个咱们心愿有返回后果，那么此时能够应用else:
#[derive(Debug)]enum UsState { Alabama, Alaska,}enum Coin { Penny, Quarter(UsState),}fn main() { let coin = Coin::Penny; let mut count = 0; if let Coin::Quarter(state) = coin { println!("State quarter from {:?}!", state); } else { count += 1; println!("没有匹配到任何后果， count将主动+1 ！！！") }}D:\learn\cargo_learn>cargo run Compiling cargo_learn v0.1.0 (D:\learn\cargo_learn)warning: variable `count` is assigned to, but never used --> src\main.rs:14:9 |14 | let mut count = 0; | ^^^^^^^^^ | = note: `#[warn(unused_variables)]` on by default = note: consider using `_count` insteadwarning: value assigned to `count` is never read --> src\main.rs:18:9 |18 | count += 1; | ^^^^^ | = note: `#[warn(unused_assignments)]` on by default = help: maybe it is overwritten before being read?warning: variant is never constructed: `Alabama` --> src\main.rs:3:5 |3 | Alabama, | ^^^^^^^ | = note: `#[warn(dead_code)]` on by defaultwarning: variant is never constructed: `Alaska` --> src\main.rs:4:5 |4 | Alaska, | ^^^^^^warning: variant is never constructed: `Quarter` --> src\main.rs:9:5 |9 | Quarter(UsState), | ^^^^^^^^^^^^^^^^warning: 5 warnings emitted Finished dev [unoptimized + debuginfo] target(s) in 0.51s Running `target\debug\cargo_learn.exe`没有匹配到任何后果， count将主动+1 ！！！