fn reduce_pre(e: &mut MirScalarExpr, column_types: &[ReprColumnType])Expand description
Pre-order rewrites, applied before children are visited.
IsNull and Not need to fire pre-order: if they push themselves inward
(e.g. Not(Not(x)) → x), the result is the new node at this position,
which the visitor will then descend into for normal post-order handling.