5. The State Machine Reveal 🟢
5. 状态机的真相 🟢

What you’ll learn:
本章将学到什么：

• How the compiler transforms async fn into an enum state machine
  编译器如何把 async fn 变成基于枚举的状态机
• Side-by-side comparison: source code vs generated states
  源代码与生成状态之间的一一对照
• Why large stack allocations in async fn blow up future sizes
  为什么 async fn 里的大栈分配会把 future 的尺寸撑爆
• The drop optimization: values drop as soon as they’re no longer needed
  drop 优化：值一旦不再需要就会立刻被释放

What the Compiler Actually Generates
编译器实际生成了什么

When you write async fn, the compiler transforms your sequential-looking code into an enum-based state machine. Understanding this transformation is the key to understanding async Rust’s performance characteristics and many of its quirks.
当写下 async fn 时，编译器会把看起来像顺序执行的代码改写成一个基于枚举的状态机。想弄懂 async Rust 的性能特征和很多古怪之处，关键就是先把这个转换过程看明白。

Side-by-Side: async fn vs State Machine
并排看：async fn 与状态机

#![allow(unused)]
fn main() {
// What you write:
async fn fetch_two_pages() -> String {
    let page1 = http_get("https://example.com/a").await;
    let page2 = http_get("https://example.com/b").await;
    format!("{page1}\n{page2}")
}
}

The compiler generates something conceptually like this:
编译器在概念上会生成类似下面这样的东西：

#![allow(unused)]
fn main() {
enum FetchTwoPagesStateMachine {
    // State 0: About to call http_get for page1
    Start,

    // State 1: Waiting for page1, holding the future
    WaitingPage1 {
        fut1: HttpGetFuture,
    },

    // State 2: Got page1, waiting for page2
    WaitingPage2 {
        page1: String,
        fut2: HttpGetFuture,
    },

    // Terminal state
    Complete,
}

impl Future for FetchTwoPagesStateMachine {
    type Output = String;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<String> {
        loop {
            match self.as_mut().get_mut() {
                Self::Start => {
                    let fut1 = http_get("https://example.com/a");
                    *self.as_mut().get_mut() = Self::WaitingPage1 { fut1 };
                }
                Self::WaitingPage1 { fut1 } => {
                    let page1 = match Pin::new(fut1).poll(cx) {
                        Poll::Ready(v) => v,
                        Poll::Pending => return Poll::Pending,
                    };
                    let fut2 = http_get("https://example.com/b");
                    *self.as_mut().get_mut() = Self::WaitingPage2 { page1, fut2 };
                }
                Self::WaitingPage2 { page1, fut2 } => {
                    let page2 = match Pin::new(fut2).poll(cx) {
                        Poll::Ready(v) => v,
                        Poll::Pending => return Poll::Pending,
                    };
                    let result = format!("{page1}\n{page2}");
                    *self.as_mut().get_mut() = Self::Complete;
                    return Poll::Ready(result);
                }
                Self::Complete => panic!("polled after completion"),
            }
        }
    }
}
}

Note: This desugaring is conceptual. The real compiler output uses unsafe pin projections — the get_mut() calls shown here require Unpin, but async state machines are !Unpin. The goal is to illustrate state transitions, not produce compilable code.
注意： 这段反语法糖代码只是 概念示意。真正的编译器输出会使用 unsafe 的 pin projection。这里展示的 get_mut() 需要 Unpin，但 async 状态机本身通常是 !Unpin。重点是说明状态如何迁移，不是给出一份可直接编译的实现。

stateDiagram-v2
    [*] --> Start
    Start --> WaitingPage1: Create http_get future #1
    WaitingPage1 --> WaitingPage1: poll() → Pending
    WaitingPage1 --> WaitingPage2: poll() → Ready(page1)
    WaitingPage2 --> WaitingPage2: poll() → Pending
    WaitingPage2 --> Complete: poll() → Ready(page2)
    Complete --> [*]: Return format!("{page1}\\n{page2}")

State contents:
各个状态里保存的内容：

• WaitingPage1 — stores fut1: HttpGetFuture (page2 not yet allocated)
  WaitingPage1：保存 fut1: HttpGetFuture，此时 page2 还没开始分配。
• WaitingPage2 — stores page1: String, fut2: HttpGetFuture (fut1 has been dropped)
  WaitingPage2：保存 page1: String 和 fut2: HttpGetFuture，这时 fut1 已经被释放了。

Why This Matters for Performance
为什么这和性能直接相关

Zero-cost: The state machine is a stack-allocated enum. No heap allocation per future, no garbage collector, no boxing — unless you explicitly use Box::pin().
零成本：这个状态机本质上是一个分配在栈上的枚举。每个 future 默认都不会额外做堆分配，也没有垃圾回收，更不会自动装箱，除非显式使用 Box::pin()。

Size: The enum’s size is the maximum of all its variants. Each .await point creates a new variant. This means:
尺寸：这个枚举的大小取决于所有变体里最大的那个。每出现一个 .await，就会多出一个新状态。因此会出现下面这种情况：

#![allow(unused)]
fn main() {
async fn small() {
    let a: u8 = 0;
    yield_now().await;
    let b: u8 = 0;
    yield_now().await;
}
// Size ≈ max(size_of(u8), size_of(u8)) + discriminant + future sizes
//      ≈ small!

async fn big() {
    let buf: [u8; 1_000_000] = [0; 1_000_000]; // 1MB on the stack!
    some_io().await;
    process(&buf);
}
// Size ≈ 1MB + inner future sizes
// ⚠️ Don't stack-allocate huge buffers in async functions!
// Use Vec<u8> or Box<[u8]> instead.
}

Drop optimization: When a state machine transitions, it drops values no longer needed. In the example above, fut1 is dropped when we transition from WaitingPage1 to WaitingPage2 — the compiler inserts the drop automatically.
Drop 优化：状态机一旦迁移，就会把后续不再需要的值立刻释放掉。上面的例子里，从 WaitingPage1 切到 WaitingPage2 时，fut1 就会被自动 drop，这个释放动作由编译器直接插进去。

Practical rule: Large stack allocations in async fn blow up the future’s size. If you see stack overflows in async code, check for large arrays or deeply nested futures. Use Box::pin() to heap-allocate sub-futures if needed.
实战规则：async fn 里的大栈分配会直接把 future 的体积顶上去。如果在 async 代码里遇到栈溢出，先去查有没有超大数组，或者 future 嵌套得过深。必要时用 Box::pin() 把子 future 放到堆上。

Exercise: Predict the State Machine
练习：预测状态机

#![allow(unused)]
fn main() {
async fn pipeline(url: &str) -> Result<usize, Error> {
    let response = fetch(url).await?;
    let body = response.text().await?;
    let parsed = parse(body).await?;
    Ok(parsed.len())
}
}

Key Takeaways — The State Machine Reveal
本章要点：状态机的真相

• async fn compiles to an enum with one variant per .await point
  async fn 会被编译成一个枚举，每个 .await 都对应一个状态变体。
• The future’s size = max of all variant sizes — large stack values blow it up
  future 的 尺寸 等于所有变体中最大的那个，因此大栈对象会把它直接撑大。
• The compiler inserts drops at state transitions automatically
  状态迁移时需要的 drop 会由编译器自动插入。
• Use Box::pin() or heap allocation when future size becomes a problem
  如果 future 体积成了问题，就用 Box::pin() 或其他堆分配方式拆分它。

See also: Ch 4 — Pin and Unpin for why the generated enum needs pinning, Ch 6 — Building Futures by Hand to build these state machines yourself
继续阅读： 第 4 章：Pin 与 Unpin 会解释为什么生成的枚举需要 pin，第 6 章：手写 Future 会带着亲手把这类状态机写出来。