Why lambda in map return (), not i32?

Question

let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x:i32| -> i32 {
    let res = x + 1;
    res;
}).rev().collect();

print!("{:?}", v)

I have this code, and get compile error

--> src\main.rs:8:68
   |
8  |       let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x:i32| -> i32 {
   |  ____________________________________________________________________^
9  | |         let res = x + 1;
10 | |         res;
   | |            - help: consider removing this semicolon
11 | |     }).rev().collect();
   | |_____^ expected i32, found ()
   |
   = note: expected type `i32`
              found type `()`

I can't understand, why found type () How it work? Why this happens?