Unsoundness: can put borrowed pointer in @Trait, and have it outlive its lifetime

[bill-myers]

Not totally sure what the root issue is, I guess it is that borrowck doesn't realize that the lifetime of "x as @mut Trait" needs to be limited to the lifetime of x.

Or perhaps it shouldn't be allowed at all to implement traits without lifetime parameters for structs with lifetime parameters.

trait R
{
    fn set_v(&mut self, v: int);
}

struct A<'self>
{
    v: &'self mut int
}

impl<'self> R for A<'self>
{
    fn set_v(&mut self, v: int)
    {
        *self.v = v;
    }
}

fn boom() -> @mut R
{
    let mut i = 3;
    let a = @mut A {v: &mut i};
    let f: @mut R = a as @mut R;
    f
}

pub fn main()
{
    let mut f = boom();
    f.set_v(66); // write to random place on stack
}

[nikomatsakis]

Dup of #5723