Memory leak in `Error::downcast` in failure 0.1.2

[Jonas Schievink]

The following code quickly leaks memory:

#[macro_use]
extern crate failure;

use failure::Error;

#[derive(Debug, Fail)]
#[fail(display = "my custom failure type")]
struct MyFail;

fn main() {
    loop {
        Error::from(MyFail).downcast::<MyFail>().unwrap();
    }
}

This happens because Error::downcast does not drop the Box<Inner<Fail>> contained within the Error, it only drops the backtrace contained in that box and moves out the error type itself.

(Thanks to @rustonaut for finding the root cause!)

Note that I've not tested this on master, but on 0.1.2, which is the most up-to-date version published on Crates.io.

[Armin Ronacher]

I can't think of a good way to fix this without making the Inner contain the Fail as Option :(

[Armin Ronacher]

Actually this cannot be fixed with making it an Option and accessing the deallocators is impossible on 1.18 which this is tested against. Taking suggestions on how to fix this.

[Philipp Korber]

It might be a bit late but I think I found a way to do it in 1.18 without any magic.

Basically you can deref "move" Boxed content (e.g.: let _: String = *Box::new("x".to_owned());).

So it should be probably possible to do something like this:

1. check if the Error contains a T
2. cast the box back the the original non-DST type used to create the DST Box
3. use box deref to move the thing back on the stack
4. use destruction to get the parts back out (we should be able to no longer
   need any ptr reads)

[Armin Ronacher]

@rustonaut if you want to give this a try I would love to pull a PR that implements this!

[Philipp Korber]

Sure, I will see when I can fit it in.
(Should be in the next one or two days).