Reverse iterating over a &vec versus vec.iter()

Question

This works because Iterator implements rev() where self is a DoubleEndedIterator:

let vec: Vec<i32> = Vec::new();
for x in vec.iter().rev() {
    //Do stuff
}

However, if I change vec.iter().rev() to &vec.rev() it won't compile because:

no method named `rev` found for type `std::vec::Vec<i32>` in the current scope

Furthermore:

the method `rev` exists but the following trait bounds were not satisfied: `std::vec::Vec<i32> : std::iter::Iterator`, `[i32] : std::iter::Iterator`

But doesn't a for loop implicitly call IntoIterator? Is &vec or vec.iter() considered idiomatic Rust?

Answer 1

If you're just looping over the Vec, then &vec is idiomatic. This works because &Vec<T> implements IntoIterator, which is what the for loop uses.

However if you want to call Iterator methods such as rev, filter, etc., you need an actual Iterator (since Vec doesn't implement Iterator, only IntoIterator).

So this:

for x in &vec.rev() {
    ...
}

is equivalent to:

for x in (&vec.rev()).into_iter() {
    ...
}

i.e. there's no chance to use IntoIterator before trying to call Iterator methods.