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    use proc_macro2::TokenStream;
    use quote::{format_ident, quote, ToTokens};
    use syn::{
        parse_quote, spanned::Spanned, visit_mut::VisitMut, Error, FnArg, GenericArgument, ImplItem,
        ItemImpl, Pat, PatIdent, Path, PathArguments, Result, ReturnType, Signature, Token, Type,
        TypePath, TypeReference,
    };

    use crate::utils::{parse_as_empty, prepend_underscore_to_self, ReplaceReceiver, SliceExt};

    pub(crate) fn attribute(args: &TokenStream, mut input: ItemImpl) -> TokenStream {
        let res = (|| -> Result<()> {
            parse_as_empty(args)?;
            validate_impl(&input)?;
            expand_impl(&mut input);
            Ok(())
        })();

        if let Err(e) = res {
            let mut tokens = e.to_compile_error();
            if let Type::Path(self_ty) = &*input.self_ty {
                let (impl_generics, _, where_clause) = input.generics.split_for_impl();

                // Generate a dummy impl of `PinnedDrop`.
                // In many cases, `#[pinned_drop] impl` is declared after `#[pin_project]`.
                // Therefore, if `pinned_drop` compile fails, you will also get an error
                // about `PinnedDrop` not being implemented.
                // This can be prevented to some extent by generating a dummy
                // `PinnedDrop` implementation.
                // We already know that we will get a compile error, so this won't
                // accidentally compile successfully.
                //
                // However, if `input.self_ty` is not Type::Path, there is a high possibility that
                // the type does not exist (since #[pin_project] can only be used on struct/enum
                // definitions), so do not generate a dummy impl.
                tokens.extend(quote! {
                    impl #impl_generics ::pin_project::__private::PinnedDrop for #self_ty
                    #where_clause
                    {
                        unsafe fn drop(self: ::pin_project::__private::Pin<&mut Self>) {}
                    }
                });
            }
            tokens
        } else {
            input.into_token_stream()
        }
    }

    /// Validates the signature of given `PinnedDrop` impl.
    fn validate_impl(item: &ItemImpl) -> Result<()> {
        const INVALID_ITEM: &str =
            "#[pinned_drop] may only be used on implementation for the `PinnedDrop` trait";

        if let Some(attr) = item.attrs.find("pinned_drop") {
            return Err(error!(attr, "duplicate #[pinned_drop] attribute"));
        }

        if let Some((_, path, _)) = &item.trait_ {
            if !path.is_ident("PinnedDrop") {
                return Err(error!(path, INVALID_ITEM));
            }
        } else {
            return Err(error!(item.self_ty, INVALID_ITEM));
        }

        if item.unsafety.is_some() {
            return Err(error!(item.unsafety, "implementing the trait `PinnedDrop` is not unsafe"));
        }
        if item.items.is_empty() {
            return Err(error!(item, "not all trait items implemented, missing: `drop`"));
        }

        match &*item.self_ty {
            Type::Path(_) => {}
            ty => {
                return Err(error!(
                    ty,
                    "implementing the trait `PinnedDrop` on this type is unsupported"
                ));
            }
        }

        item.items.iter().enumerate().try_for_each(|(i, item)| match item {
            ImplItem::Const(item) => {
                Err(error!(item, "const `{}` is not a member of trait `PinnedDrop`", item.ident))
            }
            ImplItem::Type(item) => {
                Err(error!(item, "type `{}` is not a member of trait `PinnedDrop`", item.ident))
            }
            ImplItem::Method(method) => {
                validate_sig(&method.sig)?;
                if i == 0 {
                    Ok(())
                } else {
                    Err(error!(method, "duplicate definitions with name `drop`"))
                }
            }
            _ => unreachable!("unexpected ImplItem"),
        })
    }

    /// Validates the signature of given `PinnedDrop::drop` method.
    ///
    /// The correct signature is: `(mut) self: (<path>::)Pin<&mut Self>`
    fn validate_sig(sig: &Signature) -> Result<()> {
        fn get_ty_path(ty: &Type) -> Option<&Path> {
            if let Type::Path(TypePath { qself: None, path }) = ty { Some(path) } else { None }
        }

        const INVALID_ARGUMENT: &str = "method `drop` must take an argument `self: Pin<&mut Self>`";

        if sig.ident != "drop" {
            return Err(error!(
                sig.ident,
                "method `{}` is not a member of trait `PinnedDrop", sig.ident,
            ));
        }

        if let ReturnType::Type(_, ty) = &sig.output {
            match &**ty {
                Type::Tuple(ty) if ty.elems.is_empty() => {}
                _ => return Err(error!(ty, "method `drop` must return the unit type")),
            }
        }

        match sig.inputs.len() {
            1 => {}
            0 => return Err(Error::new(sig.paren_token.span, INVALID_ARGUMENT)),
            _ => return Err(error!(sig.inputs, INVALID_ARGUMENT)),
        }

        if let Some(FnArg::Typed(arg)) = sig.receiver() {
            // (mut) self: <path>
            if let Some(path) = get_ty_path(&arg.ty) {
                let ty = path.segments.last().unwrap();
                if let PathArguments::AngleBracketed(args) = &ty.arguments {
                    // (mut) self: (<path>::)<ty><&mut <elem>..>
                    if let Some(GenericArgument::Type(Type::Reference(TypeReference {
                        mutability: Some(_),
                        elem,
                        ..
                    }))) = args.args.first()
                    {
                        // (mut) self: (<path>::)Pin<&mut Self>
                        if args.args.len() == 1
                            && ty.ident == "Pin"
                            && get_ty_path(elem).map_or(false, |path| path.is_ident("Self"))
                        {
                            if sig.unsafety.is_some() {
                                return Err(error!(
                                    sig.unsafety,
                                    "implementing the method `drop` is not unsafe"
                                ));
                            }
                            return Ok(());
                        }
                    }
                }
            }
        }

        Err(error!(sig.inputs[0], INVALID_ARGUMENT))
    }

    // from:
    //
    // fn drop(self: Pin<&mut Self>) {
    //     // ...
    // }
    //
    // into:
    //
    // unsafe fn drop(self: Pin<&mut Self>) {
    //     fn __drop_inner<T>(__self: Pin<&mut Foo<'_, T>>) {
    //         fn __drop_inner() {}
    //         // ...
    //     }
    //     __drop_inner(self);
    // }
    //
    fn expand_impl(item: &mut ItemImpl) {
        fn get_arg_pat(arg: &mut FnArg) -> Option<&mut PatIdent> {
            if let FnArg::Typed(arg) = arg {
                if let Pat::Ident(ident) = &mut *arg.pat {
                    return Some(ident);
                }
            }
            None
        }

        let path = &mut item.trait_.as_mut().unwrap().1;
        *path = parse_quote_spanned! { path.span() =>
            ::pin_project::__private::PinnedDrop
        };

        let method =
            if let ImplItem::Method(method) = &mut item.items[0] { method } else { unreachable!() };

        // `fn drop(mut self: Pin<&mut Self>)` -> `fn __drop_inner<T>(mut __self: Pin<&mut Receiver>)`
        let drop_inner = {
            let mut drop_inner = method.clone();
            let ident = format_ident!("__drop_inner");
            // Add a dummy `__drop_inner` function to prevent users call outer `__drop_inner`.
            drop_inner.block.stmts.insert(0, parse_quote!(fn #ident() {}));
            drop_inner.sig.ident = ident;
            drop_inner.sig.generics = item.generics.clone();
            let self_pat = get_arg_pat(&mut drop_inner.sig.inputs[0]).unwrap();
            prepend_underscore_to_self(&mut self_pat.ident);
            let self_ty = if let Type::Path(ty) = &*item.self_ty { ty } else { unreachable!() };
            let mut visitor = ReplaceReceiver(self_ty);
            visitor.visit_signature_mut(&mut drop_inner.sig);
            visitor.visit_block_mut(&mut drop_inner.block);
            drop_inner
        };

        // `fn drop(mut self: Pin<&mut Self>)` -> `unsafe fn drop(self: Pin<&mut Self>)`
        method.sig.unsafety = Some(<Token![unsafe]>::default());
        let self_pat = get_arg_pat(&mut method.sig.inputs[0]).unwrap();
        self_pat.mutability = None;
        let self_token = &self_pat.ident;

        method.block.stmts = parse_quote! {
            #[allow(clippy::needless_pass_by_value)] // This lint does not warn the receiver.
            #drop_inner
            __drop_inner(#self_token);
        };
    }