How to grant permission directly to a user other than a role? Our client frequently need to grant arbitrary permissions to arbitrary users. It's very boring and fault-prone for our client to create new roles, assign users to roles and then set role permissions.
The module user permissions can grant permissions directly to a user, but the grantee must have the powerful permission "administer permissions", which is not what I want.
Use the user permissions module.  Contrary to your objections to using this module, it is not the grantee who needs the  "administer permissions", but the grantor