Display

"Write to string" trait, prioritizing readability for an end user.

Derivable: ❌, without crates like derive_more.

When to implement: As-needed, for errors and other types that an end-user will see.

rust

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
#[derive(Debug)]
pub enum NetworkError {
    HttpCode(u16),
    WhaleBitTheUnderseaCable,
}

impl std::fmt::Display for NetworkError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            NetworkError::HttpCode(code) => write!(f, "HTTP Error code {code}"),
            NetworkError::WhaleBitTheUnderseaCable => {
                write!(f, "Whale attack detected – call Ishmael")
            }
        }
    }
}

impl std::error::Error for NetworkError {}

<details> - A trait similar to `Debug`, but with a focus on end-user readability.

Prerequisite for the Error trait.

If implementing for an error type, focus on providing a descriptive error for users and programmers other than you.
Same security considerations as Debug, consider the ways that sensitive data could be exposed in UI or logs.
Types that implement Display automatically have ToString implemented for them.

</details>