I'm afraid that just C++ being C++ and you are deep into UB; you can't really modify a constexpr value at runtime, and if you cast away its constness with what is effectively a const cast you are on your own. This will print "0 3" which is obviously nonsense:
constexpr int x = 3;
((int&)x) = 0;
char y[x];
std::print("{}, {}",x, sizeof(y));
You can also move the problematic sequence into a constexpr function and invoke it in a constexpr context: the compiler will reject the cast now.
Enum constants can't become lvalues, so the following also won't compile:
enum : int { x = 3};
((int&)x) = 0;
FWIW, notoriously you could modify numeric literals in early Fortrans and get into similar nonsense UB.
edit: in the end[1] it seems that you take exception with constexpr not being prvalues in C++. I guess it was found more convenient for them to have an address [1]. That doesn't make them less constant.
[1] or at least I think you do, it is not clear to me what you have been trying to claim in this discussion.
[2] C++ will materialize prvalues into temporaries when their address is needed (and give them an unique address), I guess it was thought to be wasteful for large constexpr objects, and avoids the rust pitfall you mentioned.