是否可以将实现特征的所有类型存储在列表中并在该列表上进行迭代？

Question

I have a trait implemented for many structs ( A , B , C , etc.):

pub trait ApplicableFor: Debug + Default {
    unsafe fn is_applicable_for(from: *mut u8) -> bool
    where
        Self: Sized;
}

I need a method which finds which struct returns true for this method call. I have the code:

unsafe fn check_applicable<T: ApplicableFor>(from: *mut u8) -> bool {
    T::is_applicable_for(from, to)
}

unsafe fn find_applicable(from: *mut u8) -> ApplicableFor {
    if check_applicable::<A>(from) {
        A::default()
    } else if check_applicable::<B>(from) {
        B::default()
    } else if check_applicable::<C>(from) {
        C::default()
    } else {
        panic!("Couldn't find appicable");
    }
}

In the real code, I have around 20 structs, so I want to store them somewhere and use code like this for readability:

unsafe fn find_applicable(from: *mut u8) -> ApplicableFor {
    for T in list {
        if check_applicable::<T>(from) {
            T::default()
        }
    }
    panic!("Couldn't find appicable");
}

How can I do this or how can I rewrite this better?

Answer 1

No, Rust does not directly offer the kind of metaprogramming features you need. Namely, a type is not a concrete thing that exists or can be put into a collection.

Instead, you need code generation.

Starting from a simplified version of ApplicableFor , we can write a very structured version of find_applicable :

trait ApplicableFor {
    fn is_applicable_for(from: u8) -> bool;
}

fn find_applicable(from: u8) {
    if <A>::is_applicable_for(from) {
        println!("Using {}", stringify!(A));
        return;
    }

    if <B>::is_applicable_for(from) {
        println!("Using {}", stringify!(B));
        return;
    }

    if <C>::is_applicable_for(from) {
        println!("Using {}", stringify!(C));
        return;
    }

    panic!("Couldn't find any applicable types");
}

Once we have established the structure, then we can start abstracting it with macros:

fn find_applicable(from: u8) {
    macro_rules! find_one {
        ($ty:ty) => {
            if <$ty>::is_applicable_for(from) {
                println!("Using {}", stringify!($ty));
                return;
            }
        }
    }

    find_one!(A);
    find_one!(B);
    find_one!(C);

    panic!("Couldn't find any applicable types");
}

What if we want to repeat this concept of "do something for this list of types"? Another macro:

macro_rules! each_type {
    ($one_type_macro:tt) => {
        $one_type_macro!(A);
        $one_type_macro!(B);
        $one_type_macro!(C);
    };
}

fn find_applicable(from: u8) {
    macro_rules! find_one {
        ($ty:ty) => {
            if <$ty>::is_applicable_for(from) {
                println!("Using {}", stringify!($ty));
                return;
            }
        }
    }

    each_type!(find_one);

    panic!("Couldn't find any applicable types");
}

Too much noise for the implementation of each_type! ? Create a macro that creates another macro which will be called with another macro:

macro_rules! gen_each_type {
    ($($ty:ty),*) => {
        macro_rules! each_type {
            ($one_type_macro:tt) => {
                $($one_type_macro!($ty);)*
            };
        }
    };
}

gen_each_type![A, B, C];