borrowck-write-variable-after-ref-extracted.nll

// Corresponds to:
//
// ```
// let foo = Vec { dummy: () };
// let mut p: &Vec<()> = &foo;
// let q: &&Vec<()> = &p;
// let r: &Vec<()> = &**q;
// p = ();
// use(r);
// ```
//
// Should be OK; `r` will not be invalidated by 
// and `foo` is read here.

struct Vec<+> {
  field: 0
}  

let foo: Vec<()>;
let p: &'p Vec<()>;
let q: &'q1 &'q2 Vec<()>;
let r: &'r Vec<()>;

block START {
    foo = use();
    p = &'bp foo;
    q = &'bq p;
    r = &'br **q;
    p = use();
    use(r);
    StorageDead(r);
    StorageDead(q);
    StorageDead(p);
    StorageDead(foo);
}