serde 1.0.100

on Rust Cargo

Provide serde::ser::StdError and serde::de::StdError which are either a re-export of std::error::Error (if Serde's "std" feature is enabled) or a new identical trait (otherwise).

#[cfg(feature = "std")]
pub use std::error::Error as StdError;

#[cfg(not(feature = "std"))]
pub trait StdError: Debug + Display {
    fn source(&self) -> Option<&(StdError + 'static)> { None }
}

Serde's error traits serde::ser::Error and serde::de::Error require std::error::Error as a supertrait, but only when Serde is built with "std" enabled. Data formats that don't care about no_std support should generally provide their error types with a std::error::Error impl directly:

#[derive(Debug)]
struct MySerError {...}

impl serde::ser::Error for MySerError {...}

impl std::fmt::Display for MySerError {...}

// We don't support no_std!
impl std::error::Error for MySerError {}

Data formats that do support no_std may either have a "std" feature of their own as has been required in the past:

[features]
std = ["serde/std"]

#[cfg(feature = "std")]
impl std::error::Error for MySerError {}

... or else now may provide the std Error impl unconditionally via Serde's re-export:

impl serde::ser::StdError for MySerError {}