Function duckdb_appender_destroy 

pub unsafe extern "C" fn duckdb_appender_destroy(
    appender: *mut duckdb_appender,
) -> duckdb_state

Closes the appender by flushing all intermediate states to the table and destroying it. By destroying it, this function de-allocates all memory associated with the appender. If flushing the data triggers a constraint violation, then all data is invalidated, and this function returns DuckDBError. Due to the destruction of the appender, it is no longer possible to obtain the specific error message with duckdb_appender_error. Therefore, call duckdb_appender_close before destroying the appender, if you need insights into the specific error.

@param appender The appender to flush, close and destroy. @return DuckDBSuccess on success or DuckDBError on failure.