无法将特征范围添加到结构成员

时间:2018-07-10 10:46:06

标签: generics rust traits

我正尝试添加具有通用类型std::io::Cursor的使用R,但保留Read类型限制,以便可以访问Read特性并随后可以支持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的实例,那么我希望能够调用parser.source.bytes()bytes()Read提供的一种方法。尽管R周围有注解,但编译器告诉我R不满足std::io::Read特质范围。

以下是上下文中的代码段以及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`

帮助表示感谢!

1 个答案:

答案 0 :(得分:7)

检查错误消息后的注释:

= 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> {

在该代码中您仍然会有一些其他错误。但这应该可以回答您的表面问题。