Derive Macro snafu::Snafu

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#[derive(Snafu)]
{
    // Attributes available to this derive:
    #[snafu]
}

Expand description

The Snafu macro is the entrypoint to defining your own error types. It is designed to require little configuration for the recommended and typical usecases while still offering flexibility for unique situations.

backtrace
context
crate_root
display
implicit
module
source
visibility
whatever

Attribute cheat sheet

Use this as a quick reminder of what each attribute can do and where it is valid. Detailed information on each attribute is below.

Enum

Option (inside `#[snafu(...)]`)	Description
`visibility(V)`	Sets the default visibility of the generated context selectors to `V` (e.g. `pub`)
`module`	Puts the generated context selectors into a module (module name is the enum name converted to `snake_case`)
`module(N)`	Same as above, but with the module named `N` instead
`context(suffix(N))`	Changes the default context selector suffix from `Snafu` to `N`
`crate_root(C)`	Generated code refers to a crate named `C` instead of the default `snafu`

Enum variant or struct

Option (inside `#[snafu(...)]`)	Description
`display("{field:?}: {}", foo)`	Sets the display implementation for this error variant using `format_args!` syntax. If this is omitted, the default is `"VariantName: {source}"` if there is a source or `"VariantName"` if not
`context(false)`	Skips creation of the context selector, implements `From` for the mandatory source error
`context(suffix(N))`	Changes the suffix of the generated context selector to `N`
`context(suffix(false))`	No suffix for the generated context selector
`visibility(v)`	Sets the visibility of the generated context selector to `v` (e.g. `pub`)
`visibility`	Resets visibility back to private
`whatever`	Stringly-typed error. Message field must be called `message`. Source optional, but if present must be of a specific format

Context fields

Option (inside `#[snafu(...)]`)	Description
`source`	Marks a field as the source error (even if not called `source`)
`source(from(type, transform))`	As above, plus converting from `type` to the field type by calling `transform`
`source(false)`	Marks a field that is named `source` as a regular field
`backtrace`	Marks a field as backtrace (even if not called `backtrace`)
`backtrace(false)`	Marks a field that is named `backtrace` as a regular field
`implicit`	Marks a field as implicit (Type needs to implement `GenerateImplicitData`)

Controlling `Display`

You can specify how the Display trait will be implemented for each variant. The argument is a format string and the arguments. All of the fields of the variant will be available and you can call methods on them, such as filename.display(). As an extension to the current format string capabilities, a shorthand is available for named arguments that match a field.

Example

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(display("{username} may not log in until they pay USD {amount:E}"))]
    UserMustPayForService { username: String, amount: f32 },
}
fn main() {
    assert_eq!(
        UserMustPayForServiceSnafu {
            username: "Stefani",
            amount: 1_000_000.0,
        }
        .build()
        .to_string(),
        "Stefani may not log in until they pay USD 1E6",
    );
}

The default `Display` implementation

It is recommended that you provide a value for snafu(display), but if it is omitted, the summary of the documentation comment will be used. If that is not present, the name of the variant will be used.

#[derive(Debug, Snafu)]
enum Error {
    /// No user available.
    /// You may need to specify one.
    MissingUser,
    MissingPassword,
}

fn main() {
    assert_eq!(
        MissingUserSnafu.build().to_string(),
        "No user available. You may need to specify one.",
    );
    assert_eq!(MissingPasswordSnafu.build().to_string(), "MissingPassword");
}

Controlling context

Changing the context selector suffix

When context selectors are generated, any Error suffix is removed and the suffix Snafu is added by default. If you’d prefer a different suffix, such as Ctx or Context, you can specify that with #[snafu(context(suffix(SomeIdentifier)))]. If you’d like to disable the suffix entirely, you can use #[snafu(context(suffix(false)))].

Example

#[derive(Debug, Snafu)]
enum Error {
    UsesTheDefaultSuffixError,

    #[snafu(context(suffix(Ctx)))]
    HasAnotherSuffix,

    #[snafu(context(suffix(false)))]
    DoesNotHaveASuffix,
}

fn my_code() -> Result<(), Error> {
    UsesTheDefaultSuffixSnafu.fail()?;

    HasAnotherSuffixCtx.fail()?;

    DoesNotHaveASuffix.fail()?;

    Ok(())
}

#[snafu(context(suffix))] can be specified on an enum as the default suffix for variants of the enum. In that case, if you wish to have one variant with a suffix, you will need to express it explicitly with #[snafu(context(suffix(SomeIdentifier)))].

Disabling the context selector

Sometimes, an underlying error can only occur in exactly one context and there’s no additional information that can be provided to the caller. In these cases, you can use #[snafu(context(false))] to indicate that no context selector should be created. This allows using the ? operator directly on the underlying error.

Please think about your end users before making liberal use of this feature. Adding context to an error is often what distinguishes an actionable error from a frustrating one.

Example

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(context(false))]
    NeedsNoIntroduction { source: VeryUniqueError },
}

fn my_code() -> Result<i32, Error> {
    let val = do_something_unique()?;
    Ok(val + 10)
}

fn do_something_unique() -> Result<i32, VeryUniqueError> {
    // ...
}

Controlling visibility

By default, each of the context selectors and their inherent methods will be private. It is our opinion that each module should have one or more error types that are scoped to that module, reducing the need to deal with unrelated errors when matching and increasing cohesiveness.

If you need to access the context selectors from outside of their module, you can use the #[snafu(visibility)] attribute. This can be applied to the error type as a default visibility or to specific context selectors.

There are multiple forms of the attribute:

#[snafu(visibility(X))]

X is a normal Rust visibility modifier (pub, pub(crate), pub(in some::path), etc.).
#[snafu(visibility)] will reset back to private visibility.

#[derive(Debug, Snafu)]
#[snafu(visibility(pub(crate)))] // Sets the default visibility for these context selectors
pub(crate) enum Error {
    IsPubCrate, // Uses the default
    #[snafu(visibility)]
    IsPrivate, // Will be private
}

It should be noted that API stability of context selectors is not guaranteed. Therefore, exporting them in a crate’s public API could cause semver breakage for such crates, should SNAFU internals change.

Placing context selectors in modules

When you have multiple error enums that would generate conflicting context selectors, you can choose to place the context selectors into a module using snafu(module):

use snafu::prelude::*;

#[derive(Debug, Snafu)]
#[snafu(module)]
enum ReadError {
    Opening,
}

fn example() -> Result<(), ReadError> {
    read_error::OpeningSnafu.fail()
}

#[derive(Debug, Snafu)]
enum WriteError {
    Opening, // Would conflict if `snafu(module)` was not used above.
}

By default, the module name will be the snake_case equivalent of the enum name. You can override the default by providing an argument to #[snafu(module(...))]:

use snafu::prelude::*;

#[derive(Debug, Snafu)]
#[snafu(module(read))]
enum ReadError {
    Opening,
}

fn example() -> Result<(), ReadError> {
    read::OpeningSnafu.fail()
}

As placing the context selectors in a module naturally namespaces them, you may wish to combine this option with #[snafu(context(suffix(false)))]:

use snafu::prelude::*;

#[derive(Debug, Snafu)]
#[snafu(module, context(suffix(false)))]
enum ReadError {
    Opening,
}

fn example() -> Result<(), ReadError> {
    read_error::Opening.fail()
}

The generated module starts with use super::*, so any types or traits used by the context selectors need to be in scope — complicated paths may need to be simplified or made absolute.

By default, the visibility of the generated module will be private while the context selectors inside will be pub(super). Using #[snafu(visibility)] to control the visibility will change the visibility of both the module and the context selectors.

Controlling error sources

Selecting the source field

If your error enum variant contains other errors but the field cannot be named source, or if it contains a field named source which is not actually an error, you can use #[snafu(source)] to indicate if a field is an underlying cause or not:

#[derive(Debug, Snafu)]
enum Error {
    SourceIsNotAnError {
        #[snafu(source(false))]
        source: String,
    },

    CauseIsAnError {
        #[snafu(source)]
        cause: another::Error,
    },
}

Transforming the source

If your error type contains an underlying cause that needs to be transformed, you can use #[snafu(source(from(...)))]. This takes two arguments: the real type and an expression to transform from that type to the type held by the error.

#[derive(Debug, Snafu)]
enum Error {
    SourceNeedsToBeBoxed {
        #[snafu(source(from(another::Error, Box::new)))]
        source: Box<another::Error>,
    },
}

#[derive(Debug, Snafu)]
#[snafu(source(from(Error, Box::new)))]
struct ApiError(Box<Error>);

Note: If you specify #[snafu(source(from(...)))] then the field will be treated as a source, even if it’s not named “source” - in other words, #[snafu(source(from(...)))] implies #[snafu(source)].

Controlling backtraces

If your error enum variant contains a backtrace but the field cannot be named backtrace, or if it contains a field named backtrace which is not actually a backtrace, you can use #[snafu(backtrace)] to indicate if a field is actually a backtrace or not:

#[derive(Debug, Snafu)]
enum Error {
    BacktraceIsNotABacktrace {
        #[snafu(backtrace(false))]
        backtrace: bool,
    },

    TraceIsABacktrace {
        #[snafu(backtrace)]
        trace: Backtrace,
    },
}

If your error contains other SNAFU errors which can report backtraces, you may wish to delegate returning a backtrace to those errors. To specify this, use #[snafu(backtrace)] on the source field representing the other error:

#[derive(Debug, Snafu)]
enum Error {
    MyError {
        #[snafu(backtrace)]
        source: another::Error,
    },
}

Controlling implicitly generated data

Sometimes, you can capture contextual error data without needing any arguments. Backtraces are a common example, but other global information like the current time or thread ID could also be useful. In these cases, you can use #[snafu(implicit)] on a field that implements GenerateImplicitData to remove the need to specify that data at error construction time:

use snafu::prelude::*;
use std::time::Instant;

#[derive(Debug, PartialEq)]
struct Timestamp(Instant);

impl snafu::GenerateImplicitData for Timestamp {
    fn generate() -> Self {
        Timestamp(Instant::now())
    }
}

#[derive(Debug, Snafu)]
struct RequestError {
    #[snafu(implicit)]
    timestamp: Timestamp,
}

fn do_request() -> Result<(), RequestError> {
    // ...
    ensure!(request_count < 3, RequestSnafu);

    Ok(())
}

You can use #[snafu(implicit(false))] if a field is incorrectly automatically identified as containing implicit data.

Controlling stringly-typed errors

This allows your custom error type to behave like the Whatever error type. Since it is your type, you can implement additional methods or traits. When placed on a struct or enum variant, you will be able to use the type with the whatever! macro as well as whatever_context methods, such as ResultExt::whatever_context.

#[derive(Debug, Snafu)]
enum Error {
    SpecificError {
        username: String,
    },

    #[snafu(whatever, display("{message}"))]
    GenericError {
        message: String,

        // Having a `source` is optional, but if it is present, it must
        // have this specific attribute and type:
        #[snafu(source(from(Box<dyn std::error::Error>, Some)))]
        source: Option<Box<dyn std::error::Error>>,
    },
}

Controlling how the `snafu` crate is resolved

If the snafu crate is not called snafu for some reason, you can use #[snafu(crate_root)] to instruct the macro how to find the crate root:

use my_custom_naming_of_snafu::Snafu;

#[derive(Debug, Snafu)]
#[snafu(crate_root(my_custom_naming_of_snafu))]
enum Error {
    SomeFailureMode,
}

#[derive(Debug, Snafu)]
#[snafu(crate_root(my_custom_naming_of_snafu))]
struct ApiError(Error);