尝试对预期的“usize”字节进行切片时出错，发现“u32”

Question

我试图切入缓冲区，但出现错误。

let mut buf = Vec::new();
let mut dst = [0u8; 4];
let read_index = 0;

dst.clone_from_slice(&buf[read_index..(read_index+3)]);
let length = u32::from_be_bytes(dst);
    
&buf[(read_index+4)..(read_index+length)]

error[E0308]: mismatched types
  --> src/main.rs:28:79
   |
28 |         let trade = root_as_fb_iq_feed_trade(&buf[(read_index+4)..(read_index+length)]);
   |                                                                               ^^^^^^ expected `usize`, found `u32`

error[E0277]: cannot add `u32` to `usize`
  --> src/main.rs:28:78
   |
28 |         let trade = root_as_fb_iq_feed_trade(&buf[(read_index+4)..(read_index+length)]);
   |                                                                              ^ no implementation for `usize + u32`
   |
   = help: the trait `Add<u32>` is not implemented for `usize`

Answer 1

let mut buf = Vec::new();
let read_index = 0;

dst.clone_from_slice(&buf[read_index..(read_index+3)]);
let length = usize::from_be_bytes(dst); // u32 -> usize
    
&buf[(read_index+4)..(read_index+length)]

usize 支持from_be_bytes 方法：The Rust doc of size::from_be_bytes

来自编译器的错误信息足够清晰，可以让您编写有效的代码。上面我提到的doc上也有说明，函数from_be_bytes只接受参数为[u8; 8]，所以还有一处修改：

let mut buf = Vec::new();
let mut dst = [0u8; 8]; // Expand your buffer to the size of 8
let read_index = 0;

dst.clone_from_slice(&buf[read_index..(read_index+3)]);
let length = usize::from_be_bytes(dst);
    
&buf[(read_index+4)..(read_index+length)]
 

Answer 2

let length = u32::from_be_bytes(dst);
let length_us = usize::try_from(length).unwrap();