Rust expression: inconsistent types: expected `()` "

Question

I tried to implement fizzbuzz in Rust and failed with some cryptic error:

fn main() {
    let mut i = 1;

    while i < 100 {
        println!("{}{}{}",
                 if i % 3 == 0 { "Fizz" },
                 if i % 5 == 0 { "Buzz" },
                 if !(i % 3 == 0 || i % 5 == 0) {
                     i
                 });
        i += 1;
    }
}

Error:

error: mismatched types: expected `()` but found `&'static str` (expected () but found &-ptr)
                 if i % 3 == 0 { "Fizz" },
                               ^~~~~~~~~~
error: mismatched types: expected `()` but found `&'static str` (expected () but found &-ptr)
                 if i % 5 == 0 { "Buzz" },
                               ^~~~~~~~~~
error: mismatched types: expected `()` but found `<generic integer #0>` (expected () but found integral variable)
                 if !(i % 3 == 0 || i % 5 == 0) {
                     i
                 });

Or in newer versions of Rust with a slightly modified error message:

error: if may be missing an else clause [--explain E0308]
                  if i % 3 == 0 { "Fizz" },
                  ^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found &-ptr
note: in this expansion of format_args!
note: in this expansion of print! (defined in <std macros>)
note: in this expansion of println! (defined in <std macros>)
note: expected type `()`
note:    found type `&'static str`

error: if may be missing an else clause [--explain E0308]
                 if i % 5 == 0 { "Buzz" },
                 ^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found &-ptr
note: in this expansion of format_args!
note: in this expansion of print! (defined in <std macros>)
note: in this expansion of println! (defined in <std macros>)
note: expected type `()`
note:    found type `&'static str`

error: if may be missing an else clause [--explain E0308]
                 if !(i % 3 == 0 || i % 5 == 0) {
                 ^ expected (), found integral variable
note: in this expansion of format_args!
note: in this expansion of print! (defined in <std macros>)
note: in this expansion of println! (defined in <std macros>)
note: expected type `()`
note:    found type `_`

I found why removing return gives me an error: expected '()', but found , but adding return, as suggested, did not help.

What do these errors mean and how can I avoid them in the future?

Answer 1

The problem is that it if i % 3 == 0 { "Fizz" }returns either unit ()or &'static str. Modify the if statements to return the same type in both cases, for example, by adding else { "" }.

Answer 2

, , ";" return

fn add_one(x: i32) -> i32 { 
    x + 1; // remove ";" from return
}