Qiskit
diff --git a/‎crates/circuit/src/parameter/parameter_expression.rs‎
Lines changed: 87 additions & 85 deletions b/‎crates/circuit/src/parameter/parameter_expression.rs‎
Lines changed: 87 additions & 85 deletions
diff --git a/‎crates/circuit/src/parameter/symbol_expr.rs‎
Lines changed: 11 additions & 5 deletions b/‎crates/circuit/src/parameter/symbol_expr.rs‎
Lines changed: 11 additions & 5 deletions
diff --git a/‎crates/qpy/src/formats.rs‎
Lines changed: 5 additions & 0 deletions b/‎crates/qpy/src/formats.rs‎
Lines changed: 5 additions & 0 deletions
@@ -12,6 +12,7 @@
 
 use hashbrown::hash_map::Entry;
 use hashbrown::{HashMap, HashSet};
+use indexmap::IndexSet;
 use num_complex::Complex64;
 use pyo3::exceptions::{PyRuntimeError, PyTypeError, PyValueError, PyZeroDivisionError};
 use pyo3::types::{IntoPyDict, PyComplex, PyFloat, PyInt, PyNotImplemented, PySet, PyString};
@@ -134,7 +135,62 @@ impl fmt::Display for ParameterExpression {
 impl ParameterExpression {
     pub fn qpy_replay(&self) -> Vec<OPReplay> {
         let mut replay = Vec::new();
-        qpy_replay(self, &self.name_map, &mut replay);
+        let mut unused: IndexSet<_, ahash::RandomState> = self.name_map.values().cloned().collect();
+        // The recursive inner `qpy_replay_inner` assumes it starts from a containing operation, so
+        // fails to build a complete replay in the case it starts from a single symbol or value.
+        match &self.expr {
+            SymbolExpr::Value(v) => {
+                let item = match *v {
+                    Value::Int(v) => OPReplay {
+                        op: OpCode::ADD,
+                        lhs: Some(ParameterValueType::Int(v)),
+                        rhs: Some(ParameterValueType::Int(0)),
+                    },
+                    Value::Real(v) => OPReplay {
+                        op: OpCode::ADD,
+                        lhs: Some(ParameterValueType::Float(v)),
+                        // `-0.0` is technically the identity element of floating-point addition;
+                        // `0.0 + x` is not bit-for-bit equal to `x` solely if `x` is `-0.0`.
+                        rhs: Some(ParameterValueType::Float(-0.0)),
+                    },
+                    Value::Complex(v) => OPReplay {
+                        op: OpCode::ADD,
+                        lhs: Some(ParameterValueType::Complex(v)),
+                        rhs: Some(ParameterValueType::Complex(Complex64 {
+                            re: -0.0,
+                            im: -0.0,
+                        })),
+                    },
+                };
+                replay.push(item);
+            }
+            SymbolExpr::Symbol(sym) => {
+                unused.swap_remove(sym.as_ref());
+                replay.push(OPReplay {
+                    op: OpCode::ADD,
+                    lhs: ParameterValueType::extract_from_expr(&self.expr),
+                    rhs: Some(ParameterValueType::Int(0)),
+                });
+            }
+            SymbolExpr::Unary { .. } | SymbolExpr::Binary { .. } => {
+                qpy_replay_inner(self, &self.name_map, &mut replay, &mut unused);
+            }
+        }
+        // For any unused symbols, we'll add something like `(x * 0) + expr`.  This sort of
+        // cancellation is how unused symbols appear; it doesn't matter if the _actual_ cause was
+        // `x - x` or whatever, because the end observable effect is the same.
+        for symbol in unused {
+            replay.push(OPReplay {
+                op: OpCode::MUL,
+                lhs: Some(ParameterValueType::from_symbol(symbol)),
+                rhs: Some(ParameterValueType::Int(0)),
+            });
+            replay.push(OPReplay {
+                op: OpCode::ADD,
+                lhs: None,
+                rhs: None,
+            });
+        }
         replay
     }
 }
@@ -188,21 +244,16 @@ impl ParameterExpression {
     ///
     /// This only succeeds if the underlying expression is, in fact, only a symbol.
     pub fn try_to_symbol(&self) -> Result<Symbol, ParameterError> {
-        if let SymbolExpr::Symbol(symbol) = &self.expr {
-            Ok(symbol.as_ref().clone())
-        } else {
-            Err(ParameterError::NotASymbol)
-        }
+        self.try_to_symbol_ref().cloned()
     }
 
     /// Try casting to a [Symbol], returning a reference.
     ///
     /// This only succeeds if the underlying expression is, in fact, only a symbol.
     pub fn try_to_symbol_ref(&self) -> Result<&Symbol, ParameterError> {
-        if let SymbolExpr::Symbol(symbol) = &self.expr {
-            Ok(symbol.as_ref())
-        } else {
-            Err(ParameterError::NotASymbol)
+        match &self.expr {
+            SymbolExpr::Symbol(sym) if self.name_map.len() == 1 => Ok(sym.as_ref()),
+            _ => Err(ParameterError::NotASymbol),
         }
     }
 
@@ -257,17 +308,10 @@ impl ParameterExpression {
     }
 
     /// Load from a sequence of [OPReplay]s. Used in serialization.
-    pub fn from_qpy(
-        replay: &[OPReplay],
-        subs_operations: Option<Vec<(usize, HashMap<Symbol, ParameterExpression>)>>,
-    ) -> Result<Self, ParameterError> {
+    pub fn from_qpy(replay: &[OPReplay]) -> Result<Self, ParameterError> {
         // the stack contains the latest lhs and rhs values
         let mut stack: Vec<ParameterExpression> = Vec::new();
-        let subs_operations = subs_operations.unwrap_or_default();
-        let mut current_sub_operation = subs_operations.len(); // we avoid using a queue since we only make one pass anyway
-        for (i, inst) in replay.iter().enumerate() {
-            let OPReplay { op, lhs, rhs } = inst;
-
+        for OPReplay { op, lhs, rhs } in replay {
             // put the values on the stack, if they exist
             if let Some(value) = lhs {
                 stack.push(value.clone().into());
@@ -312,15 +356,6 @@ impl ParameterExpression {
                 }
             };
             stack.push(result);
-            //now check whether any substitutions need to be applied at this stage
-            while current_sub_operation > 0 && subs_operations[current_sub_operation - 1].0 == i + 1
-            {
-                if let Some(exp) = stack.pop() {
-                    let sub_exp = exp.subs(&subs_operations[current_sub_operation - 1].1, false)?;
-                    stack.push(sub_exp);
-                }
-                current_sub_operation -= 1;
-            }
         }
 
         // once we're done, just return the last element in the stack
@@ -1370,49 +1405,18 @@ impl PyParameterExpression {
         }
     }
 
-    fn __getstate__(&self) -> PyResult<(Vec<OPReplay>, Option<ParameterValueType>)> {
-        // We distinguish in two cases:
-        //  (a) This is indeed an expression which can be rebuild from the QPY replay. This means
-        //      the replay is *not empty* and it contains all symbols.
-        //  (b) This expression is in fact only a Value or a Symbol. In this case, the QPY replay
-        //      will be empty and we instead pass a `ParameterValueType` for reconstruction.
-        let qpy = self._qpy_replay()?;
-        if !qpy.is_empty() {
-            Ok((qpy, None))
-        } else {
-            let value = ParameterValueType::extract_from_expr(&self.inner.expr);
-            if value.is_none() {
-                Err(PyValueError::new_err(format!(
-                    "Failed to serialize the parameter expression: {self:?}"
-                )))
-            } else {
-                Ok((qpy, value))
-            }
-        }
+    fn __getstate__(&self) -> Vec<OPReplay> {
+        self._qpy_replay()
     }
 
-    fn __setstate__(&mut self, state: (Vec<OPReplay>, Option<ParameterValueType>)) -> PyResult<()> {
-        // if there a replay, load from the replay
-        if !state.0.is_empty() {
-            let from_qpy = ParameterExpression::from_qpy(&state.0, None)?;
-            self.inner = from_qpy;
-        // otherwise, load from the ParameterValueType
-        } else if let Some(value) = state.1 {
-            let expr = ParameterExpression::from(value);
-            self.inner = expr;
-        } else {
-            return Err(PyValueError::new_err(
-                "Failed to read QPY replay or extract value.",
-            ));
-        }
+    fn __setstate__(&mut self, state: Vec<OPReplay>) -> PyResult<()> {
+        self.inner = ParameterExpression::from_qpy(&state)?;
         Ok(())
     }
 
     #[getter]
-    fn _qpy_replay(&self) -> PyResult<Vec<OPReplay>> {
-        let mut replay = Vec::new();
-        qpy_replay(&self.inner, &self.inner.name_map, &mut replay);
-        Ok(replay)
+    fn _qpy_replay(&self) -> Vec<OPReplay> {
+        self.inner.qpy_replay()
     }
 }
 
@@ -1872,6 +1876,13 @@ pub enum ParameterValueType {
 }
 
 impl ParameterValueType {
+    fn from_symbol(symbol: Symbol) -> Self {
+        if symbol.index.is_some() {
+            Self::VectorElement(PyParameterVectorElement { symbol })
+        } else {
+            Self::Parameter(PyParameter { symbol })
+        }
+    }
     fn extract_from_expr(expr: &SymbolExpr) -> Option<ParameterValueType> {
         if let Some(value) = expr.eval(true) {
             match value {
@@ -1880,20 +1891,7 @@ impl ParameterValueType {
                 Value::Complex(c) => Some(ParameterValueType::Complex(c)),
             }
         } else if let SymbolExpr::Symbol(symbol) = expr {
-            match symbol.index {
-                None => {
-                    let param = PyParameter {
-                        symbol: symbol.as_ref().clone(),
-                    };
-                    Some(ParameterValueType::Parameter(param))
-                }
-                Some(_) => {
-                    let param = PyParameterVectorElement {
-                        symbol: symbol.as_ref().clone(),
-                    };
-                    Some(ParameterValueType::VectorElement(param))
-                }
-            }
+            Some(Self::from_symbol(symbol.as_ref().clone()))
         } else {
             // ParameterExpressions have the value None, as they must be constructed
             None
@@ -2075,14 +2073,18 @@ fn filter_name_map(
     }
 }
 
-pub fn qpy_replay(
+fn qpy_replay_inner(
     expr: &ParameterExpression,
     name_map: &HashMap<String, Symbol>,
     replay: &mut Vec<OPReplay>,
+    unused: &mut IndexSet<Symbol, ahash::RandomState>,
 ) {
     match &expr.expr {
-        SymbolExpr::Value(_) | SymbolExpr::Symbol(_) => {
-            // nothing to do here, we only need to traverse instructions
+        // This function is written under the assumption that the top-level expression involves an
+        // operation, since `OPReplay` items correspond to operations that own their operands.
+        SymbolExpr::Value(_) => (),
+        SymbolExpr::Symbol(sym) => {
+            unused.swap_remove(sym.as_ref());
         }
         SymbolExpr::Unary { op, expr } => {
             let op = match op {
@@ -2103,7 +2105,7 @@ pub fn qpy_replay(
             let lhs = filter_name_map(expr, name_map);
 
             // recurse on the instruction
-            qpy_replay(&lhs, name_map, replay);
+            qpy_replay_inner(&lhs, name_map, replay, unused);
 
             let lhs_value = ParameterValueType::extract_from_expr(expr);
 
@@ -2129,8 +2131,8 @@ pub fn qpy_replay(
             // recurse on the parameter expressions
             let lhs = filter_name_map(lhs, name_map);
             let rhs = filter_name_map(rhs, name_map);
-            qpy_replay(&lhs, name_map, replay);
-            qpy_replay(&rhs, name_map, replay);
+            qpy_replay_inner(&lhs, name_map, replay, unused);
+            qpy_replay_inner(&rhs, name_map, replay, unused);
 
             // add the expression to the replay
             match lhs_value {
 
@@ -13,6 +13,7 @@
 use hashbrown::HashMap;
 use pyo3::IntoPyObjectExt;
 use pyo3::exceptions::PyValueError;
+use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::cmp::PartialOrd;
 use std::convert::From;
@@ -123,12 +124,17 @@ impl Symbol {
         &self.name
     }
 
+    pub fn fullname(&self) -> Cow<'_, str> {
+        self.index
+            .map(|i| Cow::Owned(format!("{}[{}]", &self.name, i)))
+            .unwrap_or(Cow::Borrowed(&self.name))
+    }
+
     pub fn repr(&self, with_uuid: bool) -> String {
-        match (self.index, with_uuid) {
-            (Some(i), true) => format!("{}[{}]_{}", self.name, i, self.uuid.as_u128()),
-            (Some(i), false) => format!("{}[{}]", self.name, i),
-            (None, true) => format!("{}_{}", self.name, self.uuid.as_u128()),
-            (None, false) => self.name.clone(),
+        if with_uuid {
+            format!("{}_{}", self.fullname(), self.uuid.as_u128())
+        } else {
+            self.fullname().into_owned()
         }
     }
     pub fn is_vector_element(&self) -> bool {
 
@@ -661,6 +661,11 @@ pub enum ParameterExpressionSymbolPack {
     Parameter(ParameterExpressionParameterSymbolPack),
     #[brw(magic = b'v')]
     ParameterVector(ParameterExpressionParameterVectorSymbolPack),
+    /// This variant _should not_ exist; it is counter to the QPY spec, and has no semantic meaning.
+    /// However, Qiskit 2.0 (with QPY 13 non-symengine serialisation but before Rust-space
+    /// `ParameterExpression` or QPY) would populate the "symbol map" with the raw dictionaries
+    /// given to `ParameterExpression.subs` calls, which include expressions.  The equivalent "read"
+    /// code would load up the entries, then immediately filter them out to make the symbol map.
     #[brw(magic = b'e')]
     ParameterExpression(ParameterExpressionParameterExpressionSymbolPack),
 }