Struct darling::Error

pub struct Error { /* fields omitted */ }

An error encountered during attribute parsing.

Given that most errors darling encounters represent code bugs in dependent crates, the internal structure of the error is deliberately opaque.

Usage

Proc-macro expansion happens very infrequently compared to runtime tasks such as deserialization, and it happens in the context of an expensive compilation taks. For that reason, darling prefers not to fail on the first error it encounters, instead doing as much work as it can, accumulating errors into a single report.

As a result, darling::Error is more of guaranteed-non-empty error collection than a single problem. These errors also have some notion of hierarchy, stemming from the hierarchical nature of darling’s input.

These characteristics make for great experiences when using darling-powered crates, provided crates using darling adhere to some best practices:

1. Do not attempt to simplify a darling::Error into some other error type, such as syn::Error. To surface compile errors, instead use darling::Error::write_errors. This preserves all span information, suggestions, etc. Wrapping a darling::Error in a custom error enum works as-expected and does not force any loss of fidelity.
2. Do not use early return (e.g. the ? operator) for custom validations. Instead, create a local Vec to collect errors as they are encountered and then use darling::Error::multiple to create an error containing all those issues if the list is non-empty after validation. This can create very complex custom validation functions; in those cases, split independent “validation chains” out into their own functions to keep the main validator manageable.
3. Use darling::Error::custom to create additional errors as-needed, then call with_span to ensure those errors appear in the right place. Use darling::util::SpannedValue to keep span information around on parsed fields so that custom diagnostics can point to the correct parts of the input AST.

Implementations

impl Error

Error creation functions

pub fn custom<T>(msg: T) -> Error where
    T: Display, 

Creates a new error with a custom message.

pub fn duplicate_field(name: &str) -> Error

Creates a new error for a field that appears twice in the input.

pub fn duplicate_field_path(path: &Path) -> Error

Creates a new error for a field that appears twice in the input. Helper to avoid repeating the syn::Path to String conversion.

pub fn missing_field(name: &str) -> Error

Creates a new error for a non-optional field that does not appear in the input.

pub fn unknown_field(name: &str) -> Error

Creates a new error for a field name that appears in the input but does not correspond to a known field.

pub fn unknown_field_path(path: &Path) -> Error

Creates a new error for a field name that appears in the input but does not correspond to a known field. Helper to avoid repeating the syn::Path to String conversion.

pub fn unknown_field_with_alts<'a, T, I>(field: &str, alternates: I) -> Error where
    T: AsRef<str> + 'a,
    I: IntoIterator<Item = &'a T>, 

Creates a new error for a field name that appears in the input but does not correspond to a known attribute. The second argument is the list of known attributes; if a similar name is found that will be shown in the emitted error message.

pub fn unsupported_shape(shape: &str) -> Error

Creates a new error for a struct or variant that does not adhere to the supported shape.

pub fn unsupported_format(format: &str) -> Error

pub fn unexpected_type(ty: &str) -> Error

Creates a new error for a field which has an unexpected literal type.

pub fn unexpected_lit_type(lit: &Lit) -> Error

Creates a new error for a field which has an unexpected literal type. This will automatically extract the literal type name from the passed-in Lit and set the span to encompass only the literal value.

Usage

This is most frequently used in overrides of the FromMeta::from_value method.

use darling::{FromMeta, Error, Result};
use syn::{Lit, LitStr};

pub struct Foo(String);

impl FromMeta for Foo {
    fn from_value(value: &Lit) -> Result<Self> {
        if let Lit::Str(ref lit_str) = *value {
            Ok(Foo(lit_str.value()))
        } else {
            Err(Error::unexpected_lit_type(value))
        }
    }
}

pub fn unknown_value(value: &str) -> Error

Creates a new error for a value which doesn’t match a set of expected literals.

pub fn too_few_items(min: usize) -> Error

Creates a new error for a list which did not get enough items to proceed.

pub fn too_many_items(max: usize) -> Error

Creates a new error when a list got more items than it supports. The max argument is the largest number of items the receiver could accept.

pub fn multiple(errors: Vec<Error, Global>) -> Error

Bundle a set of multiple errors into a single Error instance.

Panics

This function will panic if errors.is_empty() == true.

impl Error

Error instance methods

pub fn has_span(&self) -> bool

Check if this error is associated with a span in the token stream.

pub fn with_span<T>(self, node: &T) -> Error where
    T: Spanned, 

Tie a span to the error if none is already present. This is used in darling::FromMeta and other traits to attach errors to the most specific possible location in the input source code.

All darling-built impls, either from the crate or from the proc macro, will call this when appropriate during parsing, so it should not be necessary to call this unless you have overridden:

• FromMeta::from_meta
• FromMeta::from_nested_meta
• FromMeta::from_value

pub fn flatten(self) -> Error

Recursively converts a tree of errors to a flattened list.

pub fn at<T>(self, location: T) -> Error where
    T: Display, 

Adds a location to the error, such as a field or variant. Locations must be added in reverse order of specificity.

pub fn at_path(self, path: &Path) -> Error

Adds a location to the error, such as a field or variant. Locations must be added in reverse order of specificity. This is a helper function to avoid repeating path to string logic.

pub fn len(&self) -> usize

Gets the number of individual errors in this error.

This function never returns 0, as it’s impossible to construct a multi-error from an empty Vec.

pub fn write_errors(self) -> TokenStream

Write this error and any children as compile errors into a TokenStream to be returned by the proc-macro.

The behavior of this method will be slightly different if the diagnostics feature is enabled: In that case, the diagnostics will be emitted immediately by this call, and an empty TokenStream will be returned.

Return these tokens unmodified to avoid disturbing the attached span information.

Usage

// in your proc-macro function
let opts = match MyOptions::from_derive_input(&ast) {
    Ok(val) => val,
    Err(err) => {
        return err.write_errors();
    }
}

Trait Implementations

impl Debug for Error

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

Formats the value using the given formatter.

impl Display for Error

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

Formats the value using the given formatter.

impl Error for Error

pub fn description(&self) -> &str

👎 Deprecated since 1.42.0:

use the Display impl or to_string()

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

👎 Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

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

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

pub 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 From<Error> for Error

pub fn from(e: Error) -> Error

Performs the conversion.

impl IntoIterator for Error

type Item = Error

The type of the elements being iterated over.

type IntoIter = IntoIter

Which kind of iterator are we turning this into?

pub fn into_iter(self) -> IntoIter

Creates an iterator from a value.