Crate darling

Expand description

Darling

Darling is a tool for declarative attribute parsing in proc macro implementations.

Darling takes considerable design inspiration from serde. A data structure that can be read from any attribute implements FromMeta (or has an implementation automatically generated using derive). Any crate can provide FromMeta implementations, even one not specifically geared towards proc-macro authors.

Proc-macro crates should provide their own structs which implement or derive FromDeriveInput, FromField, FromVariant, FromGenerics, et alia to gather settings relevant to their operation.

Attributes

There are a number of attributes that darling exposes to enable finer-grained control over the code it generates.

Field renaming: You can use #[darling(rename="new_name")] on a field to change the name Darling looks for. You can also use #[darling(rename_all="...")] at the struct or enum level to apply a casing rule to all fields or variants.
Map function: You can use #[darling(map="path::to::function")] to run code on a field before its stored in the struct.
Default values: You can use #[darling(default)] at the type or field level to use that type’s default value to fill in values not specified by the caller.
Skipped fields: You can skip a variant or field using #[darling(skip)]. Fields marked with this will fall back to Default::default() for their value, but you can override that with an explicit default or a value from the type-level default.

Forwarded Fields

All derivable traits except FromMeta support forwarding some fields from the input AST to the derived struct. These fields are matched up by identifier before rename attribute values are considered, allowing you to use their names for your own properties. The deriving struct is responsible for making sure the types of fields it chooses to declare are compatible with this table.

A deriving struct is free to include or exclude any of the fields below.

`FromDeriveInput`

Field name	Type	Meaning
`ident`	`syn::Ident`	The identifier of the passed-in type
`vis`	`syn::Visibility`	The visibility of the passed-in type
`generics`	`T: darling::FromGenerics`	The generics of the passed-in type. This can be `syn::Generics`, `darling::ast::Generics`, or any compatible type.
`data`	`darling::ast::Data`	The body of the passed-in type
`attrs`	`Vec<syn::Attribute>`	The forwarded attributes from the passed in type. These are controlled using the `forward_attrs` attribute.

`FromField`

Field name	Type	Meaning
`ident`	`Option<syn::Ident>`	The identifier of the passed-in field, or `None` for tuple fields
`vis`	`syn::Visibility`	The visibility of the passed-in field
`ty`	`syn::Type`	The type of the passed-in field
`attrs`	`Vec<syn::Attribute>`	The forwarded attributes from the passed in field. These are controlled using the `forward_attrs` attribute.

`FromTypeParam`

Field name	Type	Meaning
`ident`	`syn::Ident`	The identifier of the passed-in type param
`bounds`	`Vec<syn::TypeParamBound>`	The bounds applied to the type param
`default`	`Option<syn::Type>`	The default type of the parameter, if one exists
`attrs`	`Vec<syn::Attribute>`	The forwarded attributes from the passed in type param. These are controlled using the `forward_attrs` attribute.

`FromVariant`

Field name	Type	Meaning
`ident`	`syn::Ident`	The identifier of the passed-in variant
`discriminant`	`Option<syn::Expr>`	For a variant such as `Example = 2`, the `2`
`fields`	`Option<darling::ast::Fields<__>>`	The fields associated with the variant
`attrs`	`Vec<syn::Attribute>`	The forwarded attributes from the passed in variant. These are controlled using the `forward_attrs` attribute.

Modules

ast

Utility types for working with the AST.

error

The darling::Error type, the multiple error Accumulator, and their internals.

usage

Traits and types used for tracking the usage of generic parameters through a proc-macro input.

util

Utility types for attribute parsing.

Macros

uses_lifetimes

Generator for UsesLifetimes impls that unions the used lifetimes of the selected fields.

uses_type_params

Generator for UsesTypeParam impls that unions the used type parameters of the selected fields.

Structs

Error

An error encountered during attribute parsing.

Traits

FromAttributes

Create an instance by parsing a list of attributes.

FromDeriveInput

Creates an instance by parsing an entire proc-macro derive input, including the, identity, generics, and visibility of the type.

FromField

Creates an instance by parsing an individual field and its attributes.

FromGenericParam

Creates an instance by parsing a specific syn::GenericParam. This can be a type param, a lifetime, or a const param.

FromGenerics

Creates an instance by parsing an entire generics declaration, including the where clause.

FromMeta

Create an instance from an item in an attribute declaration.

FromTypeParam

Creates an instance by parsing an individual type_param and its attributes.

FromVariant

Creates an instance from a specified syn::Variant.

Type Definitions

Result

An alias of Result specific to attribute parsing.