Unsoundness from mixing trait impls

[alexcrichton]

This code compiles successfully:

trait A {}                                
impl<'self> A for &'self mut A {}         

fn foo(t: &mut A) -> ~A: {                
    ~t as ~A:                             
}                                         

struct B;                                 
impl A for B {}                           

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

fn main() {                               
    let _tmp = {                          
        let mut b = B;                    
        foo(&mut b as &mut A)             
    };                                    
    println!("after conversion");         
}                                         

and yields the output of

$ ./foo       
dropping
after conversion

The value b has been dropped by the time that after conversion is printed, yet we still have a reference to it via the _tmp handle.

Nominating for backwards-compatibility

[alexcrichton]

cc @nikomatsakis

[catamorphism]

1.0, high

[nikomatsakis]

Dup of #5723

[alexcrichton]

I'd like to close this, but 5723 isn't nominated/triaged, so I'm going to leave this open until that bug is milestoned. In the meantime I will nominate #5723

[alexcrichton]

denominating because 5723 has been nominated.