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Update ch18-02-refutability.md

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@ -25,9 +25,7 @@ error[E0005]: refutable pattern in local binding: `None` not covered
因为我们没有(也不能)覆盖到模式`Some(x)`的每一个可能的值, 所以Rust会报错.
如果我们采用*refutable*模式, 使用`if let`而不是`let`. 这样当模式不匹配时, 在花括号中的代码将不执行, 这段代码只有在值匹配模式的时候才会执行, 也只在此时有意义. Listing 18-8 shows how to fix the code in Listing 18-7 with `Some(x)`
matching `some_option_value`. Using the refutable pattern `Some(x)` is allowed,
since this example uses `if let`:
如果我们采用*refutable*模式, 使用`if let`而不是`let`. 这样当模式不匹配时, 在花括号中的代码将不执行, 这段代码只有在值匹配模式的时候才会执行, 也只在此时才有意义. 例18-8显示了如何修正在例18-7中用`Some(x)`来匹配`some_option_value`的代码. 因为这个例子使用了`if let`, 因此使用*refutable*模式的`Some(x)`就没问题了:
```rust
# let some_option_value: Option<i32> = None;
@ -36,11 +34,9 @@ if let Some(x) = some_option_value {
}
```
<span class="caption">Listing 18-8: Using `if let` and a block with refutable
patterns instead of `let`</span>
<span class="caption">例18-8: 使用`if let`和一个有*refutable*模式的代码块来代替`let`</span>
Consequently, if we give `if let` an irrefutable pattern that will always match,
such as `x` as shown in Listing 18-9:
此外, 如果我们给`if let`一个绝对会匹配的*irrefutable*模式, 比如在例18-9中显示的`x`:
```rust,ignore
if let x = 5 {
@ -48,11 +44,9 @@ if let x = 5 {
};
```
<span class="caption">Listing 18-9: Attempting to use an irrefutable pattern
with `if let`</span>
<span class="caption">例18-9: 尝试把一个*irrefutable*模式用到`if let`上</span>
Rust will complain that it doesn't make sense to use `if let` with an
irrefutable pattern:
Rust将会抱怨把`if let`和一个*irrefutable*模式一起使用没有意义:
```text
error[E0162]: irrefutable if-let pattern
@ -62,10 +56,7 @@ error[E0162]: irrefutable if-let pattern
| ^ irrefutable pattern
```
Generally, match arms use refutable patterns, except for the last arm that
might match any remaining values with an irrefutable pattern. A `match` with
only one arm whose pattern is irrefutable is allowed, but it's not particularly
useful and could be replaced with a simpler `let` statement. Both the expressions
一般来说, 多数匹配使用*refutable*模式, 除非是那种可以匹配任意值的情况使用*irrefutable*模式. `match`操作符中如果只有一个*irrefutable*模式分支也没有什么问题, 但这就没什么特别的用处, 此时可以用一个更简单的`let`语句来替换. 不管是把表达式关联到`let`语句亦或是 Both the expressions
associated with a `let` statement and a single arm irrefutable match will
unconditionally be run, so the end result is the same if their expressions are.