如何为任何元素序列实现特征？

Question

I am trying to implement a trait for any sequence of elements, so that it will work for vectors, arrays and slices. So far I've tried several approaches, but I can't compile any of them :(

I have this trait, and a function that uses it, and a basic data type implementing the trait:

trait Hitable {
    fn hit(&self, val: f64) -> bool;
}

fn check_hit<T: Hitable>(world: &T) -> bool {
    world.hit(1.0)
}

struct Obj(f64);

impl Hitable for Obj {
    fn hit(&self, val: f64) -> bool {
        self.0 > val
    }
}

I'd like to be able to implement that trait for sequence of Obj 's. It works fine if I just restrict it to vectors:

impl<T> Hitable for Vec<T>
where
    T: Hitable,
{
    fn hit(&self, val: f64) -> bool {
        self.iter().any(|h| h.hit(val))
    }
}

fn main() {
    let v = vec![Obj(2.0), Obj(3.0)];
    println!("{}", check_hit(&v));
}

But I want to make it more generic so that it works for arrays and slices; how can I do that?

I tried the following four attempts:

Attempt #1: for iterator on Hitables.

// It's not clear how to call it:
//    vec.iter().hit(...) does not compile
//    vec.into_iter().hit(...) does not compile
//
impl<T, U> Hitable for T
where
    T: Iterator<Item = U>,
    U: Hitable,
{
    fn hit(&self, val: f64) -> bool {
        self.any(|h| h.hit(val))
    }
}

Attempt #2: for something which can be turned into iterator.

// Does not compile as well:
//
//         self.into_iter().any(|h| h.hit(val))
//         ^^^^ cannot move out of borrowed content
//
impl<T, U> Hitable for T
where
    T: IntoIterator<Item = U>,
    U: Hitable,
{
    fn hit(&self, val: f64) -> bool {
        self.into_iter().any(|h| h.hit(val))
    }
}

Attempt #3: for slices.

// This usage doesn't compile:
//     let v = vec![Obj(2.0), Obj(3.0)];
//     println!("{}", check_hit(&v));
//
// It says that Hitable is not implemented for vectors.
// When I convert vector to slice, i.e. &v[..], complains about
// unknown size in compilation time.
impl<T> Hitable for [T]
where
    T: Hitable,
{
    fn hit(&self, val: f64) -> bool {
        self.iter().any(|h| h.hit(val))
    }
}

Attempt #4: for Iterator + Clone

//     let v = vec![Obj(2.0), Obj(3.0)];
//     println!("{}", check_hit(&v.iter()));
//
// does not compile:
//     println!("{}", check_hit(&v.iter()));
//                    ^^^^^^^^^ `&Obj` is not an iterator
//
impl<T, U> Hitable for T
where
    T: Iterator<Item = U> + Clone,
    U: Hitable,
{
    fn hit(&self, val: f64) -> bool {
        self.clone().any(|h| h.hit(val))
    }
}

Playground link

Answer 1

1. Iterator -based

This cannot work because iterators need to be mutable in order to advance them, but your trait requires &self .

2. IntoIterator -based

I'd change the trait to take self by value and then only implement it for references to Obj . This also allows implementing it for any type that implements IntoIterator :

trait Hitable {
    fn hit(self, val: f64) -> bool;
}

fn check_hit<T: Hitable>(world: T) -> bool {
    world.hit(1.0)
}

struct Obj(f64);

impl Hitable for &'_ Obj {
    fn hit(self, val: f64) -> bool {
        self.0 > val
    }
}

impl<I> Hitable for I
where
    I: IntoIterator,
    I::Item: Hitable,
{
    fn hit(self, val: f64) -> bool {
        self.into_iter().any(|h| h.hit(val))
    }
}

fn main() {
    let o = Obj(2.0);
    let v = vec![Obj(2.0), Obj(3.0)];

    println!("{}", check_hit(&o));
    println!("{}", check_hit(&v));
}

See also:

• When should I not implement a trait for references to implementors of that trait?

3. Slice-based

I find that reading the entire error message, not just the one line summary, can help:

error[E0277]: the size for values of type `[Obj]` cannot be known at compilation time
  --> src/main.rs:28:20
   |
28 |     println!("{}", check_hit(&v[..]));
   |                    ^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `[Obj]`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
note: required by `check_hit`
  --> src/main.rs:5:1
   |
5  | fn check_hit<T: Hitable>(world: &T) -> bool {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Specifically, this bit: note: required by check_hit — check_hit requires that T be Sized . Removing that restriction allows this version to work:

fn check_hit<T: Hitable + ?Sized>(world: &T) -> bool {
//                      ^~~~~~~~
    world.hit(1.0)
}

See also:

• Why does a reference to a trait in a generic function have to implement `Sized`?
• What does the question mark mean in a type parameter bound?