You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: flatten_maybe.rs
    path of file: ./repos/serde_with/serde_with/src
    
    the code of the file until where you have to start completion: /// Support deserializing from flattened and non-flattened representation
///
/// When working with different serialization formats, sometimes it is more idiomatic to flatten
/// fields, while other formats prefer nesting. Using `#[serde(flatten)]` only the flattened form
/// is supported.
///
/// This helper creates a function, which support deserializing from either the flattened or the
/// nested 
    
/// # fn main() {
/// // Supports both flattened
/// let j = r#" {"i":1} "#;
/// assert!(serde_json::from_str::<S>(j).is_ok());
/// # // Ensure the t field is not dead code
/// # assert_eq!(serde_json::from_str::<S>(j).unwrap().t.i, 1);
///
/// // and non-flattened versions.
/// let j = r#" {"t":{"i":1}} "#;
/// assert!(serde_json::from_str::<S>(j).is_ok());
///
/// // Ensure that the value is given
/// let j = r#" {} "#;
/// assert!(serde_json::from_str::<S>(j).is_err());
///
/// // and only occurs once, not multiple times.
/// let j = r#" {"i":1,"t":{"i":1}} "#;
/// assert!(serde_json::from_str::<S>(j).is_err());
/// # }