main()1 fn main(){ }
2 
test_result() -> Result<(),()>3 fn 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>9 fn 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