PR #1497: Fixing Misleading Qubit Leak Error Messages

In my latest Quantinuum Guppy contribution, I helped fix a subtle but confusing error message.

When a qubit leaked (i.e. went out of scope without being consumed), Guppy would report an error, but in some cases the message was misleading — incorrectly blaming a branch condition when the qubit was actually leaked unconditionally.

The root cause was that Guppy’s compiler wasn’t distinguishing between two different ways of using a qubit: borrowing it temporarily (e.g. applying a gate) versus consuming it for good (e.g. measuring it). This caused the error message to point at the wrong place.

In PR #1497, I fixed this so the compiler correctly identifies whether a qubit was truly consumed on each branch of the code before deciding how to phrase the error. The result is a more accurate message that points developers straight to the real problem.

Thanks to the Guppy contributors at Quantinuum for their support.

For more technical details, refer to PR #1497.