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.

Usage

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

1. FromMetaItem 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.

Example

#[macro_use]
extern crate darling;
extern crate syn;

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

#[derive(FromDeriveInput)]
#[darling(from_ident, 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", ipsum))]
pub struct ConsumingType;

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.
• 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")] 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.