Returning qubit allocated within operation should yield an error

[adrianleh]

Is your feature request related to a problem? Please describe.

When returning qubits allocated within an operation, the compiler does not flag any issues, however the qubit will try to be deallocated. Currently this issue manifests by causing a ReleasedQubitsAreNotInZeroState exception.

A simple example program which causes this behavior can be found below:

operation OneState() : Qubit {
    use q = Qubit();
    X(q);
    return q;
}

Describe the solution you'd like
There is two solutions that I thought of for this issue:

• A check to ensure that returned qubits are still alive and will not be deallocated. Such a check should yield a compiler error/warning
• Create a system, similar to C++ unique pointers, to move a qubit's ownership to the function above the call when returning. This is of course a more extensive suggestion that would likely entail more changes than adding move semantics, as it would require ownership support across the board

Additional context
There is a related issue filed in the runtime: microsoft/qsharp-runtime#984