作者:张富春(ahfuzhang)，转载时请注明作者和引用链接，谢谢！

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1.编译期发现的数组越界

在数组下标是常量的情况下，编译期就会发现。

• cargo new arr_test
• 在arr_test/src/main.rs中写入：

fn main() {
    let mut arr : [i64;10] = [0,1,2,3,4,5,6,7,8,9];
    println!("{:?}",arr[10]);
}

• 执行cargo run
• 编译器错误信息为：

error: this operation will panic at runtime
  --> src/main.rs:10:21
   |
3 |     println!("{:?}",arr[10]);
   |                     ^^^^^^^ index out of bounds: the length is 10 but the index is 10
   |
   = note: `#[deny(unconditional_panic)]` on by default

warning: `arr_test` (bin "arr_test") generated 1 warning
error: could not compile `arr_test` due to previous error; 1 warning emitted

2.运行期的数组越界

• main.rs的内容为：

use std::time::{SystemTime, UNIX_EPOCH};

fn main() {
    let mut arr : [i64;10] = [0,1,2,3,4,5,6,7,8,9];

    let start = SystemTime::now();
    let since_the_epoch = start
        .duration_since(UNIX_EPOCH)
        .expect("Time went backwards");
    let ts = since_the_epoch.as_secs() as usize;
    let idx = ts % 10;  //下标的数据类型必须是usize

    println!("{:?}", arr[idx+9]);  //90%的概率会越界
}

• cargo run:

thread 'main' panicked at 'index out of bounds: the len is 10 but the index is 13', src/main.rs:19:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

由此说明: rust加上了运行期的越界检查，以此避免异常的行为。

那么，release版本也会做越界检查吗？

• cargo build --release
• ./arr_test/target/release/arr_test
  发现即便是release版本，也是有越界检查的。

由此证明：rust并非零成本抽象，为了做越界检查，编译器一定会在每次访问数组下标的时候加上两条越界检查指令。

3.跳过越界检查

• 直接加上unsafe并不行：

fn main(){
    //...
    unsafe {
         println!("{:?}", arr[idx+9]);
    }
}

编译器报错：

warning: unnecessary `unsafe` block
  --> src/main.rs:20:5
   |
20 |     unsafe {
   |     ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default

只有用上裸指针，rust才认为是unsafe:

use std::time::{SystemTime, UNIX_EPOCH};

fn main() {
    let mut arr : [i64;10] = [0,1,2,3,4,5,6,7,8,9];

    let start = SystemTime::now();
    let since_the_epoch = start
        .duration_since(UNIX_EPOCH)
        .expect("Time went backwards");
    let ts = since_the_epoch.as_secs() as usize;
    let idx = ts % 10;  //下标的数据类型必须是usize

    unsafe {
        let ref1_at_0 = &mut arr[0];
        let ptr2_at_0 = ref1_at_0 as *mut i64;
        let ptr3_at_1 = ptr2_at_0.add(idx+9);

        println!("{:?}", *ptr3_at_1);
    }
}

上面的代码果然成功越界，没有触发panic，打印了一个随机的整形值。

因此：

• 假设用rust数组写一个什么local cache的服务，则每次下标访问必然导致额外的越界检查的开销；
• 避免这种开销的办法就是在unsafe代码中使用裸指针

have fun

标签：10,这下,unsafe,arr,越界,let,main,rust
来源： https://www.cnblogs.com/ahfuzhang/p/16151798.html