类似于if let，while let 可以使劲匹配更多序列。例如考虑以下序列：
// Make `optional` of type `Option`
let mut optional = Some(0);
// Repeatedly try this test.
loop {
match optional {
// If `optional` destructures, evaluate the block.
Some(i) => {
if i > 9 {
println!("Greater than 9, quit!");
optional = None;
} else {
println!("`i` is `{:?}`. Try again.", i);
optional = Some(i + 1);
}
// ^ Requires 3 indentations!
},
// Quit when the destructure fails, meaning `break`.
_ => { break; }
// ^ Why should this be required? Seems superfluous.
}
}
while let 使该序列好得更多：
fn main() {
// Make `optional` of type `Option`
let mut optional = Some(0);
// This reads: "while `let` destructures `optional` into
// `Some(i)`, evaluate the block (`{}`). Else `break`.
while let Some(i) = optional {
if i > 9 {
println!("Greater than 9, quit!");
optional = None;
} else {
println!("`i` is `{:?}`. Try again.", i);
optional = Some(i + 1);
}
// ^ Less rightward drift and doesn't require
// explicitly handling the failing case.
}
// ^ `if let` had additional optional `else`/`else if`
// clauses. `while let` does not have these.
}
