fix: dt duration unit incorrectly mapped to ns (#301)

CHANGELOG.md

@@ -24,6 +24,7 @@ Types of changes:
 
 ### Fixed
 - Fixed classical register declarations not being visible inside `box` scope, causing "Missing clbit register declaration" errors for measurement statements inside box blocks. ([#306](https://github.com/qBraid/pyqasm/pull/306))
+- Fixed the backend-dependent `dt` duration unit being incorrectly relabeled as `ns` when unrolling `delay` and `box` statements without a `device_cycle_time`. Since `dt` cannot be converted to SI units without a sample rate, it is now preserved as `dt`. ([#301](https://github.com/qBraid/pyqasm/issues/301))
 
 ### Dependencies
 - Bumped `actions/configure-pages` from 5 to 6. ([#307](https://github.com/qBraid/pyqasm/pull/307))

src/pyqasm/visitor.py

@@ -2800,6 +2800,23 @@ def _evaluate_case(statements):
             default_stmts = statement.default.statements
             return _evaluate_case(default_stmts)
 
+    def _resolve_duration_unit(self, time_var) -> qasm3_ast.TimeUnit:
+        """Determine the output unit for a duration literal.
+
+        `dt` is backend-dependent and not convertible to SI units without a known
+        sample rate. Preserve it when the source unit was `dt` (or a device cycle
+        time is set). SI units are already converted to ns by the evaluator.
+        """
+        source_is_dt = (
+            isinstance(time_var, qasm3_ast.DurationLiteral)
+            and time_var.unit == qasm3_ast.TimeUnit.dt
+        )
+        return (
+            qasm3_ast.TimeUnit.dt
+            if self._module._device_cycle_time or source_is_dt
+            else qasm3_ast.TimeUnit.ns
+        )
+
     def _visit_delay_statement(
         self, statement: qasm3_ast.DelayInstruction
     ) -> list[qasm3_ast.Statement]:
@@ -2828,12 +2845,7 @@ def _visit_delay_statement(
         if duration_val:
             PulseValidator.validate_duration_literal_value(duration_val, statement)
             statement.duration = qasm3_ast.DurationLiteral(
-                duration_val,
-                unit=(
-                    qasm3_ast.TimeUnit.dt
-                    if self._module._device_cycle_time
-                    else qasm3_ast.TimeUnit.ns
-                ),
+                duration_val, unit=self._resolve_duration_unit(_delay_time_var)
             )
 
         if self._scope_manager.in_box_scope():
@@ -2903,11 +2915,7 @@ def _visit_box_statement(self, statement: qasm3_ast.Box) -> list[qasm3_ast.State
                 PulseValidator.validate_duration_literal_value(box_duration_val, statement)
                 statement.duration = qasm3_ast.DurationLiteral(
                     box_duration_val,
-                    unit=(
-                        qasm3_ast.TimeUnit.dt
-                        if self._module._device_cycle_time
-                        else qasm3_ast.TimeUnit.ns
-                    ),
+                    unit=self._resolve_duration_unit(_box_time_var),
                 )
         self._scope_manager.push_scope({})
         self._scope_manager.increment_scope_level()
@@ -2933,7 +2941,7 @@ def _visit_box_statement(self, statement: qasm3_ast.Box) -> list[qasm3_ast.State
             and box_duration_val
             and self._total_delay_duration_in_box > box_duration_val
         ):
-            time_unit = "dt" if self._module._device_cycle_time else "ns"
+            time_unit = self._resolve_duration_unit(_box_time_var).name
             raise_qasm3_error(
                 f"Total delay duration value '{self._total_delay_duration_in_box}{time_unit}' "
                 f"should be less than 'box[{box_duration_val}{time_unit}]' duration.",

tests/qasm3/test_box.py

@@ -102,6 +102,33 @@ def test_delay_instruction_device_time():
     check_unrolled_qasm(dumps(module), expected_qasm)
 
 
+def test_box_dt_unit_preserved():
+    """A ``dt`` box duration must be preserved as ``dt`` (not relabeled ``ns``)
+    when no ``device_cycle_time`` is set, since ``dt`` is backend-dependent.
+    """
+    qasm_str = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[1] q;
+    box[200dt] {
+        delay[100dt] q[0];
+        x q[0];
+    }
+    """
+    expected_qasm = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[1] q;
+    box[200.0dt] {
+      delay[100.0dt] q[0];
+      x q[0];
+    }
+    """
+    module = loads(qasm_str)
+    module.unroll()
+    check_unrolled_qasm(dumps(module), expected_qasm)
+
+
 @pytest.mark.parametrize(
     "qasm_code,error_message,error_span",
     [
@@ -199,6 +226,21 @@ def test_delay_instruction_device_time():
             r"Total delay duration value '20.0ns' should be less than 'box[10.0ns]' duration.",
             r"Error at line 6, column 12",
         ),
+        (
+            """
+            OPENQASM 3.0;
+            include "stdgates.inc";
+            qubit[5] q;
+            qubit[2] q2;
+            box[10dt] {
+              delay[20dt];
+              x q[1];
+              measure q;
+            }
+            """,
+            r"Total delay duration value '20.0dt' should be less than 'box[10.0dt]' duration.",
+            r"Error at line 6, column 12",
+        ),
         (
             """
             OPENQASM 3.0;

tests/qasm3/test_delay.py

@@ -66,6 +66,30 @@ def test_delay_instruction_device_time():
     check_unrolled_qasm(dumps(module), expected_qasm)
 
 
+def test_delay_dt_unit_preserved():
+    """A ``dt`` delay literal must be preserved as ``dt`` (not relabeled ``ns``)
+    when no ``device_cycle_time`` is set, since ``dt`` is backend-dependent. SI
+    units are still converted to ns.
+    """
+    qasm_str = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[2] q;
+    delay[100dt] q[0];
+    delay[2us] q[1];
+    """
+    expected_qasm = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[2] q;
+    delay[100.0dt] q[0];
+    delay[2000.0ns] q[1];
+    """
+    module = loads(qasm_str)
+    module.unroll()
+    check_unrolled_qasm(dumps(module), expected_qasm)
+
+
 @pytest.mark.parametrize(
     "qasm_code,error_message,error_span",
     [