Function validate_truncate_batch 

pub(crate) fn validate_truncate_batch<T: Timestamp>(
    batch: &HollowBatch<T>,
    truncate: &Description<T>,
    any_batch_rewrite: bool,
    validate_part_bounds_on_write: bool,
) -> Result<bool, InvalidUsage<T>>

Validates that batch may be appended under the desc truncate.

Returns whether truncate cuts into batch’s own desc, i.e. whether the batch’s parts may hold updates outside the desc they are registered under. Readers filter such updates out against the registered desc, but write-time part statistics still count them, so callers must record the bit (see RunMeta::bounds_truncated) to keep stats-based accounting from trusting the statistics.