Rust中的C + 11类型衰变

Question

在C++ 11中,您可以将泛型类型衰减为值类型,删除引用/右值语义和cv限定符,例如

decay<int>::type // type is `int`
decay<const int&>::type // type is `int`
decay<int&&>::type // type is `int`

是否有一种已知的机制可以在Rust中实现相同的功能,即剥离引用修饰符,生命周期和mut限定符？例如:

decay<u32>::type <--- type is `u32`
decay<&u32>::type <--- type is `u32`
decay<&mut u32>::type <--- type is `u32`
decay<&static u32>::type <--- type is `u32`

对于背景,我正在尝试编写一个宏,该宏生成一个struct存储宏匹配的一堆函数参数的值.例如,宏可能包含args foo: i32, bar: &Vec<String>,结果结构应该是:

struct GeneratedStruct {
    foo: i32,
    bar: Vec<String>,
}

Answer 1

正如Matthieu M.和kennytm在评论中所建议的那样,你可以定义一个特征并使用特化(Rust 1.15.0中的一个不稳定特征)来实现这一点.

#![feature(specialization)]

use std::any::TypeId;

trait Decay {
    type Type;
}

impl<T> Decay for T {
    default type Type = T;
}

impl<'a, T> Decay for &'a T {
    type Type = <T as Decay>::Type;
}

impl<'a, T> Decay for &'a mut T {
    type Type = <T as Decay>::Type;
}

fn foo<T: 'static>() {
    println!("{:?}", TypeId::of::<T>());
}

fn bar<T>() where <T as Decay>::Type: 'static {
    println!("{:?}", TypeId::of::<<T as Decay>::Type>());
}

fn main() {
    foo::<<i32 as Decay>::Type>();
    foo::<<&i32 as Decay>::Type>();
    foo::<<&mut i32 as Decay>::Type>();
    foo::<<&&i32 as Decay>::Type>();

    bar::<i32>();
    bar::<&i32>();
    bar::<&mut i32>();
    bar::<&&i32>();
}