presto-main/src/main/java/io/prestosql/execution/DropRoleTask.java/DropRoleTask/execute
class DropRoleTask:    @Override
    public ListenableFuture<?> execute(DropRole statement, TransactionManager transactionManager, Metadata metadata, AccessControl accessControl, QueryStateMachine stateMachine, List<Expression> parameters)
    {
        Session session = stateMachine.getSession();
        String catalog = createCatalogName(session, statement);
        String role = statement.getName().getValue().toLowerCase(ENGLISH);
        accessControl.checkCanDropRole(session.toSecurityContext(), role, catalog);
        Set<String> existingRoles = metadata.listRoles(session, catalog);
        if (!existingRoles.contains(role)) {
            throw semanticException(ROLE_NOT_FOUND, statement, "Role '%s' does not exist", role);
        }
        metadata.dropRole(session, role, catalog);
        return immediateFuture(null);
    }