Why does indexing a Vec return a value instead of a reference as promised by the Index?

Question

The documentation for the Index trait says that the .index() method returns a reference to the Output associated type (link):

fn index(&self, index: Idx) -> &Self::Output;

For Vec<T> and the usize index, Output is T. So, I expect the variable a in the following snippet to have the type &i32.

let v = vec![0];
let a = v[0];

However, the type of a is i32. Why? I am learning Rust and, as far as I understand, Rust requires you to be explicit everywhere and never performs value<->reference conversions implicitly. Hence the question.