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: flattened_maybe.rs
    path of file: ./repos/serde_with/serde_with_test/tests
    
    the code of the file until where you have to start completion: //! Test that flattened_maybe properly names all the types and traits used

// Ensure no prelude is available
#![no_implicit_prelude]
#![allow(dead_code)]

// The macro creates custom deserialization code.
// You need to specify a fun
    
fn flattened_maybe() {
    // Supports both flattened
    let j = r#" {"i":1} "#;
    ::s_json::from_str::<S>(j).unwrap();

    // and non-flattened versions.
    let j = r#" {"t":{"i":1}} "#;
    ::s_json::from_str::<S>(j).unwrap();

    // Ensure that the value is given
    let j = r#" {} "#;
    ::s_json::from_str::<S>(j).unwrap_err();

    // and only occurs once, not multiple times.
    let j = r#" {"i":1,"t":{"i":1}} "#;
    ::s_json::from_str::<S>(j).unwrap_err();
}