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    use proc_macro2::TokenStream;
    use quote::quote;
    use syn::{
        parse::{Parse, ParseStream},
        Attribute, Result, Token, Visibility,
    };

    use super::PIN;
    use crate::utils::SliceExt;

    // To generate the correct `Unpin` implementation and the projection methods,
    // we need to collect the types of the pinned fields.
    // However, since proc-macro-attribute is applied before `cfg` and `cfg_attr`
    // on fields, we cannot be collecting field types properly at this timing.
    // So instead of generating the `Unpin` implementation and the projection
    // methods here, delegate their processing to proc-macro-derive.
    //
    // At this stage, only attributes are parsed and the following attributes are
    // added to the attributes of the item.
    // * `#[derive(InternalDerive)]` - An internal helper macro that does the above
    //   processing.
    // * `#[pin(__private(#args))]` - Pass the argument of `#[pin_project]` to
    //   proc-macro-derive (`InternalDerive`).

    pub(super) fn parse_attribute(args: &TokenStream, input: TokenStream) -> Result<TokenStream> {
        let Input { attrs, body } = syn::parse2(input)?;

        Ok(quote! {
            #(#attrs)*
            #[derive(::pin_project::__private::__PinProjectInternalDerive)]
            // Use `__private` to prevent users from trying to control `InternalDerive`
            // manually. `__private` does not guarantee compatibility between patch
            // versions, so it should be sufficient for this purpose in most cases.
            #[pin(__private(#args))]
            #body
        })
    }

    #[allow(dead_code)] // false positive that fixed in Rust 1.39
    struct Input {
        attrs: Vec<Attribute>,
        body: TokenStream,
    }

    impl Parse for Input {
        fn parse(input: ParseStream<'_>) -> Result<Self> {
            let attrs = input.call(Attribute::parse_outer)?;

            let ahead = input.fork();
            let _: Visibility = ahead.parse()?;
            if !ahead.peek(Token![struct]) && !ahead.peek(Token![enum]) {
                // If we check this only on proc-macro-derive, it may generate unhelpful error
                // messages. So it is preferable to be able to detect it here.
                Err(error!(
                    input.parse::<TokenStream>()?,
                    "#[pin_project] attribute may only be used on structs or enums"
                ))
            } else if let Some(attr) = attrs.find(PIN) {
                Err(error!(attr, "#[pin] attribute may only be used on fields of structs or variants"))
            } else if let Some(attr) = attrs.find("pin_project") {
                Err(error!(attr, "duplicate #[pin_project] attribute"))
            } else {
                Ok(Self { attrs, body: input.parse()? })
            }
        }
    }