rust/src/test/ui/span/issue28498-reject-passed-to-fn.rs

// Demonstrate that a type param in negative position causes dropck to reject code
// that might indirectly access previously dropped value.
//
// Compare with run-pass/issue28498-ugeh-with-passed-to-fn.rs

#[derive(Debug)]
struct ScribbleOnDrop(String);

impl Drop for ScribbleOnDrop {
    fn drop(&mut self) {
        self.0 = format!("DROPPED");
    }
}

struct Foo<T>(u32, T, Box<for <'r> fn(&'r T) -> String>);

impl<T> Drop for Foo<T> {
    fn drop(&mut self) {
        // Use of `may_dangle` is unsound,
        // because we pass `T` to the callback in `self.2`
        // below, and thus potentially read from borrowed data.
        println!("Dropping Foo({}, {})", self.0, (self.2)(&self.1));
    }
}

fn callback(s: & &ScribbleOnDrop) -> String { format!("{:?}", s) }

fn main() {
    let (last_dropped, foo0);
    let (foo1, first_dropped);

    last_dropped = ScribbleOnDrop(format!("last"));
    first_dropped = ScribbleOnDrop(format!("first"));
    foo0 = Foo(0, &last_dropped, Box::new(callback)); // OK
    foo1 = Foo(1, &first_dropped, Box::new(callback));
    //~^ ERROR `first_dropped` does not live long enough

    println!("foo0.1: {:?} foo1.1: {:?}", foo0.1, foo1.1);
}