Why can't I run #[test]s defined in main()?

I'm learning Rust and I want to test a simple function inside main.rs as:

fn main() {
    fn adder(n: u64, m:u64) -> u64 {
        assert!(n != 0 && m !=0);
        n + m
    }

    #[test]
    fn test_gcd(){
        assert_eq!(adder(1, 10), 11);
        assert_eq!(adder(100, 33), 133);
    }
    println!("{}", adder(3, 4));
    println!("The END");
}

When I run cargo test no test are found and this warning shows up:

warning: cannot test inner items
 --> src\main.rs:7:5
  |
7 |     #[test]
  |     ^^^^^^^
  |
  = note: `#[warn(unnameable_test_items)]` on by default
  = note: this warning originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `rust_prueba` (bin "rust_prueba" test) generated 1 warning
    Finished test [unoptimized + debuginfo] target(s) in 0.58s
     Running unittests src\main.rs (target\debug\deps\rust_prueba-e1ba8a7bcfb64cfc.exe)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

But, if I define both the adder function and the test function outside the main function, it works:

fn main() {
    println!("{}", adder(3, 4));
    println!("The END");
}

fn adder(n: u64, m:u64) -> u64 {
    assert!(n != 0 && m !=0);
    n + m
}

#[test]
fn test_gcd(){
    assert_eq!(adder(1, 10), 11);
    assert_eq!(adder(100, 33), 133);
}

cargo test:

running 1 test
test test_gcd ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Why didn't my first try work?