Function trim_range_offset_len 

pub fn trim_range_offset_len(
    offset: u64,
    len: u64,
    trim_to_len: u64,
) -> Option<u64>

Trims [offset, offset + len) to [0, trim_to_len).

Returns Some(trimmed_len) on success, or None if offset > trim_to_len or on overflow.