generics - 无法将特征边界添加到结构成员

Question

我正在尝试添加使用 std::io::Cursor 和通用类型 R，但保留 Read 类型边界使得 Read trait 是可访问的，随后可以支持 bytes() 方法。

到目前为止，这是我的结构定义:

struct Parse<'parse, R: Read + BufRead + 'parse> {
    tokens: Vec<Token>,
    source: Cursor<&'parse mut R>,
}

假设我有一个变量 parser，它是 Parse 的一个实例，我希望能够调用 parser.source.bytes() . bytes()是Read提供的方法。尽管 R 周围有注释，但编译器告诉我 R 不满足 std::io::Read 特征边界。

这是上下文中的代码片段以及 direct link to the playground :

// using Cursor because it tracks position internally
use std::io::{Cursor, Read, BufRead};

struct Token {
    start: usize,
    end: usize,
}

struct Parse<'parse, R: Read + BufRead + 'parse> {
    tokens: Vec<Token>,
    source: Cursor<&'parse mut R>,
}

impl<'parse, R: Read + BufRead + 'parse> Parse <'parse, R> {
    fn new(source: &'parse mut R) -> Self {
        Parse {
            tokens: vec!(),
            source: Cursor::new(source),
        }
    }

    fn parse_primitive(&mut self) -> std::io::Result<Token> {
        let start = self.source.position();
        let bytes = self.source.bytes();                     // <- error raised here

        // dummy work
        for _ in 0..3 {
            let byte = bytes.next().unwrap().unwrap()
        }
        let end = self.source.position();
        Ok(Token { start: start as usize, end: end as usize})
    }
}

fn main() {
    //...
}

生成以下错误消息:

error[E0599]: no method named `bytes` found for type `std::io::Cursor<&'parse mut R>` in the current scope
  --> src/main.rs:24:33
   |
24 |         let bytes = self.source.bytes();
   |                                 ^^^^^
   |
   = note: the method `bytes` exists but the following trait bounds were not satisfied:
           `std::io::Cursor<&mut R> : std::io::Read`
           `&mut std::io::Cursor<&'parse mut R> : std::io::Read`

感谢帮助!

Answer 1

检查错误信息后的注释:

= note: the method `bytes` exists but the following trait bounds were not satisfied:
        `std::io::Cursor<&mut R> : std::io::Read`
        `&mut std::io::Cursor<&'parse mut R> : std::io::Read`

Read implementation for Cursor<T> 仅在 T: AsRef<[u8]> 时定义.如果添加该约束，则 Read实现将可用:

impl<'parse, R: AsRef<[u8]> + BufRead + 'parse> Parse <'parse, R> {

您在该代码中仍然会有一些其他错误。但这应该可以回答您的表面问题。