Constant HARNESS_TEMPLATE 

pub const HARNESS_TEMPLATE: &str = r#"use serde_json::Value;

{user_code}

fn main() {
    let input: Value = serde_json::from_reader(std::io::stdin()).unwrap_or(Value::Null);
    let output = run(input);
    println!("{}", serde_json::to_string(&output).unwrap());
}
"#;

The harness template that wraps user code.

The user provides fn run(input: serde_json::Value) -> serde_json::Value. The harness reads JSON from stdin, calls run(), and prints JSON to stdout.

Available to User Code

• serde_json::Value (imported at top level)
• All public items from serde_json (e.g., serde_json::json!, serde_json::Map)
• The full Rust standard library

Not Available

• External crates other than serde_json
• fn main() (provided by the harness)
• Crate-level attributes (#![...])
• Multi-file modules