Can't use on struct with type parameter

```rust
#[derive(Debug, Deserialize)]
struct RpcResponse {
jsonrpc: MustBe!("2.0"),
id: usize,
result: RS,
}
```

generates error:
`derive` cannot be used on items with type macros

Start by reproducing the reported derive invocation with the generic RpcResponse example and inspect where the `derive` cannot be used on items with type macros error is emitted. Done means the shown generic struct can derive Deserialize while retaining the MustBe!("2.0") field constraint.

Written by the indexing model from the issue text.