Update a dictionary by transferring another dictionary

destination.update(source) returns Result[(), AllocError], replacing existing values and adding new entries. Existing keys keep their positions; new keys appear in source insertion order:

def merge(destination: &mut dict[str, i64], source: dict[str, i64]) -> Result[(), AllocError]:
    destination.update(source)

def main() -> Result[(), AllocError | IoError]:
    mut scores = {"Ada": 10, "Bea": 20}?
    changes = {"Bea": 25, "Cam": 30, "Ada": 15}?
    merge(&mut scores, changes)?
    print(scores)?
    Ok(())

{"Ada": 15, "Bea": 25, "Cam": 30}

The source is consumed on both outcomes. On allocation failure, the destination stays unchanged and source owners are dropped. Use destination.update(copy(source)?) to keep the source. Storage for every new entry is reserved before any value is replaced. Old destination values are dropped during successful replacement; their destructors keep their own effects. Updating only existing keys needs no new runtime storage. Only same-typed dictionaries are accepted; pair iterables and keyword arguments are deferred.