Rust has IntoIterator trait, to create Iterator from a type. IntoIterator consumes the type, many types provides iter() method to create Iterator from immutable reference without consuming the type. But no trait for iter() method.

In most cases, such trait is not needed, because iter() method returns Iterator which implements IntoIterator trait. But consider the following case:

trait Inspector<T> {
    fn inspect(&self, v: &T);
}

inspect() method takes immutable reference of T, and T is not Copy type. If impl Inspector for any object can convert into Iterator, we can write code like this:

impl<T, I> Inspector<T> for I
    where
        I: IntoIterator<Item = T>,
        T: std::fmt::Debug,
{
    fn inspect(&self, v: &T) {
        for item in self {
            println!("{:?}", item);
        }
    }
}

But IntoIterator trait consumes the type, compiler won't let it go.

If we impl Inspector on slice:

impl<T: std::fmt::Debug> Inspector<T> for [T] {
    fn inspect(&self, v: &T) {
        for item in self {
            println!("{:?}", item);
        }
    }
}

Won't work, unless we change Inspector trait to allow unsized type:

trait Inspector<T: ?Sized> {
    fn inspect(&self, v: &T);
}

Unsized types are poison, cause much trouble to use, we don't want to use it.

Vec<T> support convert to slice very easily, but many other types don't support it, such as HashMap, LinkedList, slice version can not cover all cases.

We can not pass Iterator to inspect() method, because Iterator is a mutable object, and inspect() requires immutable reference, make it impossible to work.

iter_view crate provides IterView trait, which is similar to IntoIterator, but it doesn't consume the type,

pub trait IterView<'a> {
    type Item: 'a;
    type Iter: Iterator<Item = &'a Self::Item>;
    fn iter(&'a self) -> Self::Iter;
}

Use IterView trait, we can impl Inspector for any type which implements IterView:

impl<T, I> Inspector<T> for I
    where
        I: IterView<Item = T>,
        T: std::fmt::Debug,
{
    fn inspect(&self, v: &T) {
        for item in self.iter() {
            println!("{:?}", item);
        }
    }
}

License: MIT OR Apache-2.0