In Rust, is a vector an Iterator?

No, a vector is not an iterator.

But it implements the trait IntoIterator, which the for loop uses to convert the vector into the required iterator.

In the documentation for Vec you can see that IntoIterator is implemented in three ways:

• for Vec<T>, which is moved and the iterator returns items of type T,
• for a shared reference &Vec<T>, where the iterator returns shared references &T,
• and for &mut Vec<T>, where mutable references are returned.

iter() is just a method in Vec to convert Vec<T> directly into an iterator that returns shared references, without first converting it into a reference. There is a sibling method iter_mut() for producing mutable references.