rust - 为什么 rustc 编译会提示我的简单代码 "the trait std::io::Read is not implemented for Result"

Question

对不起，我的菜鸟使用rust 问题。

我正在做一个关于基于 avrow 的 avro 文件操作工具的项目。我看到流行的编译错误发生在标题读取时。我看到 avrow_cli 工作得很好，但我不明白为什么 avrow_cli 处理得很好。有人可以指出什么是错的吗？谢谢。

下面是一段代码和报错信息。

use std::env;
use avrow::Header;

fn main() {
    let args: Vec<String> = env::args().collect();

    println!("Hello, {}!", &args[1]);

    let mut file = std::fs::OpenOptions::new()
        .read(true)
        .open(&args[1]);

    let header = Header::from_reader(&mut file);
    println!("{}", header.schema());
}

   Compiling avro-dump-schema-rs v0.1.0 (/home/shuduo/work/avro-rs)
error[E0277]: the trait bound `Result<File, std::io::Error>: std::io::Read` is not satisfied
  --> src/main.rs:13:38
   |
13  |     let header = Header::from_reader(&mut file);
   |                                      ^^^^^^^^^ the trait `std::io::Read` is not implemented for `Result<File, std::io::Error>`
   |
  ::: /home/shuduo/.cargo/registry/src/github.com-1ecc6299db9ec823/avrow-0.2.1/src/reader.rs:638:27
   |
638 |     pub fn from_reader<R: Read>(reader: &mut R) -> Result<Self, AvrowErr> {
   |                           ---- required by this bound in `Header::from_reader`

error[E0599]: no method named `schema` found for enum `Result<Header, AvrowErr>` in the current scope
 --> src/main.rs:14:27
  |
14 |     println!("{}", header.schema());
  |                           ^^^^^^ method not found in `Result<Header, AvrowErr>`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `avro-dump-schema-rs`

To learn more, run the command again with --verbose.

Answer 1

OpenOptions::open()返回 io::Result<File>而不仅仅是 File .你必须检查 open()在能够使用该文件之前操作已成功。

获取文件的最简单方法是调用 .unwrap()或 .expect()结果，这将导致应用程序在情况下 panic (即崩溃)open()返回错误

 let mut file = std::fs::OpenOptions::new()
        .read(true)
        .open(&args[1])
        .expect(&format!("Failed to open {}", args[1]));

或者你可以尝试优雅地处理错误:

let result = std::fs::OpenOptions::new()
        .read(true)
        .open(&args[1]);

    // you can use `match` to extract the OK/Err cases
    match result{
        Ok(mut file) => {
            let header = Header::from_reader(&mut file);
            println!("{}", header.schema());
        }

        Err(e) => {
            // handle the error
        }
    }

    // or you can use `if let` if you are interested only in one of the enum variants (OK or Err)
    if let Ok(file) = result{
        let header = Header::from_reader(&mut file);
        println!("{}", header.schema());
    }
   

同样适用于您的第二条错误消息 - from_reader()返回 Result你应该处理。