[][src]Struct syn::Attribute

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

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.

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: Poundstyle: AttrStylebracket_token: Bracketpath: Pathtokens: TokenStream

Methods

impl Attribute[src]

pub fn parse_meta(&self) -> Result<Meta>[src]

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.

pub fn parse_args<T: Parse>(&self) -> Result<T>[src]

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.

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

Parse the arguments to the attribute using the given parser.

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

pub fn parse_outer(input: ParseStream) -> Result<Vec<Self>>[src]

Parses zero or more outer attributes from the stream.

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

pub fn parse_inner(input: ParseStream) -> Result<Vec<Self>>[src]

Parses zero or more inner attributes from the stream.

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

Trait Implementations

impl Clone for Attribute[src]

impl Debug for Attribute[src]

impl Eq for Attribute[src]

impl Hash for Attribute[src]

impl PartialEq<Attribute> for Attribute[src]

impl ToTokens for Attribute[src]

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

impl<T> Any for T where
    T: 'static + ?Sized
[src]

impl<T> Borrow<T> for T where
    T: ?Sized
[src]

impl<T> BorrowMut<T> for T where
    T: ?Sized
[src]

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 
[src]

impl<T> Spanned for T where
    T: Spanned + ?Sized
[src]

impl<T> ToOwned for T where
    T: Clone
[src]

type Owned = T

The resulting type after obtaining ownership.

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 
[src]

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 
[src]

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

The type returned in the event of a conversion error.