CODEWITHSUNDEEP
genericsrusttraits
Tim McNamara
Tim McNamara

Reputation: 18385

Unable to add trait bounds to member of a struct

I'm attempting to add use std::io::Cursor with a generic type R, but retain the Read type bounds so that Read trait is accessible, and subsequently can support bytes() method.

Here is my struct definition so far:

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

Assuming I have a variable parser that's an instance of Parse, I want to be able to call parser.source.bytes(). bytes() is a method provided by Read. Despite the annotation around R, the compiler is telling me that R isn't satisfying the std::io::Read trait bounds.

Here is that snippet of code in context along with a 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() {
    //...
}

Generates the following error message:

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`

Help appreciated!

Upvotes: 3

Views: 453

Answers (1)

Peter Hall
Peter Hall

Reputation: 58735

Check the notes after the error messages:

= 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`

The Read implementation for Cursor<T> is only defined when T: AsRef<[u8]>. If you add that constraint then the Read implementation will be available:

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

You will still have a few other errors in that code. But this should answer your surface question.

Upvotes: 7

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