main()1fn main(){ } 2 test_result() -> Result<(),()>3fn test_result() -> Result<(),()> { 4 let a:Option<()> = Some(()); 5 a?;//~ ERROR the `?` operator can only be used 6 Ok(()) 7 } 8 test_option() -> Option<i32>9fn test_option() -> Option<i32>{ 10 let a:Result<i32, i32> = Ok(5); 11 a?;//~ ERROR the `?` operator can only be used 12 Some(5) 13 } 14