In Stylus, custom types in Solidity (such as struct) are not supported.

[Subway2023]

struct S {
    uint16 a;
    function() external returns (uint) x;
    uint16 b;
}
contract Flow {
    S[2] t;
}

sol_storage! {
    #[entrypoint]
    pub struct Counter {
        struct S {
            uint16 a;
            function() external returns (uint) x;
            uint16 b;
        }
        S[2] t;
    }
}

compile errer

error: expected identifier, found keyword `struct`
  --> src/lib.rs:40:9
   |
40 |         struct S {
   |         ^^^^^^