what does that 'No state variables' restriction in Libraries?

Question

I read some about Libraries restriction that 'No state variables'

but, some solidity code's Libraries have struct type.

As far as I know that(may be wrong) struct type is 'storage' and 'state variables' are values which are permanently stored in contract 'storage'.

so, struct type is 'state variable'?

I'm confused.

Answer 1

A struct is just a type.

struct Foo {
    string bar;
}

There are no state variables yet because no variables have been declared. All I did is define a type.

If I do this:

uint256 myNumber;
Foo myStruct;

now I have two state variables, both existing in storage. One is a uint256, and one is a Foo.