void Method()
{
    bool hasInstanceUsage = false;
    bool hasStaticUsage = false;

    foreach (var identifier in outerClass.DescendantNodes().OfType<IdentifierNameSyntax>())
    {
        var memberSymbol = context.SemanticModel.GetSymbolInfo(identifier);
        if (memberSymbol.Symbol.Equals(propertySymbol))
        {
            var constructor = identifier.Ancestors().OfType<ConstructorDeclarationSyntax>().FirstOrDefault();
            var isInConstructor = constructor != null;
            var isAssignmentExpression = identifier.Ancestors().OfType<AssignmentExpressionSyntax>().FirstOrDefault() != null;

            // Skip anything that isn't a setter
            if (!isAssignmentExpression)
            {
                continue;
            }

            // if it is a setter but outside the constructor, we don't report any diagnostic
            if (!isInConstructor)
            {
                return;
            }

            var isStaticConstructor = context.SemanticModel.GetDeclaredSymbol(constructor).IsStatic;
            if (isStaticConstructor && isStaticProperty)
            {
                hasStaticUsage = true;
            }

            if (!isStaticConstructor && !isStaticProperty)
            {
                hasInstanceUsage = true;
            }
        }
    }

    // We can't set it to readonly if it's set in both the instance and the static constructor
    // We need a NAND operation: either it's never set, it's set in ctor 1 or it's set in ctor 2
    if (!(hasStaticUsage & hasInstanceUsage))
    {
        context.ReportDiagnostic(Diagnostic.Create(Rule, property.Identifier.GetLocation(), propertySymbol.Name));
    }
}