impl-enum

包含proc macro属性with_methods和as_dyn，使枚举使用类似于特质的对象更加方便。

使用案例

当某些值的具体类型依赖于运行时条件时，特质对象或为每个具体类型提供变体的枚举是最自然的选择，每种选择都有其优缺点。使用枚举的一个缺点是使用起来可能不方便。如果你想要使用不能转换为特质对象的类型，或者在与其他crate中定义的类型和特质一起工作时，特质对象方法也可能存在问题。这个crate旨在使这样的枚举更容易使用。

with_methods

//! The variant of the writer is dynamically selected with an environment variable.
//! Using the macro, we can use the enum with the convenience of a trait object.

use std::{
    env,
    fs::File,
    io::{Cursor, Write},
};

#[impl_enum::with_methods {
    fn write_all(&mut self, buf: &[u8]) -> std::io::Result<()>
    pub fn write(&mut self, buf: &[u8]) -> std::io::Result<usize>
}]
pub enum Writer {
    Cursor(Cursor<Vec<u8>>),
    File { file: File },
}

fn get_writer() -> Writer {
    if let Ok(path) = env::var("WRITER_FILE") {
        Writer::File {
            file: File::create(path).unwrap(),
        }
    } else {
        Writer::Cursor(Cursor::new(vec![]))
    }
}

fn main() {
    let mut writer = get_writer();
    writer.write_all(b"hello!").unwrap();
}

该宏生成与以下等效的实现块

impl Writer {
    fn write_all(&mut self, buf: &[u8]) -> std::io::Result<()> {
        match self {
            Self::Cursor(first, ..) => first.write_all(buf),
            Self::File { file, .. } => file.write_all(buf),
        }
    }
    pub fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        match self {
            Self::Cursor(first, ..) => first.write(buf),
            Self::File { file, .. } => file.write(buf),
        }
    }
}

as_dyn

//! The variant of the writer is dynamically selected with an environment variable.
//! Using the macro, we can conveniently turn the enum into a trait object when necessary.

use std::{
    fmt::Debug,
    fs::File,
    io::{Cursor, Write},
};

#[impl_enum::as_dyn(Debug, Write)]
pub enum Writer {
    Cursor(Cursor<Vec<u8>>),
    File { file: File },
}

fn get_writer() -> Writer {
    if let Ok(path) = std::env::var("WRITER_FILE") {
        Writer::File {
            file: File::create(path).unwrap(),
        }
    } else {
        Writer::Cursor(Cursor::new(vec![]))
    }
}

fn main() {
    let mut writer = get_writer();

    let dyn_debug = writer.as_dyn_debug();
    println!("{:?}", dyn_debug);

    let dyn_writer_mut = writer.as_dyn_write_mut();
    dyn_writer_mut.write_all(b"hello!").unwrap();

    let box_dyn_debug = writer.into_dyn_debug();
    println!("{:?}", box_dyn_debug);
}

该宏生成与以下等效的实现块

impl Writer {
    fn as_dyn_write(&self) -> &dyn Write {
        match self {
            Self::Cursor(first, ..) => first as &dyn Write,
            Self::File { file, .. } => file as &dyn Write,
        }
    }
    fn as_dyn_write_mut(&mut self) -> &mut dyn Write {
        match self {
            Self::Cursor(first, ..) => first as &mut dyn Write,
            Self::File { file, .. } => file as &mut dyn Write,
        }
    }
    fn into_dyn_write(self) -> Box<dyn Write> {
        match self {
            Self::Cursor(first, ..) => Box::new(first) as Box<dyn Write>,
            Self::File { file, .. } => Box::new(file) as Box<dyn Write>,
        }
    }
}

替代方案

• https://crates.io/crates/ambassador

许可协议

根据Mozilla公共许可证版本2.0授权。