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https://github.com/TedDriggs/darling

A Rust proc-macro attribute parser
https://github.com/TedDriggs/darling

proc-macro rust

Last synced: 4 months ago
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A Rust proc-macro attribute parser

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# Darling



[![Build Status](https://github.com/TedDriggs/darling/workflows/CI/badge.svg)](https://github.com/TedDriggs/darling/actions)

[![Latest Version](https://img.shields.io/crates/v/darling.svg)](https://crates.io/crates/darling)

![Rustc Version 1.56+](https://img.shields.io/badge/rustc-1.56+-lightgray.svg)



`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



```rust,ignore

use darling::{FromDeriveInput, FromMeta};



#[derive(Default, FromMeta)]

#[darling(default)]

pub struct Lorem {

    #[darling(rename = "sit")]

    ipsum: bool,

    dolor: Option,

}



#[derive(FromDeriveInput)]

#[darling(attributes(my_crate), forward_attrs(allow, doc, cfg))]

pub struct MyTraitOpts {

    ident: syn::Ident,

    attrs: Vec,

    lorem: Lorem,

}

```



The above code will then be able to parse this input:



```rust,ignore

/// 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`, then pass that to the derived `from_list` method on your argument receiver type.

This will produce a normal `darling::Result` that can be used the same as a result from parsing a `DeriveInput`.



## Macro Code



```rust,ignore

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,

    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



```rust,ignore

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` 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) -> darling::Result`, 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.



## 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.



| Name             | Description                                                               |

| ---------------- | ------------------------------------------------------------------------- |

| `any`            | Accept anything                                                           |

| `struct_any`     | Accept any struct                                                         |

| `struct_named`   | Accept structs with named fields, e.g. `struct Example { field: String }` |

| `struct_newtype` | Accept newtype structs, e.g. `struct Example(String)`                     |

| `struct_tuple`   | Accept tuple structs, e.g. `struct Example(String, String)`               |

| `struct_unit`    | Accept unit structs, e.g. `struct Example;`                               |

| `enum_any`       | Accept any enum                                                           |

| `enum_named`     | Accept enum variants with named fields                                    |

| `enum_newtype`   | Accept newtype enum variants                                              |

| `enum_tuple`     | Accept tuple enum variants                                                |

| `enum_unit`      | Accept unit enum variants                                                 |



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.