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 it's 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

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

Fields of UsualCase

backtrace: Backtrace

UsedInTightLoop

When an error is expected to be created frequently but the backtrace is rarely needed, you can wrap it in an Option. See GenerateBacktrace for instructions on how to access the backtrace in this case.q.

Fields of UsedInTightLoop

backtrace: Option<Backtrace>

SnafuErrorAsSource

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.

Fields of SnafuErrorAsSource

source: ConfigFileError

SourceErrorDoesNotHaveBacktrace

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

Fields of SourceErrorDoesNotHaveBacktrace

source: Errorbacktrace: Backtrace

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 Error where
    Self: Debug + Display, 

fn description(&self) -> &str

Deprecated since 1.42.0:

use the Display impl or to_string()

if let Err(e) = "xc".parse::<u32>() { // Print e itself, no need for description(). eprintln!("Error: {}", e); }

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

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

Returns a stack backtrace, if available, of where this error occurred.

impl ErrorCompat for Error

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

Returns a Backtrace that may be printed.

impl IntoError<Error> for UsualCase where
    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 UsedInTightLoop where
    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 SnafuErrorAsSource where
    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 SourceErrorDoesNotHaveBacktrace where
    Error: Error + ErrorCompat, 

type Source = Error

The underlying error

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

Combine the information to produce the error