[Contract]Solidityのusing xx for xxを一回分かります.

using A for*;　   # Aの関数を任意の種類に付けて使用します.
using A for BxiとはAの方法をBにつけて使うという意味です.
上記の方式を使うと、私達の契約でB型の変数を定義したら、B.xx()のようにAライブラリの関数を使うことができます.
 
局部例を挙げます.minersは直接has方法を使うことができます.

library Roles {
    struct Role {
        mapping (address => bool) bearer;
    }

    /**
     * @dev Check if an account has this role.
     * @return bool
     */
    function has(Role storage role, address account) internal view returns (bool) {
        require(account != address(0), "Roles: account is the zero address");
        return role.bearer[account];
    }
}

contract MinterRole is Context {
    using Roles for Roles.Role;

    event MinterAdded(address indexed account);
    event MinterRemoved(address indexed account);

    Roles.Role private _minters;

    constructor () internal {
        _addMinter(_msgSender());
    }

    modifier onlyMinter() {
        require(isMinter(_msgSender()), "MinterRole: caller does not have the Minter role");
        _;
    }

    function isMinter(address account) public view returns (bool) {
        return _minters.has(account);
    }
}

 
リンク:https://www.cnblogs.com/farwish/p/12560568.html