Enum snafu::guide::examples::backtrace::Error

pub enum Error {
    UsualCase {
        backtrace: Backtrace,
    },
    UsedInTightLoop {
        backtrace: Option<Backtrace>,
    },
    SnafuErrorAsSource {
        source: ConfigFileError,
    },
    SourceErrorDoesNotHaveBacktrace {
        source: Error,
        backtrace: Backtrace,
    },
}

Backtraces aren’t yet supported by the stable Rust compiler. SNAFU provides a stable-compatible way of getting backtraces as well as support for the backtrace support in the nightly compiler. By default, backtraces are disabled. It is expected that the final executable adds SNAFU as a dependency and chooses the appropriate feature flag to enable backtraces.

When using SNAFU to define error types, it’s recommended to start with a Backtrace field on every leaf error variant (those without a source). Backtraces are only captured on failure. Since backtraces are disabled by default, adding them in a library does not force any users to pay the price of backtraces if they are not used; they can be zero cost.

Certain errors are used for flow control. Those 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 would 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 Rust stabilizes backtraces.

use snafu::ErrorCompat;

fn inner_process() -> Result<(), Error> {
    // Complicated logic
}

fn main() {
    if let Err(e) = inner_process() {
        eprintln!("An error occurred: {}", e);
        if let Some(bt) = ErrorCompat::backtrace(&e) {
            eprintln!("{:?}", bt);
        }
    }
}

Variants

UsualCase

Fields

backtrace: Backtrace

The most common leaf error should always include a backtrace field.

UsedInTightLoop

Fields

backtrace: Option<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.

SnafuErrorAsSource

Fields

source: ConfigFileError

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.

SourceErrorDoesNotHaveBacktrace

Fields

source: Error

backtrace: Backtrace

This error wraps another error that does not expose a backtrace. We capture our own backtrace to provide something useful.

Trait Implementations

impl Debug for Error

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for Error

fn fmt(&self, __snafu_display_formatter: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Error for Errorwhere
    Self: Debug + Display,

fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

fn source(&self) -> Option<&(dyn Error + 'static)>

The lower-level source of this error, if any.

fn provide(&'a self, demand: &mut Demand<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type based access to context intended for error reports.

impl ErrorCompat for Error

fn backtrace(&self) -> Option<&Backtrace>

Returns a Backtrace that may be printed.

fn iter_chain(&self) -> ChainCompat<'_>where
    Self: AsErrorSource,

Returns an iterator for traversing the chain of errors, starting with the current error and continuing with recursive calls to Error::source.

impl IntoError<Error> for SnafuErrorAsSourceSnafuwhere
    Error: Error + ErrorCompat,

type Source = ConfigFileError

The underlying error

fn into_error(self, error: Self::Source) -> Error

Combine the information to produce the error

impl IntoError<Error> for SourceErrorDoesNotHaveBacktraceSnafuwhere
    Error: Error + ErrorCompat,

type Source = Error

The underlying error

fn into_error(self, error: Self::Source) -> Error

Combine the information to produce the error

impl IntoError<Error> for UsedInTightLoopSnafuwhere
    Error: Error + ErrorCompat,

type Source = NoneError

The underlying error

fn into_error(self, error: Self::Source) -> Error

Combine the information to produce the error

impl IntoError<Error> for UsualCaseSnafuwhere
    Error: Error + ErrorCompat,

type Source = NoneError

The underlying error

fn into_error(self, error: Self::Source) -> Error

Combine the information to produce the error