Function diff 

pub fn diff(
    target: &Catalog,
    source: &Catalog,
    drift: &DriftReport,
) -> ChangeSet

Compute the changes needed to converge target toward source.

drift captures any NOT VALID constraints or INVALID indexes found in the live database (from crate::catalog::read_catalog). Pass &DriftReport::default() when diffing two source-side catalogs (no live database involved).

The returned ChangeSet is unordered; ordering / dependency analysis is the planner’s responsibility (phase 5).