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#![no_std]

#[cfg(feature = "std")]
extern crate std;

use core::fmt::*;

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

#[cfg(not(feature = "std"))]
/// `Error` is a trait representing the basic expectations for error values,
/// i.e., values of type `E` in [`Result<T, E>`]. Errors must describe
/// themselves through the [`Display`] and [`Debug`] traits, and may provide
/// cause chain information:
///
/// The [`source`] method is generally used when errors cross "abstraction
/// boundaries". If one module must report an error that is caused by an error
/// from a lower-level module, it can allow access to that error via the
/// [`source`] method. This makes it possible for the high-level module to
/// provide its own errors while also revealing some of the implementation for
/// debugging via [`source`] chains.
///
/// [`source`]: trait.Error.html#method.source
pub trait Error: Debug + Display {
    /// The lower-level source of this error, if any.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::fmt;
    ///
    /// #[derive(Debug)]
    /// struct SuperError {
    ///     side: SuperErrorSideKick,
    /// }
    ///
    /// impl fmt::Display for SuperError {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "SuperError is here!")
    ///     }
    /// }
    ///
    /// impl Error for SuperError {
    ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
    ///         Some(&self.side)
    ///     }
    /// }
    ///
    /// #[derive(Debug)]
    /// struct SuperErrorSideKick;
    ///
    /// impl fmt::Display for SuperErrorSideKick {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "SuperErrorSideKick is here!")
    ///     }
    /// }
    ///
    /// impl Error for SuperErrorSideKick {}
    ///
    /// fn get_super_error() -> Result<(), SuperError> {
    ///     Err(SuperError { side: SuperErrorSideKick })
    /// }
    ///
    /// fn main() {
    ///     match get_super_error() {
    ///         Err(e) => {
    ///             println!("Error: {}", e);
    ///             println!("Caused by: {}", e.source().unwrap());
    ///         }
    ///         _ => println!("No error"),
    ///     }
    /// }
    /// ```
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        None
    }

    /// ```
    /// if let Err(e) = "xc".parse::<u32>() {
    ///     // Print `e` itself, no need for description().
    ///     eprintln!("Error: {}", e);
    /// }
    /// ```
    #[deprecated(since = "1.42.0", note = "use `Display`")]
    fn description(&self) -> &str { "description() is deprecated; use Display" }

    #[deprecated(since = "1.33.0", note = "obsoleted by Error::source, which can support downcasting")]
    #[allow(missing_docs)]
    fn cause(&self) -> Option<&dyn Error> { self.source() }
}