README.md
1# Darling
2
3[](https://github.com/TedDriggs/darling/actions)
4[](https://crates.io/crates/darling)
5
6
7`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.
8
9# Benefits
10
11- Easy and declarative parsing of macro input - make your proc-macros highly controllable with minimal time investment.
12- 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.
13
14# Usage
15
16`darling` provides a set of traits which can be derived or manually implemented.
17
181. `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.
192. `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.
203. `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.
214. `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.
225. `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.
23
24## Additional Modules
25
26- `darling::ast` provides generic types for representing the AST.
27- `darling::usage` provides traits and functions for determining where type parameters and lifetimes are used in a struct or enum.
28- `darling::util` provides helper types with special `FromMeta` implementations, such as `PathList`.
29
30# Example
31
32```rust,ignore
33use darling::{FromDeriveInput, FromMeta};
34
35#[derive(Default, FromMeta)]
36#[darling(default)]
37pub struct Lorem {
38 #[darling(rename = "sit")]
39 ipsum: bool,
40 dolor: Option<String>,
41}
42
43#[derive(FromDeriveInput)]
44#[darling(attributes(my_crate), forward_attrs(allow, doc, cfg))]
45pub struct MyTraitOpts {
46 ident: syn::Ident,
47 attrs: Vec<syn::Attribute>,
48 lorem: Lorem,
49}
50```
51
52The above code will then be able to parse this input:
53
54```rust,ignore
55/// A doc comment which will be available in `MyTraitOpts::attrs`.
56#[derive(MyTrait)]
57#[my_crate(lorem(dolor = "Hello", sit))]
58pub struct ConsumingType;
59```
60
61# Attribute Macros
62
63Non-derive attribute macros are supported.
64To 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.
65This will produce a normal `darling::Result<T>` that can be used the same as a result from parsing a `DeriveInput`.
66
67## Macro Code
68
69```rust,ignore
70use darling::{Error, FromMeta};
71use darling::ast::NestedMeta;
72use syn::ItemFn;
73use proc_macro::TokenStream;
74
75#[derive(Debug, FromMeta)]
76struct MacroArgs {
77 #[darling(default)]
78 timeout_ms: Option<u16>,
79 path: String,
80}
81
82#[proc_macro_attribute]
83pub fn your_attr(args: TokenStream, input: TokenStream) -> TokenStream {
84 let attr_args = match NestedMeta::parse_meta_list(args.into()) {
85 Ok(v) => v,
86 Err(e) => { return TokenStream::from(Error::from(e).write_errors()); }
87 };
88 let _input = syn::parse_macro_input!(input as ItemFn);
89
90 let _args = match MacroArgs::from_list(&attr_args) {
91 Ok(v) => v,
92 Err(e) => { return TokenStream::from(e.write_errors()); }
93 };
94
95 // do things with `args`
96 unimplemented!()
97}
98```
99
100## Consuming Code
101
102```rust,ignore
103use your_crate::your_attr;
104
105#[your_attr(path = "hello", timeout_ms = 15)]
106fn do_stuff() {
107 println!("Hello");
108}
109```
110
111# Features
112
113Darling's features are built to work well for real-world projects.
114
115- **Defaults**: Supports struct- and field-level defaults, using the same path syntax as `serde`.
116 Additionally, `Option<T>` and `darling::util::Flag` fields are innately optional; you don't need to declare `#[darling(default)]` for those.
117- **Field Renaming**: Fields can have different names in usage vs. the backing code.
118- **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`.
119 - **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.
120- **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.
121- **Skip fields**: Use `#[darling(skip)]` to mark a field that shouldn't be read from attribute meta-items.
122- **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`.
123- **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.
124- **"Did you mean" suggestions**: Compile errors from derived darling trait impls include suggestions for misspelled fields.
125- **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.
126
127## Shape Validation
128
129Some proc-macros only work on structs, while others need enums whose variants are either unit or newtype variants.
130Darling makes this sort of validation extremely simple.
131On the receiver that derives `FromDeriveInput`, add `#[darling(supports(...))]` and then list the shapes that your macro should accept.
132
133| Name | Description |
134| ---------------- | ------------------------------------------------------------------------- |
135| `any` | Accept anything |
136| `struct_any` | Accept any struct |
137| `struct_named` | Accept structs with named fields, e.g. `struct Example { field: String }` |
138| `struct_newtype` | Accept newtype structs, e.g. `struct Example(String)` |
139| `struct_tuple` | Accept tuple structs, e.g. `struct Example(String, String)` |
140| `struct_unit` | Accept unit structs, e.g. `struct Example;` |
141| `enum_any` | Accept any enum |
142| `enum_named` | Accept enum variants with named fields |
143| `enum_newtype` | Accept newtype enum variants |
144| `enum_tuple` | Accept tuple enum variants |
145| `enum_unit` | Accept unit enum variants |
146
147Each 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.
148
149This can also be used when deriving `FromVariant`, without the `enum_` prefix.
150
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