Fix bug preventing display of circuits where same qubit measured more…

… than once (#1939)

The logic for returning the count of measurements incorrectly reported
every qubit as being measured only once no matter how many times it was
measured. This was breaking circuit display for non-base profile
programs.

compiler/qsc/src/interpret/circuit_tests.rs

@@ -72,6 +72,36 @@ fn one_gate() {
     .assert_eq(&circ.to_string());
 }
 
+#[test]
+fn measure_same_qubit_twice() {
+    let mut interpreter = interpreter(
+        r"
+            namespace Test {
+                @EntryPoint()
+                operation Main() : Result[] {
+                    use q = Qubit();
+                    H(q);
+                    let r1 = M(q);
+                    let r2 = M(q);
+                    [r1, r2]
+                }
+            }
+        ",
+        Profile::Unrestricted,
+    );
+
+    let circ = interpreter
+        .circuit(CircuitEntryPoint::EntryPoint, false)
+        .expect("circuit generation should succeed");
+
+    expect![["
+        q_0    ── H ──── M ──── M ──
+                         ╘══════╪═══
+                                ╘═══
+    "]]
+    .assert_eq(&circ.to_string());
+}
+
 #[test]
 fn toffoli() {
     let mut interpreter = interpreter(

compiler/qsc_circuit/src/builder.rs

@@ -59,8 +59,6 @@ impl Backend for Builder {
             let mapped_q = self.map(q);
             // In the Circuit schema, result id is per-qubit
             let res_id = self.num_measurements_for_qubit(mapped_q);
-            // We don't actually need the Remapper since we're not
-            // remapping any qubits, but it's handy for keeping track of measurements
             let id = self.remapper.m(q);
 
             self.push_gate(measurement_gate(mapped_q.0, res_id));
@@ -249,21 +247,6 @@ impl Builder {
         self.circuit.operations.push(gate);
     }
 
-    fn num_measurements_by_qubit(&self) -> IndexMap<usize, usize> {
-        self.remapper.qubit_measurement_counts.iter().fold(
-            IndexMap::default(),
-            |mut map: IndexMap<usize, usize>, (q, _)| {
-                match map.get_mut(q.0) {
-                    Some(rs) => *rs += 1,
-                    None => {
-                        map.insert(q.0, 1);
-                    }
-                }
-                map
-            },
-        )
-    }
-
     fn num_measurements_for_qubit(&self, qubit: WireId) -> usize {
         self.remapper
             .qubit_measurement_counts
@@ -273,19 +256,17 @@ impl Builder {
     }
 
     fn finish_circuit(&self, mut circuit: Circuit) -> Circuit {
-        let by_qubit = self.num_measurements_by_qubit();
-
         // add deferred measurements
         if self.config.base_profile {
-            for (qubit, _) in &by_qubit {
+            for (qubit, _) in &self.remapper.qubit_measurement_counts {
                 // guaranteed one measurement per qubit, so result is always 0
-                circuit.operations.push(measurement_gate(qubit, 0));
+                circuit.operations.push(measurement_gate(qubit.0, 0));
             }
         }
 
         // add qubit declarations
         for i in 0..self.remapper.num_qubits() {
-            let num_measurements = by_qubit.get(i).map_or(0, |c| *c);
+            let num_measurements = self.num_measurements_for_qubit(WireId(i));
             circuit.qubits.push(crate::circuit::Qubit {
                 id: i,
                 num_children: num_measurements,