Derive Macro Error 

#[derive(Error)]
{
    // Attributes available to this derive:
    #[error]
}

Using #[derive(Error)]

Deriving Error will generate an Error implementation, that contains (depending on the type) a source() and a provide() method. Please note, at the time of writing provide() is only supported on nightly Rust. So you have to use that to make use of it.

For a struct, these methods always do the same. For an enum they have separate behaviour for each of the variants. The variant is first matched and then the implementation will do the same as it would have done if the variant was a struct.

Usually when you derive Error you will also want to derive Display and often From as well.

When and how does it derive source()?

1. It’s a struct/variant with named fields and one is the fields is called source. Then it would return that field as the source.
2. It’s a tuple struct/variant and there’s exactly one field that is not used as the backtrace. So either a tuple struct with one field, or one with two where one is the backtrace. Then it returns this field as the source.
3. One of the fields is annotated with #[error(source)]. Then it would return that field as the source.

When and how does it derive provide()?

1. It’s a struct/variant with named fields and one of the fields is called backtrace. Then it would return that field as the backtrace.
2. It’s a tuple struct/variant and the type of exactly one of the fields is called Backtrace. Then it would return that field as the backtrace.
3. One of the fields is annotated with #[error(backtrace)]. Then it would return that field as the backtrace.

Ignoring fields for derives

It’s possible to ignore a field or a whole enum variant completely for this derive using the #[error(ignore)] attribute. This will ignore it both for detecting backtrace and source. It’s also possible to mark a field only ignored for one of these methods by using #[error(not(backtrace))] or #[error(not(source))].

What works in no_std?

Error derive fully works on no_std environments, except the provide() method usage, because the Backtrace type is only available in std.

Optional fields

Deriving source() is supported naturally for Option<_>-typed fields.

WARNING: Field type is considered as Option<_> only syntactically, meaning that:

• Neither renaming std::option::Option would work:

  #[derive(Debug, Display, Error)]
  struct Simple;
  
  #[derive(Debug, Display, Error)]
  #[display("Oops!")]
  struct Generic(RenamedOption<Simple>); // not recognized as `Option`
  type RenamedOption<T> = Option<T>;

• Nor using any custom type named as Option would:

  #[derive(Debug, Display, Error)]
  #[display("No way!")]
  struct Generic(Option<Simple>);        // cannot use `?` syntax
  struct Option<T>(T);

TIP: If std::option::Option for some reason needs to be renamed, annotate the field with the #[error(source(optional))] attribute:

#[derive(Debug, Display, Error)]
struct Simple;

#[derive(Debug, Display, Error)]
#[display("Oops!")]
struct Generic(#[error(source(optional))] RenamedOption<Simple>);
type RenamedOption<T> = Option<T>;

Example usage

// `std::error::Error` requires `std::fmt::Debug` and `std::fmt::Display`,
// so we can also use `derive_more::Display` for fully declarative
// error-type definitions.

#[derive(Default, Debug, Display, Error)]
struct Simple;

#[derive(Default, Debug, Display, Error)]
struct WithSource {
    source: Simple,
}
#[derive(Default, Debug, Display, Error)]
#[display("WithOptionalSource")]
struct WithOptionalSource {
    source: Option<Simple>,
}
#[derive(Default, Debug, Display, Error)]
struct WithExplicitSource {
    #[error(source)]
    explicit_source: Simple,
}
#[derive(Default, Debug, Display, Error)]
#[display("WithExplicitOptionalSource")]
struct WithExplicitOptionalSource {
    #[error(source)]
    explicit_source: Option<Simple>,
}
#[derive(Default, Debug, Display, Error)]
#[display("WithExplicitOptionalSource")]
struct WithExplicitRenamedOptionalSource {
    #[error(source(optional))]
    explicit_source: RenamedOption<Simple>,
}

#[derive(Default, Debug, Display, Error)]
struct Tuple(Simple);

#[derive(Default, Debug, Display, Error)]
struct WithoutSource(#[error(not(source))] i32);

#[derive(Debug, Display, Error)]
#[display("An error with a backtrace")]
struct WithSourceAndBacktrace {
    source: Simple,
    backtrace: Backtrace,
}

// derive_more::From fits nicely into this pattern as well
#[derive(Debug, Display, Error, From)]
enum CompoundError {
    Simple,
    WithSource {
        source: Simple,
    },
    #[display("WithOptionalSource")]
    WithOptionalSource {
        source: Option<Simple>,
    },
    #[from(ignore)]
    WithBacktraceFromSource {
        #[error(backtrace)]
        source: Simple,
    },
    #[display("{source}")]
    WithDifferentBacktrace {
        source: Simple,
        backtrace: Backtrace,
    },
    WithExplicitSource {
        #[error(source)]
        explicit_source: WithSource,
    },
    #[display("WithExplicitOptionalSource")]
    WithExplicitOptionalSource {
        #[error(source)]
        explicit_source: Option<WithSource>,
    },
    #[display("WithExplicitRenamedOptionalSource")]
    WithExplicitRenamedOptionalSource {
        #[error(source(optional))]
        explicit_source: RenamedOption<WithExplicitRenamedOptionalSource>,
    },
    #[from(ignore)]
    WithBacktraceFromExplicitSource {
        #[error(backtrace, source)]
        explicit_source: WithSource,
    },
    Tuple(WithExplicitSource),
    #[display("TupleOptional")]
    TupleOptional(Option<WithExplicitSource>),
    WithoutSource(#[error(not(source))] Tuple),
}

assert!(Simple.source().is_none());
assert!(request_ref::<Backtrace>(&Simple).is_none());

assert!(WithSource::default().source().is_some());
assert!(WithOptionalSource { source: Some(Simple) }.source().is_some());

assert!(WithExplicitSource::default().source().is_some());
assert!(WithExplicitOptionalSource { explicit_source: Some(Simple) }.source().is_some());
assert!(WithExplicitRenamedOptionalSource { explicit_source: Some(Simple) }.source().is_some());

assert!(Tuple::default().source().is_some());

assert!(WithoutSource::default().source().is_none());

let with_source_and_backtrace = WithSourceAndBacktrace {
    source: Simple,
    backtrace: Backtrace::capture(),
};
assert!(with_source_and_backtrace.source().is_some());
assert!(request_ref::<Backtrace>(&with_source_and_backtrace).is_some());

assert!(CompoundError::Simple.source().is_none());

assert!(CompoundError::from(Simple).source().is_some());
assert!(CompoundError::from(Some(Simple)).source().is_some());

assert!(CompoundError::from(WithSource::default()).source().is_some());
assert!(CompoundError::from(Some(WithSource::default())).source().is_some());

assert!(CompoundError::from(WithExplicitSource::default()).source().is_some());
assert!(CompoundError::from(Some(WithExplicitSource::default())).source().is_some());
assert!(CompoundError::from(Some(WithExplicitRenamedOptionalSource { explicit_source: Some(Simple) })).source().is_some());

assert!(CompoundError::from(Tuple::default()).source().is_none());