This ticket will introduce three visitor types which derive from ‘MatchExpressionVisitor’, each of which have different responsibilities during error generation:

• ValidationErrorPreVisitor: This type will be responsible for generating errors, where each implementation of visit() will generate an error for a MatchExpression of that type. The implementations of non-leaf MatchExpressions will also allocate either a BSONObjBuilder or a BSONArrayBuilder for its children to use when generating an error.
• ValidationErrorInVisitor: This type will only have real implementations of visit() for non-leaf MatchExpressions, where each implementation is responsible for maintaining any state between visits to their children. For instance, if the current child of a ListOfMatchExpression finishes generating an error, visit() will need to add that error to the list of errors that is being tracked and create a new BSONObjBuilder for the next child error.
• ValidationErrorPostVisitor: This type will do any work needed to wrap up generating an error for the current node.

In addition to the three visitor types above, a ValidationErrorContext type will be introduced which tracks any runtime information needed by each of the visitors to produce a sensible error.