`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.
1.`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.
1.`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.
* **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`.