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< h1 id = "类型转换" > < a class = "header" href = "#类型转换" > 类型转换< / a > < / h1 >
< p > Rust 是类型安全的语言，因此在 Rust 中做类型转换不是一件简单的事，这一章节我们将对 Rust 中的类型转换进行详尽讲解。< / p >
< blockquote >
< p > 高能预警：本章节有些难，可以考虑学了进阶后回头再看< / p >
< / blockquote >
< h2 id = "as转换" > < a class = "header" href = "#as转换" > < code > as< / code > 转换< / a > < / h2 >
< p > 先来看一段代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let a: i32 = 10;
let b: u16 = 100;
if a < b {
println!("Ten is less than one hundred.");
}
}< / code > < / pre >
< p > 能跟着这本书一直学习到这里，说明你对 Rust 已经有了一定的理解，那么一眼就能看出这段代码注定会报错，因为 < code > a< / code > 和 < code > b< / code > 拥有不同的类型， < / p >
< p > 解决办法很简单，只要把 < code > b< / code > 转换成 < code > i32< / code > 类型即可， < code > as< / code > 操作符来完成： < code > if a < (b as i32) {...}< / code > 。那么为什么不把 < code > a< / code > 转换成 < code > u16< / code > 类型呢？< / p >
< p > 因为每个类型能表达的数据范围不同，如果把范围较大的类型转换成较小的类型，会造成错误，因此我们需要把范围较小的类型转换成较大的类型，来避免这些问题的发生。< / p >
< blockquote >
< p > 使用类型转换需要小心，因为如果执行以下操作 < code > 300_i32 as i8< / code > ，你将获得 < code > 44< / code > 这个值，而不是 < code > 300< / code > ，因为 < code > i8< / code > 类型能表达的的最大值为 < code > 2^7 - 1< / code > ，使用以下代码可以查看 < code > i8< / code > 的最大值：< / p >
< / blockquote >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a = i8::MAX;
println!("{}",a);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 下面列出了常用的转换形式：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let a = 3.1 as i8;
let b = 100_i8 as i32;
let c = 'a' as u8; // 将字符'a'转换为整数，
println!("{},{},{}",a,b,c)
}< / code > < / pre >
< h4 id = "内存地址转换为指针" > < a class = "header" href = "#内存地址转换为指针" > 内存地址转换为指针< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let mut values: [i32; 2] = [1, 2];
let p1: *mut i32 = values.as_mut_ptr();
let first_address = p1 as usize; // 将p1内存地址转换为一个整数
let second_address = first_address + 4; // 4 == std::mem::size_of::< i32> ()， ，
let p2 = second_address as *mut i32; // 访问该地址指向的下一个整数p2
unsafe {
*p2 += 1;
}
assert_eq!(values[1], 3);
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "强制类型转换的边角知识" > < a class = "header" href = "#强制类型转换的边角知识" > 强制类型转换的边角知识< / a > < / h4 >
< ol >
< li > 转换不具有传递性
就算 < code > e as U1 as U2< / code > 是合法的，也不能说明 < code > e as U2< / code > 是合法的（< code > e< / code > 不能直接转换成 < code > U2< / code > ）。< / li >
< / ol >
< h2 id = "tryinto-转换" > < a class = "header" href = "#tryinto-转换" > TryInto 转换< / a > < / h2 >
< p > 在一些场景中，使用 < code > as< / code > 关键字会有比较大的限制。如果你想要在类型转换上拥有完全的控制而不依赖内置的转换，例如处理转换错误，那么可以使用 < code > TryInto< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::convert::TryInto;
fn main() {
let a: u8 = 10;
let b: u16 = 1500;
let b_: u8 = b.try_into().unwrap();
if a < b_ {
println!("Ten is less than one hundred.");
}
}< / code > < / pre >
< p > 上面代码中引入了 < code > std::convert::TryInto< / code > 特征，但是却没有使用它，可能有些同学会为此困惑，主要原因在于< strong > 如果你要使用一个特征的方法，那么你需要引入该特征到当前的作用域中< / strong > ，我们在上面用到了 < code > try_into< / code > 方法，因此需要引入对应的特征。但是 Rust 又提供了一个非常便利的办法，把最常用的标准库中的特征通过< a href = "https://course.rs/appendix/prelude.html" > < code > std::prelude< / code > < / a > 模块提前引入到当前作用域中，其中包括了 < code > std::convert::TryInto< / code > ，你可以尝试删除第一行的代码 < code > use ...< / code > ，看看是否会报错。< / p >
< p > < code > try_into< / code > 会尝试进行一次转换，并返回一个 < code > Result< / code > ，此时就可以对其进行相应的错误处理。由于我们的例子只是为了快速测试，因此使用了 < code > unwrap< / code > 方法，该方法在发现错误时，会直接调用 < code > panic< / code > 导致程序的崩溃退出，在实际项目中，请不要这么使用，具体见< a href = "https://course.rs/basic/result-error/panic.html#调用-panic" > panic< / a > 部分。< / p >
< p > 最主要的是 < code > try_into< / code > 转换会捕获大类型向小类型转换时导致的溢出错误：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let b: i16 = 1500;
let b_: u8 = match b.try_into() {
Ok(b1) => b1,
Err(e) => {
println!("{:?}", e.to_string());
0
}
};
}< / code > < / pre >
< p > 运行后输出如下 < code > "out of range integral type conversion attempted"< / code > ，在这里我们程序捕获了错误，编译器告诉我们类型范围超出的转换是不被允许的，因为我们试图把 < code > 1500_i16< / code > 转换为 < code > u8< / code > 类型，后者明显不足以承载这么大的值。< / p >
< h2 id = "通用类型转换" > < a class = "header" href = "#通用类型转换" > 通用类型转换< / a > < / h2 >
< p > 虽然 < code > as< / code > 和 < code > TryInto< / code > 很强大，但是只能应用在数值类型上，可是 Rust 有如此多的类型，想要为这些类型实现转换，我们需要另谋出路，先来看看在一个笨办法，将一个结构体转换为另外一个结构体：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct Foo {
x: u32,
y: u16,
}
struct Bar {
a: u32,
b: u16,
}
fn reinterpret(foo: Foo) -> Bar {
let Foo { x, y } = foo;
Bar { a: x, b: y }
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 简单粗暴，但是从另外一个角度来看，也挺啰嗦的，好在 Rust 为我们提供了更通用的方式来完成这个目的。< / p >
< h4 id = "强制类型转换" > < a class = "header" href = "#强制类型转换" > 强制类型转换< / a > < / h4 >
< p > 在某些情况下，类型是可以进行隐式强制转换的，虽然这些转换弱化了 Rust 的类型系统，但是它们的存在是为了让 Rust 在大多数场景可以工作(说白了，帮助用户省事)，而不是报各种类型上的编译错误。< / p >
< p > 首先，在匹配特征时，不会做任何强制转换(除了方法)。一个类型 < code > T< / code > 可以强制转换为 < code > U< / code > ，不代表 < code > impl T< / code > 可以强制转换为 < code > impl U< / code > ，例如下面的代码就无法通过编译检查：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > trait Trait {}
fn foo< X: Trait> (t: X) {}
impl< 'a> Trait for & 'a i32 {}
fn main() {
let t: & mut i32 = & mut 0;
foo(t);
}< / code > < / pre >
< p > 报错如下：< / p >
< pre > < code class = "language-console" > error[E0277]: the trait bound `& mut i32: Trait` is not satisfied
--> src/main.rs:9:9
|
9 | foo(t);
| ^ the trait `Trait` is not implemented for `& mut i32`
|
= help: the following implementations were found:
< & 'a i32 as Trait>
= note: `Trait` is implemented for `& i32`, but not for `& mut i32`
< / code > < / pre >
< p > < code > & i32< / code > 实现了特征 < code > Trait< / code > ， < code > & mut i32< / code > 可以转换为 < code > & i32< / code > ，但是 < code > & mut i32< / code > 依然无法作为 < code > Trait< / code > 来使用。<!-- 这一段没读懂，代码中的例子好像和上面的文字描述关系不大 --> < / p >
< h4 id = "点操作符" > < a class = "header" href = "#点操作符" > 点操作符< / a > < / h4 >
< p > 方法调用的点操作符看起来简单，实际上非常不简单，它在调用时，会发生很多魔法般的类型转换，例如：自动引用、自动解引用，强制类型转换直到类型能匹配等。< / p >
< p > 假设有一个方法 < code > foo< / code > ，它有一个接收器(接收器就是 < code > self< / code > 、< code > & self< / code > 、< code > & mut self< / code > 参数)。如果调用 < code > value.foo()< / code > ，编译器在调用 < code > foo< / code > 之前，需要决定到底使用哪个 < code > Self< / code > 类型来调用。现在假设 < code > value< / code > 拥有类型 < code > T< / code > 。< / p >
< p > 再进一步，我们使用< a href = "https://course.rs/basic/trait/advance-trait.html#完全限定语法" > 完全限定语法< / a > 来进行准确的函数调用:< / p >
< ol >
< li > 首先，编译器检查它是否可以直接调用 < code > T::foo(value)< / code > ，称之为< strong > 值方法调用< / strong > < / li >
< li > 如果上一步调用无法完成(例如方法类型错误或者特征没有针对 < code > Self< / code > 进行实现，上文提到过特征不能进行强制转换)，那么编译器会尝试增加自动引用，例如会尝试以下调用： < code > < & T> ::foo(value)< / code > 和 < code > < & mut T> ::foo(value)< / code > ，称之为< strong > 引用方法调用< / strong > < / li >
< li > 若上面两个方法依然不工作，编译器会试着解引用 < code > T< / code > ，然后再进行尝试。这里使用了 < code > Deref< / code > 特征 —— 若 < code > T: Deref< Target = U> < / code > (< code > T< / code > 可以被解引用为 < code > U< / code > )，那么编译器会使用 < code > U< / code > 类型进行尝试，称之为< strong > 解引用方法调用< / strong > < / li >
< li > 若 < code > T< / code > 不能被解引用，且 < code > T< / code > 是一个定长类型(在编译期类型长度是已知的)，那么编译器也会尝试将 < code > T< / code > 从定长类型转为不定长类型，例如将 < code > [i32; 2]< / code > 转为 < code > [i32]< / code > < / li >
< li > 若还是不行，那…没有那了，最后编译器大喊一声：汝欺我甚，不干了！< / li >
< / ol >
< p > 下面我们来用一个例子来解释上面的方法查找算法:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let array: Rc< Box< [T; 3]> > = ...;
let first_entry = array[0];
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > array< / code > 数组的底层数据隐藏在了重重封锁之后，那么编译器如何使用 < code > array[0]< / code > 这种数组原生访问语法通过重重封锁，准确的访问到数组中的第一个元素？< / p >
< ol >
< li > 首先， < code > array[0]< / code > 只是< a href = "https://doc.rust-lang.org/std/ops/trait.Index.html" > < code > Index< / code > < / a > 特征的语法糖：编译器会将 < code > array[0]< / code > 转换为 < code > array.index(0)< / code > 调用，当然在调用之前，编译器会先检查 < code > array< / code > 是否实现了 < code > Index< / code > 特征。< / li >
< li > 接着，编译器检查 < code > Rc< Box< [T; 3]> > < / code > 是否有实现 < code > Index< / code > 特征，结果是否，不仅如此，< code > & Rc< Box< [T; 3]> > < / code > 与 < code > & mut Rc< Box< [T; 3]> > < / code > 也没有实现。< / li >
< li > 上面的都不能工作，编译器开始对 < code > Rc< Box< [T; 3]> > < / code > 进行解引用，把它转变成 < code > Box< [T; 3]> < / code > < / li >
< li > 此时继续对 < code > Box< [T; 3]> < / code > 进行上面的操作 ： < code > Box< [T; 3]> < / code > ， < code > & Box< [T; 3]> < / code > ，和 < code > & mut Box< [T; 3]> < / code > 都没有实现 < code > Index< / code > 特征，所以编译器开始对 < code > Box< [T; 3]> < / code > 进行解引用，然后我们得到了 < code > [T; 3]< / code > < / li >
< li > < code > [T; 3]< / code > 以及它的各种引用都没有实现 < code > Index< / code > 索引(是不是很反直觉:D， ， ， < code > [T; 3]< / code > 被转换成 < code > [T]< / code > ，也就是数组切片，它实现了 < code > Index< / code > 特征，因此最终我们可以通过 < code > index< / code > 方法访问到对应的元素。< / li >
< / ol >
< p > 过程看起来很复杂，但是也还好，挺好理解，如果你现在不能彻底理解，也不要紧，等以后对 Rust 理解更深了，同时需要深入理解类型转换时，再来细细品读本章。< / p >
< p > 再来看看以下更复杂的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn do_stuff< T: Clone> (value: & T) {
let cloned = value.clone();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面例子中 < code > cloned< / code > 的类型是什么？首先编译器检查能不能进行< strong > 值方法调用< / strong > ， < code > value< / code > 的类型是 < code > & T< / code > ，同时 < code > clone< / code > 方法的签名也是 < code > & T< / code > ： < code > fn clone(& T) -> T< / code > ，因此可以进行值方法调用，再加上编译器知道了 < code > T< / code > 实现了 < code > Clone< / code > ，因此 < code > cloned< / code > 的类型是 < code > T< / code > 。< / p >
< p > 如果 < code > T: Clone< / code > 的特征约束被移除呢？< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn do_stuff< T> (value: & T) {
let cloned = value.clone();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 首先，从直觉上来说，该方法会报错，因为 < code > T< / code > 没有实现 < code > Clone< / code > 特征，但是真实情况是什么呢？< / p >
< p > 我们先来推导一番。 首先通过值方法调用就不再可行，因为 < code > T< / code > 没有实现 < code > Clone< / code > 特征，也就无法调用 < code > T< / code > 的 < code > clone< / code > 方法。接着编译器尝试< strong > 引用方法调用< / strong > ，此时 < code > T< / code > 变成 < code > & T< / code > ，在这种情况下， < code > clone< / code > 方法的签名如下： < code > fn clone(& & T) -> & T< / code > ，接着我们现在对 < code > value< / code > 进行了引用。 编译器发现 < code > & T< / code > 实现了 < code > Clone< / code > 类型(所有的引用类型都可以被复制，因为其实就是复制一份地址)，因此可以推出 < code > cloned< / code > 也是 < code > & T< / code > 类型。< / p >
< p > 最终，我们复制出一份引用指针，这很合理，因为值类型 < code > T< / code > 没有实现 < code > Clone< / code > ，只能去复制一个指针了。< / p >
< p > 下面的例子也是自动引用生效的地方：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[derive(Clone)]
struct Container< T> (Arc< T> );
fn clone_containers< T> (foo: & Container< i32> , bar: & Container< T> ) {
let foo_cloned = foo.clone();
let bar_cloned = bar.clone();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 推断下上面的 < code > foo_cloned< / code > 和 < code > bar_cloned< / code > 是什么类型？提示: 关键在 < code > Container< / code > 的泛型参数，一个是 < code > i32< / code > 的具体类型，一个是泛型类型，其中 < code > i32< / code > 实现了 < code > Clone< / code > ，但是 < code > T< / code > 并没有。< / p >
< p > 首先要复习一下复杂类型派生 < code > Clone< / code > 的规则：一个复杂类型能否派生 < code > Clone< / code > ，需要它内部的所有子类型都能进行 < code > Clone< / code > 。因此 < code > Container< T> (Arc< T> )< / code > 是否实现 < code > Clone< / code > 的关键在于 < code > T< / code > 类型是否实现了 < code > Clone< / code > 特征。< / p >
< p > 上面代码中，< code > Container< i32> < / code > 实现了 < code > Clone< / code > 特征，因此编译器可以直接进行值方法调用，此时相当于直接调用 < code > foo.clone< / code > ，其中 < code > clone< / code > 的函数签名是 < code > fn clone(& T) -> T< / code > ，由此可以看出 < code > foo_cloned< / code > 的类型是 < code > Container< i32> < / code > 。< / p >
< p > 然而，< code > bar_cloned< / code > 的类型却是 < code > & Container< T> < / code > ，这个不合理啊，明明我们为 < code > Container< T> < / code > 派生了 < code > Clone< / code > 特征，因此它也应该是 < code > Container< T> < / code > 类型才对。万事皆有因，我们先来看下 < code > derive< / code > 宏最终生成的代码大概是啥样的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl< T> Clone for Container< T> where T: Clone {
fn clone(& self) -> Self {
Self(Arc::clone(& self.0))
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 从上面代码可以看出，派生 < code > Clone< / code > 能实现的根本是 < code > T< / code > 实现了< a href = "https://doc.rust-lang.org/std/clone/trait.Clone.html#derivable" > < code > Clone< / code > 特征< / a > ： < code > where T: Clone< / code > ， < code > Container< T> < / code > 就没有实现 < code > Clone< / code > 特征。< / p >
< p > 编译器接着会去尝试引用方法调用，此时 < code > & Container< T> < / code > 引用实现了 < code > Clone< / code > ，最终可以得出 < code > bar_cloned< / code > 的类型是 < code > & Container< T> < / code > 。< / p >
< p > 当然，也可以为 < code > Container< T> < / code > 手动实现 < code > Clone< / code > 特征：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl< T> Clone for Container< T> {
fn clone(& self) -> Self {
Self(Arc::clone(& self.0))
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 此时，编译器首次尝试值方法调用即可通过，因此 < code > bar_cloned< / code > 的类型变成 < code > Container< T> < / code > 。< / p >
< p > 这一块儿内容真的挺复杂，每一个坚持看完的读者都是真正的勇士，我也是：为了写好这块儿内容，作者足足花了 < strong > 4< / strong > 个小时！< / p >
< h4 id = "变形记transmutes" > < a class = "header" href = "#变形记transmutes" > 变形记(Transmutes)< / a > < / h4 >
< p > 前方危险，敬请绕行！< / p >
< p > 类型系统，你让开！我要自己转换这些类型，不成功便成仁！虽然本书大多是关于安全的内容，我还是希望你能仔细考虑避免使用本章讲到的内容。这是你在 Rust 中所能做到的真真正正、彻彻底底、最最可怕的非安全行为，在这里，所有的保护机制都形同虚设。< / p >
< p > 先让你看看深渊长什么样，开开眼，然后你再决定是否深入： < code > mem::transmute< T, U> < / code > 将类型 < code > T< / code > 直接转成类型 < code > U< / code > ，唯一的要求就是，这两个类型占用同样大小的字节数！我的天，这也算限制？这简直就是无底线的转换好吧？看看会导致什么问题：< / p >
< ol >
< li > 首先也是最重要的，转换后创建一个任意类型的实例会造成无法想象的混乱，而且根本无法预测。不要把 < code > 3< / code > 转换成 < code > bool< / code > 类型，就算你根本不会去使用该 < code > bool< / code > 类型，也不要去这样转换< / li >
< li > 变形后会有一个重载的返回类型，即使你没有指定返回类型，为了满足类型推导的需求，依然会产生千奇百怪的类型< / li >
< li > 将 < code > & < / code > 变形为 < code > & mut< / code > 是未定义的行为
< ul >
< li > 这种转换永远都是未定义的< / li >
< li > 不，你不能这么做< / li >
< li > 不要多想，你没有那种幸运< / li >
< / ul >
< / li >
< li > 变形为一个未指定生命周期的引用会导致< a href = "https://course.rs/advance/lifetime/advance.html" > 无界生命周期< / a > < / li >
< li > 在复合类型之间互相变换时，你需要保证它们的排列布局是一模一样的！一旦不一样，那么字段就会得到不可预期的值，这也是未定义的行为，至于你会不会因此愤怒， < strong > WHO CARES< / strong > ，你都用了变形了，老兄！< / li >
< / ol >
< p > 对于第 5 条，你该如何知道内存的排列布局是一样的呢？对于 < code > repr(C)< / code > 类型和 < code > repr(transparent)< / code > 类型来说，它们的布局是有着精确定义的。但是对于你自己的“普通却自信“的 Rust 类型 < code > repr(Rust)< / code > 来说，它可不是有着精确定义的。甚至同一个泛型类型的不同实例都可以有不同的内存布局。 < code > Vec< i32> < / code > 和 < code > Vec< u32> < / code > 它们的字段可能有着相同的顺序，也可能没有。对于数据排列布局来说，< strong > 什么能保证，什么不能保证< / strong > 目前还在 Rust 开发组的< a href = "https://rust-lang.github.io/unsafe-code-guidelines/layout.html" > 工作任务< / a > 中呢。< / p >
< p > 你以为你之前凝视的是深渊吗？不，你凝视的只是深渊的大门。 < code > mem::transmute_copy< T, U> < / code > 才是真正的深渊，它比之前的还要更加危险和不安全。它从 < code > T< / code > 类型中拷贝出 < code > U< / code > 类型所需的字节数，然后转换成 < code > U< / code > 。 < code > mem::transmute< / code > 尚有大小检查，能保证两个数据的内存大小一致，现在这哥们干脆连这个也丢了，只不过 < code > U< / code > 的尺寸若是比 < code > T< / code > 大，会是一个未定义行为。< / p >
< p > 当然，你也可以通过裸指针转换和 < code > unions< / code > (todo!)获得所有的这些功能，但是你将无法获得任何编译提示或者检查。裸指针转换和 < code > unions< / code > 也不是魔法，无法逃避上面说的规则。< / p >
< p > < code > transmute< / code > 虽然危险，但作为一本工具书，知识当然要全面，下面列举两个有用的 < code > transmute< / code > 应用场景 :)。< / p >
< ul >
< li > 将裸指针变成函数指针：< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn foo() -> i32 {
0
}
let pointer = foo as *const ();
let function = unsafe {
// 将裸指针转换为函数指针
std::mem::transmute::< *const (), fn() -> i32> (pointer)
};
assert_eq!(function(), 0);
< span class = "boring" > }< / span > < / code > < / pre >
< ul >
< li > 延长生命周期，或者缩短一个静态生命周期寿命：< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct R< 'a> (& 'a i32);
// 将 'b 生命周期延长至 'static 生命周期
unsafe fn extend_lifetime< 'b> (r: R< 'b> ) -> R< 'static> {
std::mem::transmute::< R< 'b> , R< 'static> > (r)
}
// 将 'static 生命周期缩短至 'c 生命周期
unsafe fn shorten_invariant_lifetime< 'b, 'c> (r: & 'b mut R< 'static> ) -> & 'b mut R< 'c> {
std::mem::transmute::< & 'b mut R< 'static> , & 'b mut R< 'c> > (r)
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上例子非常先进！但是是非常不安全的 Rust 行为！< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > Rust By Practice， ， < / p >
< ul >
< li > < a href = "https://practice-zh.course.rs/type-conversions/as.html" > as< / a >
< ul >
< li > < a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/as.md" > 习题解答< / a > < / li >
< / ul >
< / li >
< li > < a href = "https://practice-zh.course.rs/type-conversions/from-into.html" > From/Into< / a >
< ul >
< li > < a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/from-into.md" > 习题解答< / a > < / li >
< / ul >
< / li >
< li > < a href = "https://practice-zh.course.rs/type-conversions/others.html" > 其它转换< / a >
< ul >
< li > < a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/type-conversions/others.md" > 习题解答< / a > < / li >
< / ul >
< / li >
< / ul >
< / blockquote >
< / main >
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