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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. 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)` 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:
matching `some_option_value`. Using the refutable pattern `Some(x)` is allowed,
since this example uses `if let`:
```rust ```rust
# let some_option_value: Option<i32> = None; # 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 <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>
patterns instead of `let`</span>
Consequently, if we give `if let` an irrefutable pattern that will always match, 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`:
such as `x` as shown in Listing 18-9:
```rust,ignore ```rust,ignore
if let x = 5 { if let x = 5 {
@ -48,11 +44,9 @@ if let x = 5 {
}; };
``` ```
<span class="caption">Listing 18-9: Attempting to use an irrefutable pattern <span class="caption">例18-9: 尝试把一个*irrefutable*模式用到`if let`上</span>
with `if let`</span>
Rust will complain that it doesn't make sense to use `if let` with an Rust将会抱怨把`if let`和一个*irrefutable*模式一起使用没有意义:
irrefutable pattern:
```text ```text
error[E0162]: irrefutable if-let pattern error[E0162]: irrefutable if-let pattern
@ -62,10 +56,7 @@ error[E0162]: irrefutable if-let pattern
| ^ irrefutable pattern | ^ irrefutable pattern
``` ```
Generally, match arms use refutable patterns, except for the last arm that 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
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
associated with a `let` statement and a single arm irrefutable match will 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. unconditionally be run, so the end result is the same if their expressions are.