Member destructor is always executed even if self is replaced in `Drop`

[nwin]

The following code prints "Dropping!" although the corresponding variant had been removed and forgotten. Same happens if it is overwritten instead.

use std::mem;
use std::ptr;

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

enum Foo {
    Bar(Noisy),
    Baz
}
use Foo::*;

impl Drop for Foo {
    fn drop(&mut self) {
        match *self {
            Bar(_) => {
                //unsafe { ptr::write(self, Foo::Baz) }
                unsafe { mem::forget(mem::replace(self, Foo::Baz)) }
            }
            Baz => ()
        }
    }
}

fn main() {
    let _ = Foo::Bar(Noisy);
}

I’m not sure if this is a bug or intended behavior but it is not obvious to me that this would happen and might be documented in more detail.

[sfackler]

cc @pnkfelix

[steveklabnik]

(a bit of a strange tag, but I wasn't 100% sure what to put this under. kinda works, but may need recategorized once we pin down what's up here)

[nwin]

I just checked, the destructor is run also on the wrong (the new replaced one) value. The following code prints „Dropping deadbeef“

use std::mem;
use std::ptr;

struct Noisy(usize);
impl Drop for Noisy {
    fn drop(&mut self) { 
        println!("Dropping {:x}.", self.0)
    }
}

enum Foo {
    Bar(Noisy),
    Baz(usize)
}
use Foo::*;

impl Drop for Foo {
    fn drop(&mut self) {
        match *self {
            Bar(_) => {
                unsafe { mem::forget(mem::replace(self, Foo::Baz(0xdeadbeef))) }
            }
            Baz(_) => ()
        }
    }
}

fn main() {
    let _ = Foo::Bar(Noisy(0));
}

[pnkfelix]

hmm, I assume this has something to do with the drop glue assuming that the drop implementation will not change which variant of enum we are looking at.

I'm not exactly sure what the best thing to do is. Ideally we would ensure the drop method would not be able to change the enum variant in this fashion.

Nominating for discussion at triage meeting. Probably an I-needs-decision; probably can be decided by 1.0 (i.e. need not be resolved by the beta release itself.)

[pnkfelix]

Current plan: Change the drop glue to inspect the discriminant after the destructor runs.

1.0 polish, P-backcompat-lang.

[pnkfelix]

(i think i have a plausible patch for this worked out.)