snafu 0.9.0

//! Exposing complete backtraces to the location of the error.
//!
//! Start by looking at the error type [`Error`].

use crate::{Snafu, Backtrace, ErrorCompat, GenerateImplicitData};

/// Rust 1.65 stabilized the [`std::backtrace::Backtrace`] type, but
/// there's not yet a stable abstraction for accessing a backtrace
/// from an arbitrary error value. SNAFU provides a stable-compatible
/// way of accessing backtraces on a SNAFU-created error type. SNAFU
/// also supports environments where backtraces are not available,
/// such as `no_std` projects.
///
/// When defining error types which include backtraces, it's
/// recommended to start with a [`Backtrace`] field on every leaf
/// error variant (those without a `source`). Backtraces are only
/// captured on failure.
///
/// Certain errors are used for flow control. These don't need a
/// backtrace as they don't represent actual failures. However,
/// sometimes an error is *mostly* used for flow control but might
/// also indicate an error. In those cases, you can use
/// `Option<Backtrace>` to avoid capturing a backtrace unless an
/// environment variable is set by the end user to provide additional
/// debugging.
///
/// For variants that do have a source, you need to evaluate if the
/// source error provides a backtrace of some kind. If it is another
/// SNAFU error, for example, you can *delegate* retrieval of the
/// backtrace to the source error. If the source error doesn't provide
/// its own backtrace, you should capture your own backtrace. This
/// backtrace will not be as useful as one captured by the source
/// error, but it's as useful as you can get.
///
/// When you wish to display the backtrace of an error, you can use
/// the [`ErrorCompat::backtrace`] method. It's recommended to always
/// use this in the fully-qualified form so it will be easy to find
/// and replace when there's a stable way to access backtraces.
///
/// ```
/// # use snafu::guide::examples::backtrace::*;
/// use snafu::ErrorCompat;
///
/// fn inner_process() -> Result<(), Error> {
///     // Complicated logic
///     # UsualCaseSnafu.fail()
/// }
///
/// fn main() {
///     if let Err(e) = inner_process() {
///         eprintln!("An error occurred: {}", e);
///         if let Some(bt) = ErrorCompat::backtrace(&e) {
///             eprintln!("{:?}", bt);
///         }
///     }
/// }
/// ```
#[derive(Debug, Snafu)]
// This line is only needed to generate documentation; it is not
// needed in most cases:
#[snafu(crate_root(crate), visibility(pub))]
pub enum Error {
    /// The most common case: leaf errors should always include a
    /// backtrace field.
    UsualCase {
        backtrace: Backtrace,
    },

    /// When an error is expected to be created frequently but the
    /// backtrace is rarely needed, you can wrap it in an
    /// `Option`. See [the instructions][] on how to access the
    /// backtrace in this case.
    ///
    /// [the instructions]: GenerateImplicitData#impl-GenerateImplicitData-for-Option<Backtrace>
    UsedInTightLoop {
        backtrace: Option<Backtrace>,
    },

    /// This error wraps another error that already has a
    /// backtrace. Instead of capturing our own, we forward the
    /// request for the backtrace to the inner error. This gives a
    /// more accurate backtrace.
    SnafuErrorAsSource {
        #[snafu(backtrace)]
        source: ConfigFileError,
    },

    /// This error wraps another error that does not expose a
    /// backtrace. We capture our own backtrace to provide something
    /// useful.
    SourceErrorDoesNotHaveBacktrace {
        source: std::io::Error,
        backtrace: Backtrace,
    },
}

/// This is a placeholder example and can be ignored.
#[derive(Debug, Snafu)]
#[snafu(crate_root(crate))]
pub enum ConfigFileError {
    Dummy { backtrace: Backtrace },
}