8.7.2 Rust 建模 on 编程那些事儿

8.7.2.1 使用原始指针

Tue, 11 Aug 2026 11:30:00 +0800

译文 · 基于 Comprehensive Rust

原文链接: https://google.github.io/comprehensive-rust/unsafe-deep-dive/pinning/self-referential-buffer/rust-raw-pointers.html

8.7.2.1 使用原始指针

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// Copyright 2026 Google LLC
// SPDX-License-Identifier: Apache-2.0
#[derive(Debug)]
pub struct SelfReferentialBuffer {
 data: [u8; 1024],
 cursor: *mut u8,
}

impl SelfReferentialBuffer {
 pub fn new() -> Self {
 let mut buffer =
 SelfReferentialBuffer { data: [0; 1024], cursor: std::ptr::null_mut() };

 buffer.update_cursor();
 buffer
 }

 // 危险：每次移动后都必须调用
 pub fn update_cursor(&mut self) {
 self.cursor = self.data.as_mut_ptr();
 }

 pub fn read(&self, n_bytes: usize) -> &[u8] {
 unsafe {
 let start = self.data.as_ptr();
 let end = start.add(1024);
 let cursor = self.cursor as *const u8;

 assert!((start..=end).contains(&cursor), "cursor is out of bounds");

 let available = end.offset_from(cursor) as usize;
 let len = n_bytes.min(available);
 std::slice::from_raw_parts(cursor, len)
 }
 }

 pub fn write(&mut self, bytes: &[u8]) {
 unsafe {
 let start = self.data.as_mut_ptr();
 let end = start.add(1024);

 assert!((start..=end).contains(&self.cursor), "cursor is out of bounds");
 let available = end.offset_from(self.cursor) as usize;
 let len = bytes.len().min(available);

 std::ptr::copy_nonoverlapping(bytes.as_ptr(), self.cursor, len);
 self.cursor = self.cursor.add(len);
 }
 }
}

不要在此停留太久。

8.7.2.2 使用整数偏移

Tue, 11 Aug 2026 11:30:00 +0800

译文 · 基于 Comprehensive Rust

原文链接: https://google.github.io/comprehensive-rust/unsafe-deep-dive/pinning/self-referential-buffer/rust-offset.html

8.7.2.2 使用整数偏移

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// Copyright 2026 Google LLC
// SPDX-License-Identifier: Apache-2.0
#[derive(Debug)]
pub struct SelfReferentialBuffer {
 data: [u8; 1024],
 position: usize,
}

impl SelfReferentialBuffer {
 pub fn new() -> Self {
 SelfReferentialBuffer { data: [0; 1024], position: 0 }
 }

 pub fn read(&self, n_bytes: usize) -> &[u8] {
 let available = self.data.len().saturating_sub(self.position);
 let len = n_bytes.min(available);
 &self.data[self.position..self.position + len]
 }

 pub fn write(&mut self, bytes: &[u8]) {
 let available = self.data.len().saturating_sub(self.position);
 let len = bytes.len().min(available);
 self.data[self.position..self.position + len].copy_from_slice(&bytes[..len]);
 self.position += len;
 }
}

在 Rust 中，更惯用的做法是使用偏移变量，并在需要时创建引用。

8.7.2.3 使用 `Pin`

Tue, 11 Aug 2026 11:30:00 +0800

译文 · 基于 Comprehensive Rust

原文链接: https://google.github.io/comprehensive-rust/unsafe-deep-dive/pinning/self-referential-buffer/rust-pin.html

8.7.2.3 使用 `Pin<Ptr>`

Pinning 允许 Rust 程序员创建与 C++ 类更为相似的类型。

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// Copyright 2026 Google LLC
// SPDX-License-Identifier: Apache-2.0
use std::marker::PhantomPinned;
use std::pin::Pin;

/// 不可移动的自引用缓冲区。
#[derive(Debug)]
pub struct SelfReferentialBuffer {
 data: [u8; 1024],
 cursor: *mut u8,
 _pin: PhantomPinned,
}

impl SelfReferentialBuffer {
 pub fn new() -> Pin<Box<Self>> {
 let buffer = SelfReferentialBuffer {
 data: [0; 1024],
 cursor: std::ptr::null_mut(),
 _pin: PhantomPinned,
 };
 let mut pinned = Box::pin(buffer);

 unsafe {
 let mut_ref = Pin::get_unchecked_mut(pinned.as_mut());
 mut_ref.cursor = mut_ref.data.as_mut_ptr();
 }

 pinned
 }

 pub fn read(&self, n_bytes: usize) -> &[u8] {
 unsafe {
 let start = self.data.as_ptr();
 let end = start.add(self.data.len());
 let cursor = self.cursor as *const u8;

 assert!((start..=end).contains(&cursor), "cursor is out of bounds");

 let offset = cursor.offset_from(start) as usize;
 let available = self.data.len().saturating_sub(offset);
 let len = n_bytes.min(available);

 &self.data[offset..offset + len]
 }
 }

 pub fn write(mut self: Pin<&mut Self>, bytes: &[u8]) {
 let this = unsafe { self.as_mut().get_unchecked_mut() };
 unsafe {
 let start = this.data.as_mut_ptr();
 let end = start.add(1024);

 assert!((start..=end).contains(&this.cursor), "cursor is out of bounds");
 let available = end.offset_from(this.cursor) as usize;
 let len = bytes.len().min(available);

 std::ptr::copy_nonoverlapping(bytes.as_ptr(), this.cursor, len);
 this.cursor = this.cursor.add(len);
 }
 }
}

注意函数签名现已改变。例如 ::new() 返回 Pin<Box<Self>> 而非 Self。这会产生堆分配，因为 Pin<Ptr> 必须与 Box 等指针类型配合使用。

8.7.2 Rust 建模 on 编程那些事儿

8.7.2.1 使用原始指针

8.7.2.1 使用原始指针

8.7.2.2 使用整数偏移

8.7.2.2 使用整数偏移

8.7.2.3 使用 `Pin`

8.7.2.3 使用 Pin<Ptr>

8.7.2.3 使用 `Pin<Ptr>`