# Attribute in syn - Rust
An attribute like `#[repr(transparent)]`.
Keywords: rust, rustlang, rust-lang, Attribute

# Struct syn::Attribute

source · [−]

    pub struct Attribute {
        pub pound_token: Pound,
        pub style: AttrStyle,
        pub bracket_token: Bracket,
        pub path: Path,
        pub tokens: TokenStream,
    }

Expand description

An attribute like `#[repr(transparent)]`.

This type is available only if Syn is built with the `"derive"` or `"full"` feature.

## Syntax

Rust has six types of attributes.

- Outer attributes like `#[repr(transparent)]`. These appear outside or in front of the item they describe.
- Inner attributes like `#![feature(proc_macro)]`. These appear inside of the item they describe, usually a module.
- Outer doc comments like `/// # Example`.
- Inner doc comments like `//! Please file an issue`.
- Outer block comments `/** # Example */`.
- Inner block comments `/*! Please file an issue */`.

The `style` field of type `AttrStyle` distinguishes whether an attribute is outer or inner. Doc comments and block comments are promoted to attributes, as this is how they are processed by the compiler and by `macro_rules!` macros.

The `path` field gives the possibly colon-delimited path against which the attribute is resolved. It is equal to `"doc"` for desugared doc comments. The `tokens` field contains the rest of the attribute body as tokens.

    #[derive(Copy)]      #[crate::precondition x < 5]
      ^^^^^^~~~~~~         ^^^^^^^^^^^^^^^^^^^ ~~~~~
      path  tokens                 path        tokens

## Parsing from tokens to Attribute

This type does not implement the `Parse` trait and thus cannot be parsed directly by `ParseStream::parse`. Instead use `ParseStream::call` with one of the two parser functions `Attribute::parse_outer` or `Attribute::parse_inner` depending on which you intend to parse.

    use syn::{Attribute, Ident, Result, Token};
    use syn::parse::{Parse, ParseStream};

    // Parses a unit struct with attributes.
    //
    //     #[path = "s.tmpl"]
    //     struct S;
    struct UnitStruct {
        attrs: Vec<Attribute>,
        struct_token: Token![struct],
        name: Ident,
        semi_token: Token![;],
    }

    impl Parse for UnitStruct {
        fn parse(input: ParseStream) -> Result<Self> {
            Ok(UnitStruct {
                attrs: input.call(Attribute::parse_outer)?,
                struct_token: input.parse()?,
                name: input.parse()?,
                semi_token: input.parse()?,
            })
        }
    }

## Parsing from Attribute to structured arguments

The grammar of attributes in Rust is very flexible, which makes the syntax tree not that useful on its own. In particular, arguments of the attribute are held in an arbitrary `tokens: TokenStream`. Macros are expected to check the `path` of the attribute, decide whether they recognize it, and then parse the remaining tokens according to whatever grammar they wish to require for that kind of attribute.

If the attribute you are parsing is expected to conform to the conventional structured form of attribute, use `parse_meta()` to obtain that structured representation. If the attribute follows some other grammar of its own, use `parse_args()` to parse that into the expected data structure.

## Doc comments

The compiler transforms doc comments, such as `/// comment` and `/*! comment */`, into attributes before macros are expanded. Each comment is expanded into an attribute of the form `#[doc = r"comment"]`.

As an example, the following `mod` items are expanded identically:

    let doc: ItemMod = parse_quote! {
        /// Single line doc comments
        /// We write so many!
        /**
         * Multi-line comments...
         * May span many lines
         */
        mod example {
            //! Of course, they can be inner too
            /*! And fit in a single line */
        }
    };
    let attr: ItemMod = parse_quote! {
        #[doc = r" Single line doc comments"]
        #[doc = r" We write so many!"]
        #[doc = r"
         * Multi-line comments...
         * May span many lines
         "]
        mod example {
            #![doc = r" Of course, they can be inner too"]
            #![doc = r" And fit in a single line "]
        }
    };
    assert_eq!(doc, attr);

## Fields

`pound_token: Pound` `style: AttrStyle` `bracket_token: Bracket` `path: Path` `tokens: TokenStream`

## Implementations

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### impl Attribute

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#### pub fn parse_meta(&self) -> Result<Meta>

Parses the content of the attribute, consisting of the path and tokens, as a `Meta` if possible.

This function is available only if Syn is built with the `"parsing"` feature.

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#### pub fn parse_args<T: Parse>(&self) -> Result<T>

Parse the arguments to the attribute as a syntax tree.

This is similar to `syn::parse2::<T>(attr.tokens)` except that:

- the surrounding delimiters are not included in the input to the parser; and
- the error message has a more useful span when `tokens` is empty.

    #[my_attr(value < 5)]
              ^^^^^^^^^ what gets parsed

This function is available only if Syn is built with the `"parsing"` feature.

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#### pub fn parse_args_with<F: Parser>(&self, parser: F) -> Result<F::Output>

Parse the arguments to the attribute using the given parser.

This function is available only if Syn is built with the `"parsing"` feature.

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#### pub fn parse_outer(input: ParseStream<'_>) -> Result<Vec<Self>>

Parses zero or more outer attributes from the stream.

This function is available only if Syn is built with the `"parsing"` feature.

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#### pub fn parse_inner(input: ParseStream<'_>) -> Result<Vec<Self>>

Parses zero or more inner attributes from the stream.

This function is available only if Syn is built with the `"parsing"` feature.

## Trait Implementations

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### impl Clone for Attribute

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#### fn clone(&self) -> Self

Returns a copy of the value. Read more

1.0.0 · source

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more

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### impl ToTokens for Attribute

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#### fn to_tokens(&self, tokens: &mut TokenStream)

Write `self` to the given `TokenStream`. Read more

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#### fn to_token_stream(&self) -> TokenStream

Convert `self` directly into a `TokenStream` object. Read more

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#### fn into_token_stream(self) -> TokenStream

Convert `self` directly into a `TokenStream` object. Read more

## Auto Trait Implementations

### impl RefUnwindSafe for Attribute

### impl !Send for Attribute

### impl !Sync for Attribute

### impl Unpin for Attribute

### impl UnwindSafe for Attribute

## Blanket Implementations

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### impl<T> Any for T where
T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more

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### impl<T> Borrow<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

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### impl<T> BorrowMut<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more

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### impl<T> From<T> for T

const: unstable · source

#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T where
U: From<T>,

const: unstable · source

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> Spanned for T where
T: Spanned + ?Sized,

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#### fn span(&self) -> Span

Returns a `Span` covering the complete contents of this syntax tree node, or `Span::call_site()` if this node is empty. Read more

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### impl<T> ToOwned for T where
T: Clone,

#### type Owned = T

The resulting type after obtaining ownership.

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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more

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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more

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### impl<T, U> TryFrom<U> for T where
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.

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### impl<T, U> TryInto<U> for T where
U: TryFrom<T>,

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.