Darling

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.

Benefits

• 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.

Usage

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 PathList.

Example

use darling::{FromDeriveInput, FromMeta};

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

#[derive(FromDeriveInput)]
#[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`.
#[derive(MyTrait)]
#[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 use darling::ast::NestedMeta::parse_meta_list to convert the arguments TokenStream to a Vec<NestedMeta>, then pass that to the derived from_list method on your argument receiver type. 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 {
    #[darling(default)]
    timeout_ms: Option<u16>,
    path: String,
}

#[proc_macro_attribute]
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`
    unimplemented!()
}

Consuming Code

use your_crate::your_attr;

#[your_attr(path = "hello", timeout_ms = 15)]
fn do_stuff() {
    println!("Hello");
}

Features

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.
  • Transformation of forwarded attributes: You can add #[darling(with=path)] to the attrs field to use a custom function to transform the forwarded attributes before they're provided to your struct. The function signature is fn(Vec<Attribute>) -> darling::Result<T>, where T is the type you declared for the attrs field. Returning an error from this function will propagate with all other parsing errors.
• 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.
• Struct flattening: Use #[darling(flatten)] to remove one level of structure when presenting your meta item to users. Fields that are not known to the parent struct will be forwarded to the flatten field.
• Custom shorthand: Use #[darling(from_word = ...)] on a struct or enum to override how a simple word is interpreted. By default, it is an error for your macro's user to fail to specify the fields of your struct, but with this you can choose to instead produce a set of default values. This takes either a path or a closure whose signature matches FromMeta::from_word.
• Custom handling for missing fields: When a field is not present and #[darling(default)] is not used, derived impls will call FromMeta::from_none on that field's type to try and get the fallback value for the field. Usually, there is not a fallback value, so a missing field error is generated. Option<T: FromMeta> uses this to make options optional without requiring #[darling(default)] declarations, and structs and enums can use this themselves with #[darling(from_none = ...)]. This takes either a path or a closure whose signature matches FromMeta::from_none.

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.

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.