How does the new database design used by Bitcrust work?
My understanding after a quick look at the code is that they use this sequential scan of stacked records of spends only for the recent 3 blocks (https://github.com/tomasvdw/bitcrust/blob/556375ccfaa81ca241b25f7b5b7f00fb97d5bd6b/src/store/spend_tree/record.rs#L296)

If the sequential scan did not find a match (if the input is from older blocks), they use this huge 2^58-bit bitmap, that is sparsely mapped to sparse files (if the filesystem does not support sparse files, they will use 16GB of storage for each 'range' that contains at least one bit set)

the code that uses 58-bit index:
https://github.com/tomasvdw/bitcrust/blob/556375ccfaa81ca241b25f7b5b7f00fb97d5bd6b/src/store/spend_index.rs#L67 (more exactly, a 52-bit index to a map of U64 values)

the 'hash' variable there does not contain a true hash, it is just a mapping of the internal storage indexes to 58-bit value: https://github.com/tomasvdw/bitcrust/blob/556375ccfaa81ca241b25f7b5b7f00fb97d5bd6b/src/store/spend_tree/record.rs#L168