use proc_macro2::{Span, TokenStream};
use quote::quote;
use syn::{
    parse::{Parse, ParseStream},
    *,
};

use crate::utils::{SliceExt, CURRENT_PRIVATE_MODULE};

use super::PIN;

// 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 { mut attrs, body } = syn::parse2(input)?;

    let private = Ident::new(CURRENT_PRIVATE_MODULE, Span::call_site());
    attrs.push(syn::parse_quote! {
        #[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.
    attrs.push(syn::parse_quote! {
        #[pin(#private(#args))]
    });

    Ok(quote! {
        #(#attrs)*
        #body
    })
}

#[allow(dead_code)] // https://github.com/rust-lang/rust/issues/56750
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 _vis: 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, "only one #[pin_project] attribute is allowed"))
        } else {
            Ok(Self { attrs, body: input.parse()? })
        }
    }
}