Do not hoist declarations of mutated variables

Declarations/initializations of non-constant variables must not be hoisted from loops because it changes the semantics of the code.

For example:

for (...) {
  int i = 0; // <- rhs is invariant but lhs is not
  for (...) {
    i += 1;
  }
}

Previously got falsly transformed to:

int i = 0;
for (...) {
  for (...) {
    i += 1;
  }
}

src/pystencils/backend/transformations/hoist_loop_invariant_decls.py

@@ -27,6 +27,7 @@ class HoistContext:
     def __init__(self) -> None:
         self.hoisted_nodes: list[PsDeclaration] = []
         self.assigned_symbols: set[PsSymbol] = set()
+        self.mutated_symbols: set[PsSymbol] = set()
         self.invariant_symbols: set[PsSymbol] = set()
 
     def _is_invariant(self, expr: PsExpression) -> bool:
@@ -123,6 +124,7 @@ class HoistLoopInvariantDeclarations:
         """Hoist invariant declarations out of the given loop."""
         hc = HoistContext()
         hc.assigned_symbols.add(loop.counter.symbol)
+        hc.mutated_symbols.add(loop.counter.symbol)
         self._prepare_hoist(loop.body, hc)
         self._hoist_from_block(loop.body, hc)
         return hc
@@ -134,8 +136,12 @@ class HoistLoopInvariantDeclarations:
             case PsExpression():
                 return
 
+            case PsDeclaration(PsSymbolExpr(lhs_symb), _):
+                hc.assigned_symbols.add(lhs_symb)
+
             case PsAssignment(PsSymbolExpr(lhs_symb), _):
                 hc.assigned_symbols.add(lhs_symb)
+                hc.mutated_symbols.add(lhs_symb)
 
             case PsAssignment(_, _):
                 return
@@ -147,6 +153,7 @@ class HoistLoopInvariantDeclarations:
                         loop = stmt
                         nested_hc = self._hoist(loop)
                         hc.assigned_symbols |= nested_hc.assigned_symbols
+                        hc.mutated_symbols |= nested_hc.mutated_symbols
                         statements_new += nested_hc.hoisted_nodes
                         if loop.body.statements:
                             statements_new.append(loop)
@@ -169,7 +176,8 @@ class HoistLoopInvariantDeclarations:
 
         for node in block.statements:
             if isinstance(node, PsDeclaration):
-                if hc._is_invariant(node.rhs):
+                lhs_symb = cast(PsSymbolExpr, node.lhs).symbol
+                if lhs_symb not in hc.mutated_symbols and hc._is_invariant(node.rhs):
                     hc.hoisted_nodes.append(node)
                     hc.invariant_symbols.add(node.declared_symbol)
                 else: