#darling #proc-macro #serde #derive #derive-macro


Helper crate for proc-macro library for reading attributes into structs when implementing custom derives. Use https://crates.io/crates/darling in your code.

56 releases

new 0.20.8 Feb 23, 2024
0.20.5 Jan 31, 2024
0.20.3 Jul 12, 2023
0.14.4 Mar 9, 2023
0.2.0 Jun 19, 2017

#2 in #darling

Download history 1055858/week @ 2023-11-03 1049415/week @ 2023-11-10 995039/week @ 2023-11-17 987212/week @ 2023-11-24 1041849/week @ 2023-12-01 1052033/week @ 2023-12-08 983720/week @ 2023-12-15 534679/week @ 2023-12-22 706744/week @ 2023-12-29 1026262/week @ 2024-01-05 1059705/week @ 2024-01-12 1206918/week @ 2024-01-19 1168802/week @ 2024-01-26 1235237/week @ 2024-02-02 1218165/week @ 2024-02-09 959971/week @ 2024-02-16

4,785,245 downloads per month
Used in 6,204 crates (4 directly)

MIT license



Build Status Latest Version [Rustc Version 1.56+]

darling is a crate for proc macro authors, which enables parsing attributes into structs. It is heavily inspired by serde both in its internals and in its API.


  • Easy and declarative parsing of macro input - make your proc-macros highly controllable with minimal time investment.
  • Great validation and errors, no work required. When users of your proc-macro make a mistake, darling makes sure they get error markers at the right place in their source, and provides "did you mean" suggestions for misspelled fields.


darling provides a set of traits which can be derived or manually implemented.

  1. FromMeta is used to extract values from a meta-item in an attribute. Implementations are likely reusable for many libraries, much like FromStr or serde::Deserialize. Trait implementations are provided for primitives, some std types, and some syn types.
  2. FromDeriveInput is implemented or derived by each proc-macro crate which depends on darling. This is the root for input parsing; it gets access to the identity, generics, and visibility of the target type, and can specify which attribute names should be parsed or forwarded from the input AST.
  3. FromField is implemented or derived by each proc-macro crate which depends on darling. Structs deriving this trait will get access to the identity (if it exists), type, and visibility of the field.
  4. FromVariant is implemented or derived by each proc-macro crate which depends on darling. Structs deriving this trait will get access to the identity and contents of the variant, which can be transformed the same as any other darling input.
  5. FromAttributes is a lower-level version of the more-specific FromDeriveInput, FromField, and FromVariant traits. Structs deriving this trait get a meta-item extractor and error collection which works for any syntax element, including traits, trait items, and functions. This is useful for non-derive proc macros.

Additional Modules

  • darling::ast provides generic types for representing the AST.
  • darling::usage provides traits and functions for determining where type parameters and lifetimes are used in a struct or enum.
  • darling::util provides helper types with special FromMeta implementations, such as IdentList.


extern crate darling;
extern crate syn;

#[derive(Default, FromMeta)]
pub struct Lorem {
    #[darling(rename = "sit")]
    ipsum: bool,
    dolor: Option<String>,

#[darling(attributes(my_crate), forward_attrs(allow, doc, cfg))]
pub struct MyTraitOpts {
    ident: syn::Ident,
    attrs: Vec<syn::Attribute>,
    lorem: Lorem,

The above code will then be able to parse this input:

/// A doc comment which will be available in `MyTraitOpts::attrs`.
#[my_crate(lorem(dolor = "Hello", sit))]
pub struct ConsumingType;

Attribute Macros

Non-derive attribute macros are supported. To parse arguments for attribute macros, derive FromMeta on the argument receiver type, then pass &syn::AttributeArgs to the from_list method. This will produce a normal darling::Result<T> that can be used the same as a result from parsing a DeriveInput.

Macro Code

use darling::{Error, FromMeta};
use darling::ast::NestedMeta;
use syn::ItemFn;
use proc_macro::TokenStream;

#[derive(Debug, FromMeta)]
struct MacroArgs {
    timeout_ms: Option<u16>,
    path: String,

pub fn your_attr(args: TokenStream, input: TokenStream) -> TokenStream {
    let attr_args = match NestedMeta::parse_meta_list(args.into()) {
        Ok(v) => v,
        Err(e) => { return TokenStream::from(Error::from(e).write_errors()); }
    let _input = syn::parse_macro_input!(input as ItemFn);

    let _args = match MacroArgs::from_list(&attr_args) {
        Ok(v) => v,
        Err(e) => { return TokenStream::from(e.write_errors()); }

    // do things with `args`

Consuming Code

use your_crate::your_attr;

#[your_attr(path = "hello", timeout_ms = 15)]
fn do_stuff() {


Darling's features are built to work well for real-world projects.

  • Defaults: Supports struct- and field-level defaults, using the same path syntax as serde. Additionally, Option<T> and darling::util::Flag fields are innately optional; you don't need to declare #[darling(default)] for those.
  • Field Renaming: Fields can have different names in usage vs. the backing code.
  • Auto-populated fields: Structs deriving FromDeriveInput and FromField can declare properties named ident, vis, ty, attrs, and generics to automatically get copies of the matching values from the input AST. FromDeriveInput additionally exposes data to get access to the body of the deriving type, and FromVariant exposes fields.
  • Mapping function: Use #[darling(map="path")] or #[darling(and_then="path")] to specify a function that runs on the result of parsing a meta-item field. This can change the return type, which enables you to parse to an intermediate form and convert that to the type you need in your struct.
  • Skip fields: Use #[darling(skip)] to mark a field that shouldn't be read from attribute meta-items.
  • Multiple-occurrence fields: Use #[darling(multiple)] on a Vec field to allow that field to appear multiple times in the meta-item. Each occurrence will be pushed into the Vec.
  • Span access: Use darling::util::SpannedValue in a struct to get access to that meta item's source code span. This can be used to emit warnings that point at a specific field from your proc macro. In addition, you can use darling::Error::write_errors to automatically get precise error location details in most cases.
  • "Did you mean" suggestions: Compile errors from derived darling trait impls include suggestions for misspelled fields.

Shape Validation

Some proc-macros only work on structs, while others need enums whose variants are either unit or newtype variants. Darling makes this sort of validation extremely simple. On the receiver that derives FromDeriveInput, add #[darling(supports(...))] and then list the shapes that your macro should accept.

Name Description
any Accept anything
struct_any Accept any struct
struct_named Accept structs with named fields, e.g. struct Example { field: String }
struct_newtype Accept newtype structs, e.g. struct Example(String)
struct_tuple Accept tuple structs, e.g. struct Example(String, String)
struct_unit Accept unit structs, e.g. struct Example;
enum_any Accept any enum
enum_named Accept enum variants with named fields
enum_newtype Accept newtype enum variants
enum_tuple Accept tuple enum variants
enum_unit Accept unit enum variants

Each one is additive, so listing #[darling(supports(struct_any, enum_newtype))] would accept all structs and any enum where every variant is a newtype variant.

This can also be used when deriving FromVariant, without the enum_ prefix.


~19K SLoC