Support classically controlled gates in Stim

[joao-boechat]

Support classically-controlled gates (rec targets) in the Stim → QIR compiler

What this is (in Stim)

Stim lets certain two-qubit controlled gates take a measurement record target
(rec[-k]) in place of a qubit control. This turns the gate into a
classically-controlled (feed-forward) operation: if the recorded measurement
bit is set, apply the target Pauli to the target qubit.

M 0
CX rec[-1] 1    # if the last measurement was 1, apply X to qubit 1
CZ 0 rec[-1]    # CZ is symmetric, so the rec may go on either side
XCZ 1 rec[-1]   # reversed order: rec on the 2nd target, X applied to qubit 1

A rec is only valid where the gate has a Z-basis control, which limits it to:

Gate (+ aliases)	rec allowed on	Classical action
CX / CNOT / ZCX	1st (control)	X on target
CY / ZCY	1st (control)	Y on target
CZ / ZCZ	either (symmetric)	Z on the other qubit
XCZ	2nd (the Z side)	X on the 1st qubit
YCZ	2nd (the Z side)	Y on the 1st qubit

Docs: Stim gate reference
(see the CX,
CZ, and
XCZ entries).

What changed

Previously these gates went through broadcast_pair, which assumes both targets
are qubits and rejects a rec as an unsupported target. The changes:

• broadcast_controlled — a new dispatch for controlled gates. For each target
  pair it either emits the fully-quantum decomposition (both qubits) or, when a
  target is a rec, emits a classically-controlled operation.
• classical_control — emits the feed-forward pattern (reusing the existing
  REQUIRE machinery): read the measurement result, branch, apply the gate's
  single-qubit Pauli to the target, then rejoin.
• AllowedRecPosition (First / Second / Either) — encodes which side of a
  gate may carry a rec, so CX accepts it only on the control, XCZ only on the
  second target, and CZ on either.
• Call sites — CX/CY/CZ/XCZ/YCZ now call broadcast_controlled, each
  supplying its quantum decomposition and its single-qubit classical action.
• Dedicated diagnostics for the invalid cases, instead of a generic
  "unsupported target":
  • MisplacedMeasurementRecord — rec on a side the gate doesn't allow.
  • NegatedMeasurementRecord — a negated control (!rec[-1]).
  • MeasurementRecordWithoutQubit — both targets are records (no qubit to act on).
• Tests — qir/tests/measurement_record_targets.rs covers every gate and every
  error case.