Destructor for `@Trait` is not run in certain circumstances

[Blei]

I'm not sure if this is a duplicate of an existing issue, there are many destructor related issues open already. But anyways, here it is:

#[feature(managed_boxes)];

struct DroppableStruct { a: int }
impl Drop for DroppableStruct {
    fn drop(&mut self) {
        println!("Dropping");
    }
}

trait MyTrait { }
impl MyTrait for DroppableStruct {}


struct Whatever<'a> { w: &'a MyTrait }
impl <'a> Whatever<'a> {
    fn new<'b>(w: &'b MyTrait) -> Whatever<'b> {
        Whatever { w: w }
    }
}

fn main() {
    let mut f = DroppableStruct;
    let _a = Whatever::new(@f as @MyTrait);
}

This doesn't run the destructor of DroppableStruct. This is pretty much as minimal as I could get the example, many changes make it work. E.g. changing the main function body to:

    let f = DroppableStruct { a: 1 };
    let b = @f as @MyTrait;
    let _a = Whatever::new(b);

makes the destructor work. Also, not saving the Whatever struct in a variable makes it work. Also, not saving the @MyTrait in Whatever makes it work. Good luck.

Update: removed mut, was not relevant.
Update2: added a field to the struct to show it's not an issue about zero-sized structs.

[emberian]

cc @nikomatsakis

[flaper87]

It's no longer possible to cast to @-traits as of b2d30b7

I tried to reproduced this bug using ~ and & pointers but it worked as expected.

[emberian]

@flaper87 is there a testcase which covers this?

[flaper87]

@cmr I had seen it before but I can't find it anymore. Re-opening and flagging as needs test:

struct DroppableStruct;

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

trait MyTrait { }
impl MyTrait for ~DroppableStruct {}

struct Whatever { w: ~MyTrait }
impl  Whatever {
    fn new(w: ~MyTrait) -> Whatever {
        Whatever { w: w }
    }
}

fn main() {
    let f = ~DroppableStruct;
    let _a = Whatever::new(~f as ~MyTrait);
}