Passing storage of one library to another library

Question

I have two libraries: A and B as well as contract C using them.

Library B, in one of its functions, needs to call a couple of functions defined for A.Storage.

The code below compiles, tests are passing, but I have no idea how should I interpret storage modifier when passing library as a parameter: A.Storage storage a. Is this setup safe? Am I referencing the memory of contract C and not creating a copy of A.Storage?

From what I tested, passing A.Storage memory a does not work, the compiler complains:

TypeError: Member "someFuncDefinedForA" is not available in struct A.Storage memory outside of storage.

library A {
  struct Storage { ... }
}

library B {
  using A for A.Storage;

  struct Storage { ... }

  function myFunc(
      Storage storage self,
      A.Storage storage a // <-- ?
  ) public { 
    // (...)
    a.someFuncDefinedForA();
  }
}

contract C {
  using A for A.Storage;
  using B for B.Storage;

  A.Storage internal a;
  B.Storage internal b;

  function doIt() public {
    b.myFunc(a);
  }
}

Answer 1

Neither B nor A have storage of their own, they use the storage provided by C.

I write a code similar and it works with both storage and memory.

pragma solidity ^0.5.0;

library A {
    
    struct S {
        uint256 a;
    }
    
    function foo(A.S memory a) internal view returns (uint256) {
        return a.a;
    }
}

library B {
    
    using A for A.S;
    
    struct T {
        uint256 b;
    }
    
    function bar(B.T memory t, A.S memory s) internal view returns (uint256) {
        return s.foo() * 1000 + t.b;
    }
}

contract C {
    using B for B.T;
    
    A.S public k;
    B.T public l;
    
    function baz() public returns (uint256) {
        k = A.S(121);
        l = B.T(333);
        return l.bar(k);
    }
    
}