EventList::From for Into<Event> slices can cause double-drop

[ammaraskar]

Hi there, we (Rust group @sslab-gatech) are scanning crates on crates.io for potential soundness bugs. We noticed that in the EventList::From implementation for slices of types that implement Into<Event>:

ocl/ocl/src/standard/event.rs

Lines 1040 to 1043 in 0308686

	for idx in 0..events.len() {
	let event = unsafe { ptr::read(events.get_unchecked(idx)) };
	el.push(event.into());
	}

It grabs the event using ptr::read duplicating the ownership and then calls event.into() which can potentially panic. This can lead to the event being double freed as shown in the example below:

#![forbid(unsafe_code)]

use ocl::{Event, EventList};

struct MyIntoEventType(u32);

impl Drop for MyIntoEventType {
    fn drop(&mut self) {
        println!("Dropping the MyIntoEventType");
    }
}

impl Into<Event> for MyIntoEventType {
    fn into(self) -> Event {
        panic!("Panicking in Into");
    }
}

fn main() {
    let slice = [MyIntoEventType(1)];
    let event_list = EventList::from(slice);
}

Output:

thread 'main' panicked at 'Panicking in Into', src/main.rs:28:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Dropping the MyIntoEventType
Dropping the MyIntoEventType

[ammaraskar]

Aah this was already reported as part of #194