pub trait MapVisitor {
    fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error> where S: Serializer;
    fn len(&self) -> Option<usize> { ... }
}
            Required Methods
            
            Provided Methods
            
        Return the length of the map if known.
 
        Implementors
        
    
        
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