mozilla-central/third_party/rust/darling/src
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lib.rs # Darling Darling is a tool for declarative attribute parsing in proc macro implementations. ## Design Darling takes considerable design inspiration from [`serde`](https://serde.rs). 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>` (or anything, using `#[darling(with = ...)]`)|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>` (or anything, using `#[darling(with = ...)]`)|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>` (or anything, using `#[darling(with = ...)]`)|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`|`darling::ast::Fields<T> where T: FromField`|The fields associated with the variant| |`attrs`|`Vec<syn::Attribute>` (or anything, using `#[darling(with = ...)]`)|The forwarded attributes from the passed in variant. These are controlled using the `forward_attrs` attribute.| 5521
macros_public.rs Macros that should be exported from both `darling_core` and `darling`. Note that these are **sym-linked** into the main code, and so cannot declare on items that are exported differently in `darling_core` vs. `darling`. 3586