对于一些使用情况，匹配很尴尬。例如：
// Make `optional` of type `Option`
let optional = Some(7);
match optional {
Some(i) => {
println!("This is a really long string and `{:?}`", i);
// ^ Needed 2 indentations just so we could destructure
// `i` from the option.
},
_ => {},
// ^ Required because `match` is exhaustive. Doesn't it seem
// like wasted space?
};
if let 是清洁器用例，另外允许指定的各种失败选项：
fn main() {
// All have type `Option`
let number = Some(7);
let letter: Option = None;
let emoticon: Option = None;
// The `if let` construct reads: "if `let` destructures `number` into
// `Some(i)`, evaluate the block (`{}`). Else do nothing.
if let Some(i) = number {
println!("Matched {:?}!", i);
}
// If you need to specify a failure, use an else:
if let Some(i) = letter {
println!("Matched {:?}!", i);
} else {
// Destructure failed. Change the failure case.
println!("Didn't match a number. Let's go with a letter!");
};
// Provide an altered failing condition.
let i_like_letters = false;
if let Some(i) = emoticon {
println!("Matched {:?}!", i);
// Destructure failed. Evaluated the condition to see if this branch
// should be taken.
} else if i_like_letters {
println!("Didn't match a number. Let's go with a letter!");
// The condition evaluated false. This branch is the default.
} else {
println!("I don't like letters. Let's go with an emoticon :)!");
};
}
