Use after free bug in lru crate about lru-rs

[jeromefroe]

Thanks for the bug report @oherrala! I just made #121 to address the issue. Your suspicion is exactly correct, the example code shouldn't be allowed to compile because we can't modify the cache while we also have an iterator.

[oherrala]

Thanks for quick fix @jeromefroe! I tested #121 and it indeed now gives compiler error for my test code:

error[E0502]: cannot borrow `cache` as mutable because it is also borrowed as immutable
  --> examples/use-after-free.rs:13:9
   |
12 |     for (key, value) in cache.into_iter() {
   |                         -----------------
   |                         |
   |                         immutable borrow occurs here
   |                         immutable borrow later used here
13 |         cache.pop(key);
   |         ^^^^^^^^^^^^^^ mutable borrow occurs here

I think this is worth reporting to RustSec after release.