Unsound behavior when returning &-pointers

[alexcrichton]

This code:

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

impl<'self> Iterator<&'self int> for A<'self> {
    fn next(&mut self) -> Option<&'self int> {
        *self.cur += 1;
        return Some(&*self.cur);
    }
}

fn main() {
    let mut a = 3;
    let mut a = A { cur: &mut a };
    let b = a.next();
    println!("{:?}", b);
    let c = a.next();
    println!("{:?}", b);
    println!("{:?}", c);
}

Runs with:

$ rustc foo.rs
warning: no debug symbols in executable (-arch x86_64)
$ ./foo
Some(&4)
Some(&5)
Some(&5)

I thought something like this should be a compiler error? It seems to me that the lifetime of b would conflict with attempting to re-borrow a as mut.

cc @nikomatsakis

@blake2-ppc discovered this in a bug in a test he wrote for #9062

[nikomatsakis]

Dup #8624