What type is the "type ()" in Rust?

Question

As a simple exercise to learn Rust, I've decided to implement a simple binary search:

pub fn binary_search(arr: &[i32], key: i32) -> usize {
    let min: usize = 0;
    let max: usize = arr.len();
    while max >= min {
        let mid: usize = (max - min) / 2 as usize;
        if key == arr[mid] {
            mid as usize
        }

        if key < arr[mid] {
            min = mid + 1;
            continue;
        }

        max = mid - 1;
    }
    -1 as usize
}

#[cfg(test)]
mod tests {

    use super::binary_search;

    #[test]
    fn binary_search_works() {
        let arr: [i32; 8] = [1, 2, 3, 4, 5, 6, 7, 8];
        let index: usize = binary_search(&arr, 2);
        assert_eq!(1, index);
    }
}

At build time, I get this error which I do not understand. What is the () type? Variable mid is always usize but even with the as cast I'm getting this compilation error.

error: mismatched types [E0308]
            mid as usize
            ^~~~~~~~~~~~
help: run `rustc --explain E0308` to see a detailed explanation
note: expected type `()`
note:    found type `usize`

Answer 1

() is the unit type, analogous to a void return type in other languages.

You're getting it here:

if key == arr[mid] {
    mid as usize
}

Rust is expecting that if expression to return (), but you're returning usize for that expression. Since virtually everything in Rust is an expression, you can usually implicit return like you're trying to here, but in this specific case you can't because the if expression is not the only expression in the while expression. You could fix the immediate problem by using return mid as usize; instead.