Rust anyhow 简明示例教程

#[cfg_attr(not(doc), repr(transparent))]
pub struct Error {
    inner: Own<ErrorImpl>,
}

#[repr(C)]
pub(crate) struct ErrorImpl<E = ()> {
    vtable: &'static ErrorVTable,
    backtrace: Option<Backtrace>,
    // NOTE: Don't use directly. Use only through vtable. Erased type may have
    // different alignment.
    _object: E,
}

impl<E> StdError for ErrorImpl<E>
where
    E: StdError,
{
    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        unsafe { ErrorImpl::error(self.erase()).source() }
    }
    #[cfg(error_generic_member_access)]
    fn provide<'a>(&'a self, request: &mut Request<'a>) {
        unsafe { ErrorImpl::provide(self.erase(), request) }
    }
}

pub type Result<T, E = Error> = core::result::Result<T, E>;

use anyhow::{Result, Context};
use std::{fs, io};
// 1. 读取文件、解析数据和执行数据操作都可能出现错误，
// 所以我们需要返回 Result 来兼容异常情况。
// 这里我们使用 anyhow::Result 来简化和传播错误。
fn read_and_process_file(file_path: &str) -> Result<()> {
    // 尝试读取文件
    let data = fs::read_to_string(file_path)
        // 2. 使用 with_context 来附加错误信息，然后利用 ? 语法糖传播错误。
        .with_context(||format!("failed to read file `{}`", file_path))?;
    // 解析数据
    let processed_data = parse_data(&data)
        .with_context(||format!("failed to parse data from file `{}`", file_path))?;
    // 执行数据操作
    perform_some_operation(processed_data)
        .with_context(|| "failed to perform operation based on file data")?;
    Ok(())
}
fn parse_data(data: &str) -> Result<String> {
    Ok(data.to_uppercase())
}
fn perform_some_operation(data: String) -> Result<()> {
    println!("processed data: {}", data);
    Ok(())
}
fn main() {
    let file_path = "./anyhow.txt";
  	// 执行处理逻辑
    let res =  read_and_process_file(file_path);
  	// 处理结果
    match res {
        Ok(_) => println!("successfully!"),
        Err(e) => {
            // 3. 使用 downcast 来反解出实际的错误实例，本案例中可能出现的异常是 io::Error。
            if let Some(my_error) = e.downcast_ref::<io::Error>() {
                println!("has io error: {:#}", my_error);
            } else {
                println!("unknown error: {:?}", e);
            }
        }
    }
}