public override void VisitIdentifierName(IdentifierNameSyntax identifierName, bool lvalue)
 {
     if (!lvalue || identifierName.ReferencedSymbol.MutableBinding)
     {
         return;
     }
     diagnostics.Add(SemanticError.CantAssignToImmutable(function.File, identifierName.Span));
     function.Poison();
 }