如何通过Crate或宏实现Rust泛型类型的匹配？

我发现可以通过以下方式对泛型类型进行「匹配」：

use std::any::Any;

trait SomeTrait {}

struct SomeStruct<T: SomeTrait> {
  atr: T,
}

struct Bar {
  name: String,
}

impl SomeTrait for Bar {}

struct Baz;
impl SomeTrait for Baz {}

fn foo<T: SomeTrait + 'static>(s: SomeStruct<T>) {
    let any: Box<dyn Any> = Box::new(s.atr);
    if let Some(bar) = any.downcast_ref::<Bar>() {
        println!("got bar! {:?}", bar.name);
    } else {
        println!("did not get bar");
    }
}

fn main() {
  foo::<Bar>(SomeStruct {
    atr: Bar { name: "hello".to_string() }
  });
  foo::<Baz>(SomeStruct { atr: Baz {} });
}

请问是否有现成的Crate或宏可以实现该功能？我设想的宏使用方式如下：

fn foo<T: SomeTrait + 'static>(s: SomeStruct<T>) {
    generic_match! s.ty {
      Bar(bar) => println!("got bar! {:?}", bar.name),
      Baz => println!("did not get bar"),
    }
}

现成Crate推荐 #

1. downcast-rs：这个Crate简化了Any trait的向下转换逻辑，支持为自定义trait添加向下转换能力。你可以为trait SomeTrait实现Downcast trait，之后就能直接对&dyn SomeTrait进行类型匹配，省去手动封装Box<dyn Any>的步骤。

2. match-type：这个Crate提供的match_type!宏语法和你设想的高度契合，能直接针对泛型类型做分支匹配，用法示例如下：

use match_type::match_type;

fn foo<T: SomeTrait + 'static>(s: SomeStruct<T>) {
    match_type!(T {
        Bar => {
            let bar = &s.atr as &Bar;
            println!("got bar! {:?}", bar.name);
        },
        Baz => println!("did not get bar"),
        _ => println!("unhandled type"),
    });
}

手动实现宏 #

如果不想引入额外依赖，也可以自己实现一个轻量宏，核心是自动生成downcast_ref的匹配分支：

macro_rules! generic_match {
    ($target:expr, {
        $($ty:ident($var:ident) => $body:block,)*
        $($ty2:ident => $body2:block,)*
    }) => {
        let any_ref = &$target as &dyn std::any::Any;
        $(
            if let Some($var) = any_ref.downcast_ref::<$ty>() {
                $body
            } else
        )*
        $(
            if any_ref.is::<$ty2>() {
                $body2
            } else
        )*
        {
            // 默认分支，不需要可删除
            unreachable!("unexpected type")
        }
    };
}

// 使用示例
fn foo<T: SomeTrait + 'static>(s: SomeStruct<T>) {
    generic_match!(s.atr, {
        Bar(bar) => {
            println!("got bar! {:?}", bar.name);
        },
        Baz => {
            println!("did not get bar");
        },
    });
}

内容的提问来源于stack exchange，提问作者0x5DA