# Inference

The type inference engine is pretty smart. It does more than looking at the type of the r-value during an initialization. It also looks at how the variable is used afterwards to infer its type. Here's an advanced example of type inference:

    fn main() {
        // Because of the annotation, the compiler knows that `elem` has type u8.
        let elem = 5u8;

        // Create an empty vector (a growable array).
        let mut vec = Vec::new();
        // At this point the compiler doesn't know the exact type of `vec`, it
        // just knows that it's a vector of something (`Vec<_>`).

        // Insert `elem` in the vector.
        vec.push(elem);
        // Aha! Now the compiler knows that `vec` is a vector of `u8`s (`Vec<u8>`)
        // TODO ^ Try commenting out the `vec.push(elem)` line

        println!("{:?}", vec);
    }

No type annotation of variables was needed, the compiler is happy and so is the programmer!