Wax_utils.Source_mapSourceMap generated_offset to the start of original_location.
Map generated_offset to a specific source position — used to attach the closing end opcode of a block or expression to its end position.
add_absent_mapping t ~generated_offset records that the code at generated_offset has no original location, emitting a segment that resets the mapping so the previous location does not bleed into it.
checkpoint t captures the set of mappings recorded so far, for a later shift_since. Mappings are recorded with generated offsets relative to whichever buffer is being encoded; a checkpoint plus shift_since rebases the ones added afterwards once their absolute file offset is known.
shift_since t cp ~delta adds delta to the generated offset of every mapping recorded since the checkpoint cp.