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PART 4
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Xingyu Wang
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@ -185,92 +185,89 @@ fn enable_register(&mut reg) {
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在上面的代码中，预期结果是什么？好吧，代码会将启用位设置为 0，因为 `10＆1 = 0`。那真不幸；最好在尝试写入之前知道你要写入字段的值是否适合该字段。事实上，我会考虑放弃错误字段值的高位*未定义行为*（喘气）。
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### Using Rust with safety in mind
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### 出于安全考虑使用 Rust
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How can you check that a field's value fits in its prescribed position in a general way? More type-level numbers!
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You can add a `Width` parameter to `Field` and use it to verify that a given value can fit into the field:
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如何以一般方式检查字段的值是否适合其规定的位置？需要更多类型级别的数字！
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你可以在 `Field` 中添加 `Width` 参数，并使用它来验证给定的值是否适合该字段：
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```
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struct Field<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> {
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value: u8,
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_mask: PhantomData<Mask>,
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_offset: PhantomData<Offset>,
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_width: PhantomData<Width>,
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struct Field<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> {
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value: u8,
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_mask: PhantomData<Mask>,
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_offset: PhantomData<Offset>,
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_width: PhantomData<Width>,
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}
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type RegEnabled = Field<U1,U1, U0>;
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type RegInterrupt = Field<U1, U2, U1>;
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type RegKind = Field<U3, op!(U7 << U2), U2>;
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type RegEnabled = Field<U1,U1, U0>;
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type RegInterrupt = Field<U1, U2, U1>;
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type RegKind = Field<U3, op!(U7 << U2), U2>;
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impl<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> Field<Width, Mask, Offset> {
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fn new(val: u8) -> Option<Self> {
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if val <= (1 << Width::U8) - 1 {
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Some(Field {
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value: (val << Offset::U8) & Mask::U8,
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_mask: PhantomData,
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_offset: PhantomData,
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_width: PhantomData,
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})
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} else {
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None
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}
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}
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impl<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> Field<Width, Mask, Offset> {
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fn new(val: u8) -> Option<Self> {
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if val <= (1 << Width::U8) - 1 {
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Some(Field {
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value: (val << Offset::U8) & Mask::U8,
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_mask: PhantomData,
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_offset: PhantomData,
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_width: PhantomData,
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})
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} else {
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None
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}
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}
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}
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```
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Now you can construct a `Field` only if the given value fits! Otherwise, you have `None`, which signals that an error has occurred, rather than lopping off the high bits of the value and silently writing an unexpected value.
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现在，只有给定值适合时，您才能构造一个 `Field`！否则，你将得到 `None` 信号，该信号指示发生了错误，而不是放弃该值的高位并静默写入意外的值。
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Note, though, this will raise an error at runtime. However, we knew the value we wanted to write beforehand, remember? Given that, we can teach the compiler to reject entirely a program which has an invalid field value—we don’t have to wait until we run it!
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This time, you'll add a _trait bound_ (the `where` clause) to a new realization of new, called `new_checked`, that asks the incoming value to be less than or equal to the maximum possible value a field with the given `Width` can hold:
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但是请注意，这将在运行时引发错误。但是，我们知道我们想事先写入的值，还记得吗？鉴于此，我们可以教编译器完全拒绝具有无效字段值的程序 —— 我们不必等到运行它！
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这次，您将向 `new` 的新实现 `new_checked` 中添加一个特征绑定（`where` 子句），该函数要求输入值小于或等于给定字段用 ``Width` 所控制的最大可能值：
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```
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struct Field<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> {
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value: u8,
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_mask: PhantomData<Mask>,
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_offset: PhantomData<Offset>,
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_width: PhantomData<Width>,
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struct Field<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> {
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value: u8,
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_mask: PhantomData<Mask>,
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_offset: PhantomData<Offset>,
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_width: PhantomData<Width>,
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}
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type RegEnabled = Field<U1, U1, U0>;
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type RegInterrupt = Field<U1, U2, U1>;
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type RegKind = Field<U3, op!(U7 << U2), U2>;
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type RegEnabled = Field<U1, U1, U0>;
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type RegInterrupt = Field<U1, U2, U1>;
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type RegKind = Field<U3, op!(U7 << U2), U2>;
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impl<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> Field<Width, Mask, Offset> {
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const fn new_checked<V: Unsigned>() -> Self
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where
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V: IsLessOrEqual<op!((U1 << Width) - U1), Output = True>,
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{
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Field {
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value: (V::U8 << Offset::U8) & Mask::U8,
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_mask: PhantomData,
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_offset: PhantomData,
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_width: PhantomData,
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}
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}
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impl<Width: Unsigned, Mask: Unsigned, Offset: Unsigned> Field<Width, Mask, Offset> {
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const fn new_checked<V: Unsigned>() -> Self
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where
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V: IsLessOrEqual<op!((U1 << Width) - U1), Output = True>,
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{
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Field {
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value: (V::U8 << Offset::U8) & Mask::U8,
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_mask: PhantomData,
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_offset: PhantomData,
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_width: PhantomData,
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}
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}
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}
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```
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Only numbers for which this property holds has an implementation of this trait, so if you use a number that does not fit, it will fail to compile. Take a look!
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只有拥有此属性的数字才具有此特征的实现，因此，如果使用不适合的数字，它将无法编译。让我们看一看！
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```
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fn enable_register(&mut reg) {
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reg.update(RegEnabled::new_checked::<U10>());
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fn enable_register(&mut reg) {
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reg.update(RegEnabled::new_checked::<U10>());
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}
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12 | reg.update(RegEnabled::new_checked::<U10>());
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| ^^^^^^^^^^^^^^^^ expected struct `typenum::B0`, found struct `typenum::B1`
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= note: expected type `typenum::B0`
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found type `typenum::B1`
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12 | reg.update(RegEnabled::new_checked::<U10>());
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| ^^^^^^^^^^^^^^^^ expected struct `typenum::B0`, found struct `typenum::B1`
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= note: expected type `typenum::B0`
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found type `typenum::B1`
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```
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`new_checked` will fail to produce a program that has an errant too-high value for a field. Your typo won't blow up at runtime because you could never have gotten an artifact to run.
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`new_checked` 将无法生成一个程序，因为该字段的值错误地过高。你的输入错误不会在运行时爆炸，因为你永远无法获得一个可以运行的工件。
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You're nearing Peak Rust in terms of how safe you can make memory-mapped hardware interactions. However, what you wrote back in the first example in C was far more succinct than the type parameter salad you ended up with. Is doing such a thing even tractable when you're talking about potentially hundreds or even thousands of registers?
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就使内存映射的硬件进行交互的安全性而言，你已经接近 Peak Rust。但是，你在 C 的第一个示例中所写的内容比最终得到的一锅粥类型参数更简洁。当你谈论潜在的数百甚至数千个寄存器时，这样做是否容易处理？
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### Just right with Rust: both safe and accessible
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