Function checked_pruning_expr 

pub fn checked_pruning_expr(
    expr: &ExprRef,
    available_stats: &FieldPathSet,
) -> Option<(ExprRef, RequiredStats)>

Build a pruning expr mask, using an existing set of stats. The available stats are provided as a set of FieldPath.

A pruning expression is one that returns true for all positions where the original expression cannot hold, and false if it cannot be determined from stats alone whether the positions can be pruned.

If the falsification logic attempts to access an unknown stat, this function will return None.