如何创建使用指定关联类型的默认函数？

Question

pub trait Builder<W: Write> {
    type Error: Error;

    fn build<'re>(
        writer: W,
        records: impl Iterator<Item = &'re Record>,
    ) -> Result<(), Self::Error> {
        let mut state = Self::create(writer)?;
        for record in records {
            state.next(record)?;
        }

        state.finalize()
    }

    fn build_string<'re>(
        records: impl Iterator<Item = &'re Record>,
    ) -> Result<String, <Self as Builder<&'??? mut Vec<u8>>>::Error> { // what type should I use?
        let mut buffer = Vec::new();
        Self::<???>::build(&mut buffer, records)?; // same question
        Ok(String::from_utf8_lossy(buffer.as_slice()).to_string())
    }

    fn create(writer: W) -> Result<Self, Self::Error>;
    fn next(&mut self, record: &Record) -> Result<(), Self::Error>;
    fn finalize(&mut self) -> Result<(), Self::Error>;
}

据我所知，不可能实现我在这里尝试做的事情。如果那是正确的，那么实现默认

build_string

方法的最佳方法是什么，以便我可以在实现

Builder

的类型上访问它？

Answer 1

解决方案：

pub trait Builder<W: Write>: Sized {
    type Error: std::error::Error;

    fn build<'re>(writer: W, records: impl Iterator<Item = &'re Record>) -> Result<W, Self::Error> {
        let mut state = Self::create(writer)?;
        for record in records {
            state.next(record)?;
        }

        state.finalize()
    }

    fn create(writer: W) -> Result<Self, Self::Error>;
    fn next(&mut self, record: &Record) -> Result<(), Self::Error>;
    fn finalize(self) -> Result<W, Self::Error>;
}

pub trait BuilderString: Builder<Vec<u8>> {
    fn build_string<'re>(records: impl Iterator<Item = &'re Record>) -> Result<String, Self::Error>;
}

impl<T> BuilderString for T
where
    T: Builder<Vec<u8>>,
{
    fn build_string<'re>(records: impl Iterator<Item = &'re Record>) -> Result<String, Self::Error> {
        let buffer = Vec::new();
        let buffer = Self::build(buffer, records)?;
        Ok(String::from_utf8_lossy(buffer.as_slice()).to_string())
    }
}

感谢 Seaish on Rust 编程语言 Discord 服务器。