Parse a set of square brackets and expose their content to subsequent parsers.

Example

# use quote::quote;
#
use proc_macro2::TokenStream;
use syn::{bracketed, token, Result, Token};
use syn::parse::{Parse, ParseStream};

// Parse an outer attribute like:
//
//     #[repr(C, packed)]
struct OuterAttribute {
    pound_token: Token![#],
    bracket_token: token::Bracket,
    content: TokenStream,
}

impl Parse for OuterAttribute {
    fn parse(input: ParseStream) -> Result<Self> {
        let content;
        Ok(OuterAttribute {
            pound_token: input.parse()?,
            bracket_token: bracketed!(content in input),
            content: content.parse()?,
        })
    }
}
#
# fn main() {
#     let input = quote! {
#         #[repr(C, packed)]
#     };
#     syn::parse2::<OuterAttribute>(input).unwrap();
# }