Constant DEFAULT_MAX_NAMESPACE_BINDINGS 

pub const DEFAULT_MAX_NAMESPACE_BINDINGS: usize = 128;

Default limit on the number of xmlns / xmlns:* namespace bindings allowed in scope at once in a NamespaceResolver, not counting the two reserved bindings (xml and xmlns) that are always present.

Real-world XML dialects (XHTML, SVG, SOAP, RSS, RRDP, …) declare a handful of namespaces, almost always on the root element; 128 is significantly more than what most legitimate documents would declare, while bounding both the heap allocated and the cost of prefix resolution (which scans the binding stack).