It's way too easy to do something like this in c++:

    auto v = vector<int> { 1, 2, 3 };
    auto &e = v[0]; // plan to use element later
    // ...
    v.push_back(v.size());
    // ...
    cout << e << "\n"; // boom

As a real example of this: suppose you're managing a vector of connected users. v is large enough to hold 64 user structs. e is a reference to the last user who produced an event. If a 65th user connects then the vector will need to be reallocated, thereby invalidating e.

Rust saves you here. The compiler will let you have a reference to the element if the vector is marked as immutable. If the vector is marked as mutable the compiler will yell at you and you'll need to store an index instead.