Reason of the error expected (), found struct `std::vec::Vec` in Rust?

Question

I am new to rust programming. I wanted to implement merge sort with recursion. Here is my code:

fn merge(a: &mut Vec<u32>, b: &mut Vec<u32>) -> Vec<u32> {
    let mut temp: Vec<u32> = Vec::new();

    println!("The digit is {}", a[0]);
    while a.len() > 0 && b.len() > 0 {
        if a[0] > b[0] {
            temp.push(a[0]);
            a.pop();
        } else {
            temp.push(b[0]);
            b.pop();
        }
    }

    while a.len() > 0 {
        temp.push(a[0]);
        a.pop();
    }

    while b.len() > 0 {
        temp.push(b[0]);
        b.pop();
    }

    temp
}

fn merge_sort(v: &mut Vec<u32>) -> Vec<u32> {
    println!("The divided vector is: {:?}", v);
    let n = v.len();

    if n == 1 {
        println!("The divided vector is: {:?}", v.to_vec());
        let t: Vec<u32> = Vec::new();
        t.push(v[0]);
        t
    }

    if n == 0 {
        panic!("Alas!! NULL");
    }

    merge(
        &mut merge_sort(&mut v[0..n / 2].to_vec()),
        &mut merge_sort(&mut v[n / 2 + 1..n].to_vec()),
    )
    .to_vec()
}

fn main() {
    let mut v = vec![23, 78, 89, 64, 23, 12, 79, 45, 64];
    println!("The vector is: {:?}", v);
    println!("The length {}", v.len());

    let v = merge_sort(&mut v);

    println!("The sorted vector is: {:?}", v);
}

The problem is, when I am trying to compile it, I am getting the following error:

error[E0308]: mismatched types
  --> src/main.rs:36:9
   |
32 | /     if n == 1 {
33 | |         println!("The divided vector is: {:?}", v.to_vec());
34 | |         let t: Vec<u32> = Vec::new();
35 | |         t.push(v[0]);
36 | |         t
   | |         ^ expected `()`, found struct `std::vec::Vec`
37 | |     }
   | |     -- help: consider using a semicolon here
   | |_____|
   |       expected this to be `()`
   |
   = note: expected unit type `()`
                 found struct `std::vec::Vec<u32>`

Do you have any idea why am I getting this strange error! It seems, I am missing something.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b94db91b44c8f593bcd1fdd5d52c2ae4 — Stargateur, Mar 20 '20 at 10:26
Does this answer your question? [What's the difference between using the return statement and omitting the semicolon in Rust?](https://stackoverflow.com/questions/59013389/whats-the-difference-between-using-the-return-statement-and-omitting-the-semico) — trent, Mar 20 '20 at 13:18
Relevant quote: "it is not the fact that an expression does not have a semicolon that makes it the return value, it is the fact that it is the final expression in the function." You cannot return from a function early by merely omitting the semicolon from some arbitrary expression; you have to use the `return` keyword. — trent, Mar 20 '20 at 13:20

peterulb · Answer 1 · 2020-03-20T10:58:29.750

In Rust, the block type is the type of the final expression or () if there is none. Also combined blocks needs to be same type like if{...} else if{...} else{...}. Without an else the return type of an if expression must be () as this is the type that is returned when the expression evaluates to false.

Additionally the result is not the final expression in your code. What you need instead is to use return. Also be aware Vec::push requires a mutable reference of instance(&mut self).

if n == 1 {
    println!("The divided vector is: {:?}", v.to_vec());
    let mut t: Vec<u32> = Vec::new();
    t.push(v[0]);
    return t;
}

Reason of the error expected (), found struct `std::vec::Vec` in Rust?

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