Rust E0277 — a required trait is not implemented

A real compile-time requirement

A generic operation can require a trait such as Debug, Display, Clone or Send. A type without that trait cannot be used, even if its fields seem straightforward.

// struct Record { number: i32 }
// println!("{:?}", Record { number: 3 }); // E0277: Debug missing

#[derive(Debug)]
struct Record { number: i32 }

fn main() {
    let record = Record { number: 3 };
    println!("{:?}", record);
}

Match the requested trait

Debug and Display are not interchangeable; the former is primarily intended for diagnostic formatting and the latter for user-facing values. A function that requires Display can express that constraint:

fn show<T: std::fmt::Display>(value: T) -> String {
    format!("{value}")
}

Try show(42) and show("Ada"), then try show(Record { number: 3 }) as a compile-fail example until Display is explicitly implemented.

Safety and review

Deriving Debug on a type with secrets can leak data into logs. Before adding a trait, ask whether it expresses the intended API. Do not add unsafe implementations to satisfy thread-bound traits without proving their invariants.

Reference: https://doc.rust-lang.org/error_codes/E0277.html

Implement the missing contract deliberately

Try implementing std::fmt::Display only for the safe public field of the record:

impl std::fmt::Display for Record {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "record #{}", self.number)
    }
}

Now show(Record { number: 3 }) can compile and yield "record #3". Verify this separately from {:?}, which uses Debug. Do not derive Debug for a type that holds credentials unless sensitive members are intentionally omitted from logs. A trait error might also arise from missing bounds on an enclosing generic function; implementing a trait globally is not always the correct API repair.