Crate darling

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Darling

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

Design

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 nameTypeMeaning
identsyn::IdentThe identifier of the passed-in type
vissyn::VisibilityThe visibility of the passed-in type
genericsT: darling::FromGenericsThe generics of the passed-in type. This can be syn::Generics, darling::ast::Generics, or any compatible type.
datadarling::ast::DataThe body of the passed-in type
attrsVec<syn::Attribute>The forwarded attributes from the passed in type. These are controlled using the forward_attrs attribute.

FromField

Field nameTypeMeaning
identOption<syn::Ident>The identifier of the passed-in field, or None for tuple fields
vissyn::VisibilityThe visibility of the passed-in field
tysyn::TypeThe type of the passed-in field
attrsVec<syn::Attribute>The forwarded attributes from the passed in field. These are controlled using the forward_attrs attribute.

FromTypeParam

Field nameTypeMeaning
identsyn::IdentThe identifier of the passed-in type param
boundsVec<syn::TypeParamBound>The bounds applied to the type param
defaultOption<syn::Type>The default type of the parameter, if one exists
attrsVec<syn::Attribute>The forwarded attributes from the passed in type param. These are controlled using the forward_attrs attribute.

FromVariant

Field nameTypeMeaning
identsyn::IdentThe identifier of the passed-in variant
discriminantOption<syn::Expr>For a variant such as Example = 2, the 2
fieldsOption<darling::ast::Fields<__>>The fields associated with the variant
attrsVec<syn::Attribute>The forwarded attributes from the passed in variant. These are controlled using the forward_attrs attribute.

Modules

Utility types for working with the AST.
The darling::Error type and its internals.
Traits and types used for tracking the usage of generic parameters through a proc-macro input.
Utility types for attribute parsing.

Macros

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

Structs

An error encountered during attribute parsing.

Traits

Creates an instance by parsing an entire proc-macro derive input, including the, identity, generics, and visibility of the type.
Creates an instance by parsing an individual field and its attributes.
Creates an instance by parsing a specific syn::GenericParam. This can be a type param, a lifetime, or a const param.
Creates an instance by parsing an entire generics declaration, including the where clause.
Create an instance from an item in an attribute declaration.
Creates an instance by parsing an individual type_param and its attributes.
Creates an instance from a specified syn::Variant.

Type Definitions

An alias of Result specific to attribute parsing.