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README.md
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.
FromMetaItem
is used to extract values from a meta-item in an attribute. Implementations are likely reusable for many libraries, much likeFromStr
orserde::Deserialize
. Trait implementations are provided for primitives, some std types, and somesyn
types.FromDeriveInput
is implemented or derived by each proc-macro crate which depends ondarling
. 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.FromField
is implemented or derived by each proc-macro crate which depends ondarling
. 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
andFromField
can declare properties namedident
,vis
,ty
,attrs
, andgenerics
to automatically get copies of the matching values from the input AST. - 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 aVec
field to allow that field to appear multiple times in the meta-item. Each occurrence will be pushed into theVec
.