[−][src]Module snafu::guide::attributes

Attributes understood by the `Snafu` macro

Controlling `Display`

There are a number of ways you can specify how the Display trait will be implemented for each variant:

#[snafu(display("a format string with arguments: {}", info))]

The argument is a format string and the arguments. Available in Rust 1.34.
#[snafu(display = r#"("a format string with arguments: {}", info)"#)]

The same argument as above, but wrapped in a raw string to support previous Rust versions.

Each choice has the same capabilities. All of the fields of the variant will be available and you can call methods on them, such as filename.display().

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!(
        Error::MissingUser.to_string(),
        "No user available. You may need to specify one.",
    );
    assert_eq!(
        Error::MissingPassword.to_string(),
        "MissingPassword",
    );
}

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 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 a number of forms of the attribute:

#[snafu(visibility(X))]

X is a normal Rust visibility modifier (pub, pub(crate), pub(in some::path), etc.). Supported in Rust 1.34.
#[snafu(visibility = "X")]

The same argument as above, but wrapped in a string to support previous Rust versions.
#[snafu(visibility)] will reset back to private visibility.

#[derive(Debug, Snafu)]
#[snafu(visibility = "pub(crate)")] // Default
enum Error {
    IsPubCrate, // Uses the default
    #[snafu(visibility)]
    IsPrivate, // Will be private
}

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>);

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. Use #[snafu(backtrace(delegate))] to specify this:

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