从brotly-sys迁移至Dropbox Brotli实现Coder trait遇类型错误

问题背景 #

原使用已停止维护的Brotly-sys库，转用Dropbox的Brotli库后，为压缩/解压实现Coder接口时出现编译类型不匹配错误，涉及DecompressorWriter和CompressorWriter的finish方法实现。

原实现代码 #

use std::io::{self, Write};

pub use brotli::writer::DecompressorWriter;
pub use brotli::enc::writer::CompressorWriter;

use super::Coder;

impl<W: Write> Coder<W> for DecompressorWriter<W> {
    fn get_mut(&mut self) -> &mut W {
        DecompressorWriter::get_mut(self)
    }

    fn finish(self) -> std::io::Result<W> {
        DecompressorWriter::flush(&mut self).map_err(|_| {
            io::Error::new(io::ErrorKind::Other, "brotli decoder failed to finalize stream")
        });
    }

    fn finish_boxed(self: Box<Self>) -> io::Result<W> {
        self.finish()
    }
}

impl<W: Write> Coder<W> for CompressorWriter<W> {
    fn get_mut(&mut self) -> &mut W {
        CompressorWriter::get_mut(self)
    }

    fn finish(self) -> std::io::Result<W> {
        self.flush()?;
        CompressorWriter::flush(&mut self).map_err(|_| {
            io::Error::new(io::ErrorKind::Other, "brotli encoder failed to finalize stream")
        })
    }

    fn finish_boxed(self: Box<Self>) -> io::Result<W> {
        self.finish()
    }
}

编译错误 #

error[E0308]: mismatched types
  --> lib/src/codecs/brotli.rs:13:24
   |
13 |     fn finish(self) -> std::io::Result<W> {
   |        ------          ^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found `()`
   |        |
   |        implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::result::Result<W, std::io::Error>`
           found unit type `()`

error[E0308]: mismatched types
  --> lib/src/codecs/brotli.rs:31:9
   |
24 |   impl<W: Write> Coder<W> for CompressorWriter<W> {
   |        - this type parameter
...
29 |       fn finish(self) -> std::io::Result<W> {
   |                          ------------------ expected `std::result::Result<W, std::io::Error>` because of return type
30 |           self.flush()?;
31 | /         CompressorWriter::flush(&mut self).map_err(|_| {
32 | |             io::Error::new(io::ErrorKind::Other, "brotli encoder failed to finalize stream")
33 | |         })
   | |__________^ expected type parameter `W`, found `()`
   |
   = note: expected enum `std::result::Result<W, _>`
              found enum `std::result::Result<(), _>`

补充：flush仅返回Result<()>，无法提供底层W实例；Coder接口要求finish返回std::io::Result<W>。

修复方案 #

错误根源 #

1. DecompressorWriter的finish：代码末尾多了分号，导致返回()而非Result；且未提取底层的W实例。
2. CompressorWriter的finish：仅调用flush得到Result<()>，未获取底层W，不符合返回类型要求。

修改后的代码 #

use std::io::{self, Write};

pub use brotli::writer::DecompressorWriter;
pub use brotli::enc::writer::CompressorWriter;

use super::Coder;

impl<W: Write> Coder<W> for DecompressorWriter<W> {
    fn get_mut(&mut self) -> &mut W {
        DecompressorWriter::get_mut(self)
    }

    fn finish(mut self) -> std::io::Result<W> {
        // 完成flush并处理错误
        DecompressorWriter::flush(&mut self).map_err(|_| {
            io::Error::new(io::ErrorKind::Other, "brotli decoder failed to finalize stream")
        })?;
        // 提取底层writer并返回
        Ok(self.into_inner())
    }

    fn finish_boxed(self: Box<Self>) -> io::Result<W> {
        self.finish()
    }
}

impl<W: Write> Coder<W> for CompressorWriter<W> {
    fn get_mut(&mut self) -> &mut W {
        CompressorWriter::get_mut(self)
    }

    fn finish(mut self) -> std::io::Result<W> {
        // 执行flush操作
        self.flush()?;
        CompressorWriter::flush(&mut self).map_err(|_| {
            io::Error::new(io::ErrorKind::Other, "brotli encoder failed to finalize stream")
        })?;
        // 提取底层writer并返回
        Ok(self.into_inner())
    }

    fn finish_boxed(self: Box<Self>) -> io::Result<W> {
        self.finish()
    }
}

关键修改点 #

• 移除DecompressorWriter::flush后的分号，改用?传播错误，确保错误正确向上传递。
• 使用into_inner方法从Brotli的writer中提取底层W实例，包装为Ok(W)返回，符合Coder接口的返回类型要求。
• 对CompressorWriter先完成所有flush操作，确保压缩流完全写入底层writer后，再提取并返回W。

内容的提问来源于stack exchange，提问作者Vana