fn main() {
  println!("hello.");
}
  
/// panic if passed `false`:
///
fn dont_panic(really: bool) -> i32 {
   if !really { panic!("Aaaaaaaah!"); }
   42
}


#[test]
fn test_cause_a_panic() {

    assert_eq!( dont_panic(true), 42 );

    // Now check that dont_panic(false) really does panic:
    let my_result = std::panic::catch_unwind( || dont_panic(false) );
    assert!(my_result.is_err());

    assert_eq!( dont_panic(true), 42 );
 
    /* Explanation: the `||` has NOTHING to do with boolean-OR;
     * It is creating an anonymous function.
     * In this case, an anonymous function that takes 0 inputs (a "thunk").
     * It's a classic way to have a chunk of code that you want somebody else to run.
     *
     * The problem if we just call `dont_panic(false)` is that control will never reach
     * `assert!` -- the panic(exception) will have already occurred!
     * So instead we'll wrap our code-that-might-panic inside a thunk,
     * and give that thunk to somebody else and let them invoke it.  (Kinda like a hot potato.)
     *
     * So catch_unwind takes our thunk, and (you can imagine) calls it inside a try/catch.
     * - if the provided code runs normally and returns some value `r`, catch_unwind returns `Ok(r)`
     * - if the provided code panics, it catches that panic and wraps it into an `Err(msg)`.
     *
     * Btw, in Java/JUnit `assertThrows` also has to be passed a thunk 
     * (`java.util.concurrent.Callable` or `java.util.function.Supplier`).
     *
     * Minor note: the panic still prints a message, BUT the program continues.
     *   Though you won't see that message if you're in 'test' mode, 
     *   since that hides stdout by default.
     *
     *
     * tl;dr: to test code you *expect* to panic: 
     * precede it with `||` to turn it into a function-of-0-inputs,
     * pass that function to catch_unwind, 
     * and then assert that catch_unwind returned an Err.
     */

     // This can be done on a single-line, of course:
     assert!( std::panic::catch_unwind( || dont_panic(false) ).is_err() );

     //   h/t https://stackoverflow.com/a/42649833/320830   user U007D
}