Add reno

Author: ElePT

releasenotes/notes/add-defcals-openqasm3-7b3542297a0ff487.yaml

@@ -0,0 +1,40 @@
+---
+features_qasm:
+  - |
+    Added partial support for ``defcal`` symbols in the OpenQASM3 exporter. This enables downstream packages to export custom instructions 
+    that operate on both quantum and classical bits using :meth:`qiskit.qasm3.dumps`. 
+    Users can now define custom instructions (e.g., a ``CustomMeasure`` that acts on a qubit and returns a classical bit) and specify their behavior using 
+    :class:`.DefcalInstruction`. These defcals are passed to the exporter via the ``implicit_defcals`` argument in :meth:`qiskit.qasm3.dumps`.
+
+    For example::
+
+      from qiskit.circuit import Instruction, QuantumCircuit
+      from qiskit.qasm3 import dumps
+      from qiskit.qasm3.exporter import DefcalInstruction, types
+
+      custom_measure = Instruction("measure_2", 1, 1, [])
+      qc = QuantumCircuit(1, 1)
+      qc.h(0)
+      qc.append(custom_measure, [0], [0])
+      qc.measure(0, 0)
+
+      defcals = {
+          "measure_2": DefcalInstruction("measure_2", 0, 1, types.Bool()),
+      }
+
+      out_qasm = dumps(qc, implicit_defcals=defcals)
+      print(out_qasm)
+
+    Would output the following valid OpenQASM3 string::
+
+      OPENQASM 3.0;
+      bit[1] c;
+      qubit[1] q;
+      h q[0];
+      c[0] = measure_2 q[0];
+      c[0] = measure q[0];
+
+    This approach assumes that the grammar definition for the defcal is provided externally (e.g., in a header file), 
+    although such a file is not strictly required for the exporter to function.
+
+