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< h1 id = "定海神针-pin-和-unpin" > < a class = "header" href = "#定海神针-pin-和-unpin" > 定海神针 Pin 和 Unpin< / a > < / h1 >
< p > 在 Rust 异步编程中，有一个定海神针般的存在，它就是 < code > Pin< / code > ，作用说简单也简单，说复杂也非常复杂，当初刚出来时就连一些 Rust 大佬都一头雾水，何况瑟瑟发抖的我。好在今非昔比，目前网上的资料已经很全，而我就借花献佛，给大家好好讲讲这个 < code > Pin< / code > 。< / p >
< p > 在 Rust 中，所有的类型可以分为两类:< / p >
< ul >
< li > < strong > 类型的值可以在内存中安全地被移动< / strong > ，例如数值、字符串、布尔值、结构体、枚举，总之你能想到的几乎所有类型都可以落入到此范畴内< / li >
< li > < strong > 自引用类型< / strong > ，大魔王来了，大家快跑，在之前章节我们已经见识过它的厉害< / li >
< / ul >
< p > 下面就是一个自引用类型< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct SelfRef {
value: String,
pointer_to_value: *mut String,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在上面的结构体中，< code > pointer_to_value< / code > 是一个裸指针，指向第一个字段 < code > value< / code > 持有的字符串 < code > String< / code > 。很简单对吧？现在考虑一个情况， 若 < code > value< / code > 被移动了怎么办？< / p >
< p > 此时一个致命的问题就出现了：< code > value< / code > 的内存地址变了，而 < code > pointer_to_value< / code > 依然指向 < code > value< / code > 之前的地址，一个重大 bug 就出现了！< / p >
< p > 灾难发生，英雄在哪？只见 < code > Pin< / code > 闪亮登场，它可以防止一个类型在内存中被移动。再来回忆下之前在 < code > Future< / code > 章节中，我们提到过在 < code > poll< / code > 方法的签名中有一个 < code > self: Pin< & mut Self> < / code > ，那么为何要在这里使用 < code > Pin< / code > 呢？< / p >
< h2 id = "为何需要-pin" > < a class = "header" href = "#为何需要-pin" > 为何需要 Pin< / a > < / h2 >
< p > 其实 < code > Pin< / code > 还有一个小伙伴 < code > UnPin< / code > ，与前者相反，后者表示类型可以在内存中安全地移动。在深入之前，我们先来回忆下 < code > async/.await< / code > 是如何工作的:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let fut_one = /* ... */; // Future 1
let fut_two = /* ... */; // Future 2
async move {
fut_one.await;
fut_two.await;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在底层，< code > async< / code > 会创建一个实现了 < code > Future< / code > 的匿名类型，并提供了一个 < code > poll< / code > 方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // `async { ... }`语句块创建的 `Future` 类型
struct AsyncFuture {
fut_one: FutOne,
fut_two: FutTwo,
state: State,
}
// `async` 语句块可能处于的状态
enum State {
AwaitingFutOne,
AwaitingFutTwo,
Done,
}
impl Future for AsyncFuture {
type Output = ();
fn poll(mut self: Pin< & mut Self> , cx: & mut Context< '_> ) -> Poll< ()> {
loop {
match self.state {
State::AwaitingFutOne => match self.fut_one.poll(..) {
Poll::Ready(()) => self.state = State::AwaitingFutTwo,
Poll::Pending => return Poll::Pending,
}
State::AwaitingFutTwo => match self.fut_two.poll(..) {
Poll::Ready(()) => self.state = State::Done,
Poll::Pending => return Poll::Pending,
}
State::Done => return Poll::Ready(()),
}
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当 < code > poll< / code > 第一次被调用时，它会去查询 < code > fut_one< / code > 的状态，若 < code > fut_one< / code > 无法完成，则 < code > poll< / code > 方法会返回。未来对 < code > poll< / code > 的调用将从上一次调用结束的地方开始。该过程会一直持续，直到 < code > Future< / code > 完成为止。< / p >
< p > 然而，如果我们的 < code > async< / code > 语句块中使用了引用类型，会发生什么？例如下面例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > async {
let mut x = [0; 128];
let read_into_buf_fut = read_into_buf(& mut x);
read_into_buf_fut.await;
println!("{:?}", x);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码会编译成下面的形式：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct ReadIntoBuf< 'a> {
buf: & 'a mut [u8], // 指向下面的`x`字段
}
struct AsyncFuture {
x: [u8; 128],
read_into_buf_fut: ReadIntoBuf< 'what_lifetime?> ,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里，< code > ReadIntoBuf< / code > 拥有一个引用字段，指向了结构体的另一个字段 < code > x< / code > ，一旦 < code > AsyncFuture< / code > 被移动，那 < code > x< / code > 的地址也将随之变化，此时对 < code > x< / code > 的引用就变成了不合法的，也就是 < code > read_into_buf_fut.buf< / code > 会变为不合法的。< / p >
< p > 若能将 < code > Future< / code > 在内存中固定到一个位置，就可以避免这种问题的发生，也就可以安全的创建上面这种引用类型。< / p >
< h2 id = "unpin" > < a class = "header" href = "#unpin" > Unpin< / a > < / h2 >
< p > 事实上，绝大多数类型都不在意是否被移动(开篇提到的第一种类型)，因此它们都< strong > 自动实现< / strong > 了 < code > Unpin< / code > 特征。< / p >
< p > 从名字推测，大家可能以为 < code > Pin< / code > 和 < code > Unpin< / code > 都是特征吧？实际上，< code > Pin< / code > 不按套路出牌，它是一个结构体：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub struct Pin< P> {
pointer: P,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 它包裹一个指针，并且能确保该指针指向的数据不会被移动，例如 < code > Pin< & mut T> < / code > , < code > Pin< & T> < / code > , < code > Pin< Box< T> > < / code > ，都能确保 < code > T< / code > 不会被移动。< / p >
< img alt = "" src = "https://pic1.zhimg.com/80/v2-de79f3a7a401588d671ecd121916cd90_1440w.png" class = "center" / >
< p > 而 < code > Unpin< / code > 才是一个特征，它表明一个类型可以随意被移动，那么问题来了，可以被 < code > Pin< / code > 住的值，它有没有实现什么特征呢？ 答案很出乎意料，可以被 < code > Pin< / code > 住的值实现的特征是 < code > !Unpin< / code > ，大家可能之前没有见过，但是它其实很简单，< code > !< / code > 代表没有实现某个特征的意思，< code > !Unpin< / code > 说明类型没有实现 < code > Unpin< / code > 特征，那自然就可以被 < code > Pin< / code > 了。< / p >
< p > 那是不是意味着类型如果实现了 < code > Unpin< / code > 特征，就不能被 < code > Pin< / code > 了？其实，还是可以 < code > Pin< / code > 的，毕竟它只是一个结构体，你可以随意使用，< strong > 但是不再有任何效果而已，该值一样可以被移动< / strong > ！ < / p >
< p > 例如 < code > Pin< & mut u8> < / code > ，显然 < code > u8< / code > 实现了 < code > Unpin< / code > 特征，它可以在内存中被移动，因此 < code > Pin< & mut u8> < / code > 跟 < code > & mut u8< / code > 实际上并无区别，一样可以被移动。< / p >
< p > 因此，一个类型如果不能被移动，它必须实现 < code > !Unpin< / code > 特征。如果大家对 < code > Pin< / code > 、 < code > Unpin< / code > 还是模模糊糊，建议再重复看一遍之前的内容，理解它们对于我们后面要讲到的内容非常重要！< / p >
< p > 如果将 < code > Unpin< / code > 与之前章节学过的 < a href = "https://course.rs/advance/concurrency-with-threads/send-sync.html" > < code > Send/Sync< / code > < / a > 进行下对比，会发现它们都很像：< / p >
< ul >
< li > 都是标记特征( marker trait )，该特征未定义任何行为，非常适用于标记< / li >
< li > 都可以通过< code > !< / code > 语法去除实现< / li >
< li > 绝大多数情况都是自动实现, 无需我们的操心< / li >
< / ul >
< h2 id = "深入理解-pin" > < a class = "header" href = "#深入理解-pin" > 深入理解 Pin< / a > < / h2 >
< p > 对于上面的问题，我们可以简单的归结为如何在 Rust 中处理自引用类型(果然，只要是难点，都和自引用脱离不了关系)，下面用一个稍微简单点的例子来理解下 < code > Pin< / code > :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[derive(Debug)]
struct Test {
a: String,
b: *const String,
}
impl Test {
fn new(txt: & str) -> Self {
Test {
a: String::from(txt),
b: std::ptr::null(),
}
}
fn init(& mut self) {
let self_ref: *const String = & self.a;
self.b = self_ref;
}
fn a(& self) -> & str {
& self.a
}
fn b(& self) -> & String {
assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
unsafe { & *(self.b) }
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > Test< / code > 提供了方法用于获取字段 < code > a< / code > 和 < code > b< / code > 的值的引用。这里< code > b< / code > 是 < code > a< / code > 的一个引用， ， ： < code > Test< / code > 就是一个自引用结构体。< / p >
< p > 如果不移动任何值，那么上面的例子将没有任何问题，例如:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mut test1 = Test::new("test1");
test1.init();
let mut test2 = Test::new("test2");
test2.init();
println!("a: {}, b: {}", test1.a(), test1.b());
println!("a: {}, b: {}", test2.a(), test2.b());
}< / code > < / pre >
< p > 输出非常正常：< / p >
< pre > < code class = "language-console" > a: test1, b: test1
a: test2, b: test2
< / code > < / pre >
< p > 明知山有虎，偏向虎山行，这才是我辈年轻人的风华。既然移动数据会导致指针不合法，那我们就移动下数据试试，将 < code > test1< / code > 和 < code > test2< / code > 进行下交换：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mut test1 = Test::new("test1");
test1.init();
let mut test2 = Test::new("test2");
test2.init();
println!("a: {}, b: {}", test1.a(), test1.b());
std::mem::swap(& mut test1, & mut test2);
println!("a: {}, b: {}", test2.a(), test2.b());
}< / code > < / pre >
< p > 按理来说，这样修改后，输出应该如下:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > a: test1, b: test1
a: test1, b: test1
< span class = "boring" > }< / span > < / code > < / pre >
< p > 但是实际运行后，却产生了下面的输出:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > a: test1, b: test1
a: test1, b: test2
< span class = "boring" > }< / span > < / code > < / pre >
< p > 原因是 < code > test2.b< / code > 指针依然指向了旧的地址，而该地址对应的值现在在 < code > test1< / code > 里，最终会打印出意料之外的值。< / p >
< p > 如果大家还是将信将疑，那再看看下面的代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mut test1 = Test::new("test1");
test1.init();
let mut test2 = Test::new("test2");
test2.init();
println!("a: {}, b: {}", test1.a(), test1.b());
std::mem::swap(& mut test1, & mut test2);
test1.a = "I've totally changed now!".to_string();
println!("a: {}, b: {}", test2.a(), test2.b());
}< / code > < / pre >
< p > 下面的图片也可以帮助更好的理解这个过程：< / p >
< img alt = "" src = "https://pica.zhimg.com/80/v2-eaeb33da283dc1063b862d2307821976_1440w.jpg" class = "center" / >
< h2 id = "pin-在实践中的运用" > < a class = "header" href = "#pin-在实践中的运用" > Pin 在实践中的运用< / a > < / h2 >
< p > 在理解了 < code > Pin< / code > 的作用后，我们再来看看它怎么帮我们解决问题。< / p >
< h4 id = "将值固定到栈上" > < a class = "header" href = "#将值固定到栈上" > 将值固定到栈上< / a > < / h4 >
< p > 回到之前的例子，我们可以用 < code > Pin< / code > 来解决指针指向的数据被移动的问题:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::pin::Pin;
use std::marker::PhantomPinned;
#[derive(Debug)]
struct Test {
a: String,
b: *const String,
_marker: PhantomPinned,
}
impl Test {
fn new(txt: & str) -> Self {
Test {
a: String::from(txt),
b: std::ptr::null(),
_marker: PhantomPinned, // 这个标记可以让我们的类型自动实现特征`!Unpin`
}
}
fn init(self: Pin< & mut Self> ) {
let self_ptr: *const String = & self.a;
let this = unsafe { self.get_unchecked_mut() };
this.b = self_ptr;
}
fn a(self: Pin< & Self> ) -> & str {
& self.get_ref().a
}
fn b(self: Pin< & Self> ) -> & String {
assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
unsafe { & *(self.b) }
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面代码中，我们使用了一个标记类型 < code > PhantomPinned< / code > 将自定义结构体 < code > Test< / code > 变成了 < code > !Unpin< / code > (编译器会自动帮我们实现)，因此该结构体无法再被移动。< / p >
< p > 一旦类型实现了 < code > !Unpin< / code > ，那将它的值固定到栈( < code > stack< / code > )上就是不安全的行为，因此在代码中我们使用了 < code > unsafe< / code > 语句块来进行处理，你也可以使用 < a href = "https://docs.rs/pin-utils/" > < code > pin_utils< / code > < / a > 来避免 < code > unsafe< / code > 的使用。< / p >
< blockquote >
< p > BTW, Rust 中的 unsafe 其实没有那么可怕，虽然听上去很不安全，但是实际上 Rust 依然提供了很多机制来帮我们提升了安全性，因此不必像对待 Go 语言的 < code > unsafe< / code > 那样去畏惧于使用 Rust 中的 < code > unsafe< / code > ，大致使用原则总结如下：没必要用时，就不要用，当有必要用时，就大胆用，但是尽量控制好边界，让 < code > unsafe< / code > 的范围尽可能小< / p >
< / blockquote >
< p > 此时，再去尝试移动被固定的值，就会导致< strong > 编译错误< / strong > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > pub fn main() {
// 此时的`test1`可以被安全的移动
let mut test1 = Test::new("test1");
// 新的`test1`由于使用了`Pin`，因此无法再被移动，这里的声明会将之前的`test1`遮蔽掉(shadow)
let mut test1 = unsafe { Pin::new_unchecked(& mut test1) };
Test::init(test1.as_mut());
let mut test2 = Test::new("test2");
let mut test2 = unsafe { Pin::new_unchecked(& mut test2) };
Test::init(test2.as_mut());
println!("a: {}, b: {}", Test::a(test1.as_ref()), Test::b(test1.as_ref()));
std::mem::swap(test1.get_mut(), test2.get_mut());
println!("a: {}, b: {}", Test::a(test2.as_ref()), Test::b(test2.as_ref()));
}< / code > < / pre >
< p > 注意到之前的粗体字了吗？ ， < / p >
< pre > < code class = "language-shell" > error[E0277]: `PhantomPinned` cannot be unpinned
--> src/main.rs:47:43
|
47 | std::mem::swap(test1.get_mut(), test2.get_mut());
| ^^^^^^^ within `Test`, the trait `Unpin` is not implemented for `PhantomPinned`
< / code > < / pre >
< blockquote >
< p > 需要注意的是固定在栈上非常依赖于你写出的 < code > unsafe< / code > 代码的正确性。我们知道 < code > & 'a mut T< / code > 可以固定的生命周期是 < code > 'a< / code > ，但是我们却不知道当生命周期 < code > 'a< / code > 结束后，该指针指向的数据是否会被移走。如果你的 < code > unsafe< / code > 代码里这么实现了，那么就会违背 < code > Pin< / code > 应该具有的作用！< / p >
< p > 一个常见的错误就是忘记去< a href = "https://course.rs/basic/variable.html#%E5%8F%98%E9%87%8F%E9%81%AE%E8%94%BDshadowing" > 遮蔽( shadow )< / a > 初始的变量，因为你可以 < code > drop< / code > 掉 < code > Pin< / code > ，然后在 < code > & 'a mut T< / code > 结束后去移动数据:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mut test1 = Test::new("test1");
let mut test1_pin = unsafe { Pin::new_unchecked(& mut test1) };
Test::init(test1_pin.as_mut());
drop(test1_pin);
println!(r#"test1.b points to "test1": {:?}..."#, test1.b);
let mut test2 = Test::new("test2");
mem::swap(& mut test1, & mut test2);
println!("... and now it points nowhere: {:?}", test1.b);
}
< span class = "boring" > use std::pin::Pin;
< / span > < span class = "boring" > use std::marker::PhantomPinned;
< / span > < span class = "boring" > use std::mem;
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(Debug)]
< / span > < span class = "boring" > struct Test {
< / span > < span class = "boring" > a: String,
< / span > < span class = "boring" > b: *const String,
< / span > < span class = "boring" > _marker: PhantomPinned,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl Test {
< / span > < span class = "boring" > fn new(txt: & str) -> Self {
< / span > < span class = "boring" > Test {
< / span > < span class = "boring" > a: String::from(txt),
< / span > < span class = "boring" > b: std::ptr::null(),
< / span > < span class = "boring" > // This makes our type `!Unpin`
< / span > < span class = "boring" > _marker: PhantomPinned,
< / span > < span class = "boring" > }
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > fn init< 'a> (self: Pin< & 'a mut Self> ) {
< / span > < span class = "boring" > let self_ptr: *const String = & self.a;
< / span > < span class = "boring" > let this = unsafe { self.get_unchecked_mut() };
< / span > < span class = "boring" > this.b = self_ptr;
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > fn a< 'a> (self: Pin< & 'a Self> ) -> & 'a str {
< / span > < span class = "boring" > & self.get_ref().a
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > fn b< 'a> (self: Pin< & 'a Self> ) -> & 'a String {
< / span > < span class = "boring" > assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
< / span > < span class = "boring" > unsafe { & *(self.b) }
< / span > < span class = "boring" > }
< / span > < span class = "boring" > }< / span > < / code > < / pre >
< / blockquote >
< h4 id = "固定到堆上" > < a class = "header" href = "#固定到堆上" > 固定到堆上< / a > < / h4 >
< p > 将一个 < code > !Unpin< / code > 类型的值固定到堆上，会给予该值一个稳定的内存地址，它指向的堆中的值在 < code > Pin< / code > 后是无法被移动的。而且与固定在栈上不同，我们知道堆上的值在整个生命周期内都会被稳稳地固定住。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::pin::Pin;
use std::marker::PhantomPinned;
#[derive(Debug)]
struct Test {
a: String,
b: *const String,
_marker: PhantomPinned,
}
impl Test {
fn new(txt: & str) -> Pin< Box< Self> > {
let t = Test {
a: String::from(txt),
b: std::ptr::null(),
_marker: PhantomPinned,
};
let mut boxed = Box::pin(t);
let self_ptr: *const String = & boxed.as_ref().a;
unsafe { boxed.as_mut().get_unchecked_mut().b = self_ptr };
boxed
}
fn a(self: Pin< & Self> ) -> & str {
& self.get_ref().a
}
fn b(self: Pin< & Self> ) -> & String {
unsafe { & *(self.b) }
}
}
pub fn main() {
let test1 = Test::new("test1");
let test2 = Test::new("test2");
println!("a: {}, b: {}",test1.as_ref().a(), test1.as_ref().b());
println!("a: {}, b: {}",test2.as_ref().a(), test2.as_ref().b());
}< / code > < / pre >
< h4 id = "将固定住的-future-变为-unpin" > < a class = "header" href = "#将固定住的-future-变为-unpin" > 将固定住的 < code > Future< / code > 变为 < code > Unpin< / code > < / a > < / h4 >
< p > 之前的章节我们有提到 < code > async< / code > 函数返回的 < code > Future< / code > 默认就是 < code > !Unpin< / code > 的。< / p >
< p > 但是，在实际应用中，一些函数会要求它们处理的 < code > Future< / code > 是 < code > Unpin< / code > 的，此时，若你使用的 < code > Future< / code > 是 < code > !Unpin< / code > 的，必须要使用以下的方法先将 < code > Future< / code > 进行固定:< / p >
< ul >
< li > < code > Box::pin< / code > ， < code > Pin< Box< T> > < / code > < / li >
< li > < code > pin_utils::pin_mut!< / code > ， < code > Pin< & mut T> < / code > < / li >
< / ul >
< p > 固定后获得的 < code > Pin< Box< T> > < / code > 和 < code > Pin< & mut T> < / code > 既可以用于 < code > Future< / code > ， < strong > 又会自动实现 < code > Unpin< / code > < / strong > 。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use pin_utils::pin_mut; // `pin_utils` 可以在crates.io中找到
// 函数的参数是一个`Future`，但是要求该`Future`实现`Unpin`
fn execute_unpin_future(x: impl Future< Output = ()> + Unpin) { /* ... */ }
let fut = async { /* ... */ };
// 下面代码报错: 默认情况下，`fut` 实现的是`!Unpin`，并没有实现`Unpin`
// execute_unpin_future(fut);
// 使用`Box`进行固定
let fut = async { /* ... */ };
let fut = Box::pin(fut);
execute_unpin_future(fut); // OK
// 使用`pin_mut!`进行固定
let fut = async { /* ... */ };
pin_mut!(fut);
execute_unpin_future(fut); // OK
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 相信大家看到这里，脑袋里已经快被 < code > Pin< / code > 、 < code > Unpin< / code > 、 < code > !Unpin< / code > 整爆炸了，没事，我们再来火上浇油下:)< / p >
< ul >
< li > 若 < code > T: Unpin< / code > ( Rust 类型的默认实现)，那么 < code > Pin< 'a, T> < / code > 跟 < code > & 'a mut T< / code > 完全相同，也就是 < code > Pin< / code > 将没有任何效果, 该移动还是照常移动< / li >
< li > 绝大多数标准库类型都实现了 < code > Unpin< / code > ，事实上，对于 Rust 中你能遇到的绝大多数类型，该结论依然成立
，其中一个例外就是：< code > async/await< / code > 生成的 < code > Future< / code > 没有实现 < code > Unpin< / code > < / li >
< li > 你可以通过以下方法为自己的类型添加 < code > !Unpin< / code > 约束：
< ul >
< li > 使用文中提到的 < code > std::marker::PhantomPinned< / code > < / li >
< li > 使用< code > nightly< / code > 版本下的 < code > feature flag< / code > < / li >
< / ul >
< / li >
< li > 可以将值固定到栈上，也可以固定到堆上
< ul >
< li > 将 < code > !Unpin< / code > 值固定到栈上需要使用 < code > unsafe< / code > < / li >
< li > 将 < code > !Unpin< / code > 值固定到堆上无需 < code > unsafe< / code > ，可以通过 < code > Box::pin< / code > 来简单的实现< / li >
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