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    use proc_macro2::{Delimiter, Group, Span, TokenStream};
    use quote::{format_ident, quote, quote_spanned, ToTokens};
    use syn::{
        parse_quote, token, visit_mut::VisitMut, Attribute, Data, DataEnum, DeriveInput, Error, Field,
        Fields, FieldsNamed, FieldsUnnamed, Generics, Ident, Index, Lifetime, LifetimeDef, Meta,
        MetaList, NestedMeta, Result, Token, Type, Variant, Visibility, WhereClause,
    };

    use super::{
        args::{parse_args, Args, ProjReplace, UnpinImpl},
        PIN,
    };
    use crate::utils::{
        determine_lifetime_name, determine_visibility, insert_lifetime_and_bound, ReplaceReceiver,
        SliceExt, Variants,
    };

    pub(super) fn parse_derive(input: TokenStream) -> Result<TokenStream> {
        let mut input: DeriveInput = syn::parse2(input)?;

        let mut cx;
        let mut generate = GenerateTokens::default();

        let ident = &input.ident;
        let ty_generics = input.generics.split_for_impl().1;
        let self_ty = parse_quote!(#ident #ty_generics);
        let mut visitor = ReplaceReceiver(&self_ty);
        visitor.visit_generics_mut(&mut input.generics);
        visitor.visit_data_mut(&mut input.data);

        match &input.data {
            Data::Struct(data) => {
                cx = Context::new(&input.attrs, &input.vis, ident, &mut input.generics, Struct)?;
                parse_struct(&mut cx, &data.fields, &mut generate)?;
            }
            Data::Enum(data) => {
                cx = Context::new(&input.attrs, &input.vis, ident, &mut input.generics, Enum)?;
                parse_enum(&mut cx, data, &mut generate)?;
            }
            Data::Union(_) => {
                return Err(error!(
                    input,
                    "#[pin_project] attribute may only be used on structs or enums"
                ));
            }
        }

        Ok(generate.into_tokens(&cx))
    }

    #[derive(Default)]
    struct GenerateTokens {
        exposed: TokenStream,
        scoped: TokenStream,
    }

    impl GenerateTokens {
        fn extend(&mut self, expose: bool, tokens: TokenStream) {
            if expose {
                self.exposed.extend(tokens);
            } else {
                self.scoped.extend(tokens);
            }
        }

        fn into_tokens(self, cx: &Context<'_>) -> TokenStream {
            let mut tokens = self.exposed;
            let scoped = self.scoped;

            let unpin_impl = make_unpin_impl(cx);
            let drop_impl = make_drop_impl(cx);
            let allowed_lints = global_allowed_lints();

            tokens.extend(quote! {
                // All items except projected types are generated inside a `const` scope.
                // This makes it impossible for user code to refer to these types.
                // However, this prevents Rustdoc from displaying docs for any
                // of our types. In particular, users cannot see the
                // automatically generated `Unpin` impl for the '__UnpinStruct' types
                //
                // Previously, we provided a flag to correctly document the
                // automatically generated `Unpin` impl by using def-site hygiene,
                // but it is now removed.
                //
                // Refs:
                // * https://github.com/rust-lang/rust/issues/63281
                // * https://github.com/taiki-e/pin-project/pull/53#issuecomment-525906867
                // * https://github.com/taiki-e/pin-project/pull/70
                #allowed_lints
                #[allow(clippy::used_underscore_binding)]
                const _: () = {
                    #scoped
                    #unpin_impl
                    #drop_impl
                };
            });
            tokens
        }
    }

    /// Returns attributes that should be applied to all generated code.
    fn global_allowed_lints() -> TokenStream {
        quote! {
            #[allow(box_pointers)] // This lint warns use of the `Box` type.
            #[allow(deprecated)]
            #[allow(explicit_outlives_requirements)] // https://github.com/rust-lang/rust/issues/60993
            #[allow(single_use_lifetimes)] // https://github.com/rust-lang/rust/issues/55058
            #[allow(unreachable_pub)] // This lint warns `pub` field in private struct.
            // This lint warns of `clippy::*` generated by external macros.
            // We allow this lint for compatibility with older compilers.
            #[allow(clippy::unknown_clippy_lints)]
            #[allow(clippy::pattern_type_mismatch)]
            #[allow(clippy::redundant_pub_crate)] // This lint warns `pub(crate)` field in private struct.
        }
    }

    /// Returns attributes used on projected types.
    fn proj_allowed_lints(kind: TypeKind) -> (TokenStream, TokenStream, TokenStream) {
        let large_enum_variant = if kind == Enum {
            Some(quote! {
                #[allow(variant_size_differences)]
                #[allow(clippy::large_enum_variant)]
            })
        } else {
            None
        };
        let global_allowed_lints = global_allowed_lints();
        let proj_mut = quote! {
            #global_allowed_lints
            #[allow(dead_code)] // This lint warns unused fields/variants.
            #[allow(clippy::mut_mut)] // This lint warns `&mut &mut <ty>`.
            #[allow(clippy::type_repetition_in_bounds)] // https://github.com/rust-lang/rust-clippy/issues/4326}
        };
        let proj_ref = quote! {
            #global_allowed_lints
            #[allow(dead_code)] // This lint warns unused fields/variants.
            #[allow(clippy::ref_option_ref)] // This lint warns `&Option<&<ty>>`.
            #[allow(clippy::type_repetition_in_bounds)] // https://github.com/rust-lang/rust-clippy/issues/4326
        };
        let proj_own = quote! {
            #global_allowed_lints
            #[allow(dead_code)] // This lint warns unused fields/variants.
            #large_enum_variant
        };
        (proj_mut, proj_ref, proj_own)
    }

    struct Context<'a> {
        /// The original type.
        orig: OriginalType<'a>,
        /// The projected types.
        proj: ProjectedType,
        /// Types of the pinned fields.
        pinned_fields: Vec<Type>,
        /// Kind of the original type: struct or enum
        kind: TypeKind,

        /// `PinnedDrop` argument.
        pinned_drop: Option<Span>,
        /// `UnsafeUnpin` or `!Unpin` argument.
        unpin_impl: UnpinImpl,
        /// `project` argument.
        project: bool,
        /// `project_ref` argument.
        project_ref: bool,
        /// `project_replace [= <ident>]` argument.
        project_replace: ProjReplace,
    }

    impl<'a> Context<'a> {
        fn new(
            attrs: &'a [Attribute],
            vis: &'a Visibility,
            ident: &'a Ident,
            generics: &'a mut Generics,
            kind: TypeKind,
        ) -> Result<Self> {
            let Args { pinned_drop, unpin_impl, project, project_ref, project_replace } =
                parse_args(attrs)?;

            if let Some(name) = [project.as_ref(), project_ref.as_ref(), project_replace.ident()]
                .iter()
                .filter_map(Option::as_ref)
                .find(|name| **name == ident)
            {
                return Err(error!(name, "name `{}` is the same as the original type name", name));
            }

            let mut lifetime_name = String::from("'pin");
            determine_lifetime_name(&mut lifetime_name, generics);
            let lifetime = Lifetime::new(&lifetime_name, Span::call_site());

            let ty_generics = generics.split_for_impl().1;
            let ty_generics_as_generics = parse_quote!(#ty_generics);
            let mut proj_generics = generics.clone();
            let pred = insert_lifetime_and_bound(
                &mut proj_generics,
                lifetime.clone(),
                &ty_generics_as_generics,
                ident,
            );
            let mut where_clause = generics.make_where_clause().clone();
            where_clause.predicates.push(pred);

            let own_ident = project_replace
                .ident()
                .cloned()
                .unwrap_or_else(|| format_ident!("__{}ProjectionOwned", ident));

            Ok(Self {
                kind,
                pinned_drop,
                unpin_impl,
                project: project.is_some(),
                project_ref: project_ref.is_some(),
                project_replace,
                proj: ProjectedType {
                    vis: determine_visibility(vis),
                    mut_ident: project.unwrap_or_else(|| format_ident!("__{}Projection", ident)),
                    ref_ident: project_ref.unwrap_or_else(|| format_ident!("__{}ProjectionRef", ident)),
                    own_ident,
                    lifetime,
                    generics: proj_generics,
                    where_clause,
                },
                orig: OriginalType { attrs, vis, ident, generics },
                pinned_fields: Vec::new(),
            })
        }
    }

    #[derive(Copy, Clone, Eq, PartialEq)]
    enum TypeKind {
        Enum,
        Struct,
    }

    use TypeKind::{Enum, Struct};

    struct OriginalType<'a> {
        /// Attributes of the original type.
        attrs: &'a [Attribute],
        /// Visibility of the original type.
        vis: &'a Visibility,
        /// Name of the original type.
        ident: &'a Ident,
        /// Generics of the original type.
        generics: &'a Generics,
    }

    struct ProjectedType {
        /// Visibility of the projected types.
        vis: Visibility,
        /// Name of the projected type returned by `project` method.
        mut_ident: Ident,
        /// Name of the projected type returned by `project_ref` method.
        ref_ident: Ident,
        /// Name of the projected type returned by `project_replace` method.
        own_ident: Ident,
        /// Lifetime on the generated projected types.
        lifetime: Lifetime,
        /// Generics of the projected types.
        generics: Generics,
        /// `where` clause of the projected types. This has an additional
        /// bound generated by `insert_lifetime_and_bound`
        where_clause: WhereClause,
    }

    struct ProjectedVariants {
        proj_variants: TokenStream,
        proj_ref_variants: TokenStream,
        proj_own_variants: TokenStream,
        proj_arms: TokenStream,
        proj_ref_arms: TokenStream,
        proj_own_arms: TokenStream,
    }

    #[derive(Default)]
    struct ProjectedFields {
        proj_pat: TokenStream,
        proj_body: TokenStream,
        proj_own_body: TokenStream,
        proj_fields: TokenStream,
        proj_ref_fields: TokenStream,
        proj_own_fields: TokenStream,
    }

    fn validate_struct(ident: &Ident, fields: &Fields) -> Result<()> {
        if fields.is_empty() {
            let msg = "#[pin_project] attribute may not be used on structs with zero fields";
            if let Fields::Unit = fields { Err(error!(ident, msg)) } else { Err(error!(fields, msg)) }
        } else {
            Ok(())
        }
    }

    fn validate_enum(brace_token: token::Brace, variants: &Variants) -> Result<()> {
        if variants.is_empty() {
            return Err(Error::new(
                brace_token.span,
                "#[pin_project] attribute may not be used on enums without variants",
            ));
        }
        let has_field = variants.iter().try_fold(false, |has_field, v| {
            if let Some((_, e)) = &v.discriminant {
                Err(error!(e, "#[pin_project] attribute may not be used on enums with discriminants"))
            } else if let Some(attr) = v.attrs.find(PIN) {
                Err(error!(attr, "#[pin] attribute may only be used on fields of structs or variants"))
            } else if v.fields.is_empty() {
                Ok(has_field)
            } else {
                Ok(true)
            }
        })?;
        if has_field {
            Ok(())
        } else {
            Err(error!(variants, "#[pin_project] attribute may not be used on enums with zero fields"))
        }
    }

    fn parse_struct(
        cx: &mut Context<'_>,
        fields: &Fields,
        generate: &mut GenerateTokens,
    ) -> Result<()> {
        // Do this first for a better error message.
        let packed_check = ensure_not_packed(&cx.orig, fields)?;

        validate_struct(cx.orig.ident, fields)?;

        let ProjectedFields {
            proj_pat,
            proj_body,
            proj_fields,
            proj_ref_fields,
            proj_own_fields,
            proj_own_body,
        } = match fields {
            Fields::Named(_) => visit_fields(cx, None, fields, Delimiter::Brace)?,
            Fields::Unnamed(_) => visit_fields(cx, None, fields, Delimiter::Parenthesis)?,
            Fields::Unit => unreachable!(),
        };

        let proj_ident = &cx.proj.mut_ident;
        let proj_ref_ident = &cx.proj.ref_ident;
        let proj_own_ident = &cx.proj.own_ident;
        let vis = &cx.proj.vis;
        let mut orig_generics = cx.orig.generics.clone();
        let orig_where_clause = orig_generics.where_clause.take();
        let proj_generics = &cx.proj.generics;
        let proj_where_clause = &cx.proj.where_clause;

        // For tuple structs, we need to generate `(T1, T2) where Foo: Bar`
        // For non-tuple structs, we need to generate `where Foo: Bar { field1: T }`
        let (where_clause_fields, where_clause_ref_fields, where_clause_own_fields) = match fields {
            Fields::Named(_) => (
                quote!(#proj_where_clause #proj_fields),
                quote!(#proj_where_clause #proj_ref_fields),
                quote!(#orig_where_clause #proj_own_fields),
            ),
            Fields::Unnamed(_) => (
                quote!(#proj_fields #proj_where_clause;),
                quote!(#proj_ref_fields #proj_where_clause;),
                quote!(#proj_own_fields #orig_where_clause;),
            ),
            Fields::Unit => unreachable!(),
        };

        let (proj_attrs, proj_ref_attrs, proj_own_attrs) = proj_allowed_lints(cx.kind);
        generate.extend(cx.project, quote! {
            #proj_attrs
            #vis struct #proj_ident #proj_generics #where_clause_fields
        });
        generate.extend(cx.project_ref, quote! {
            #proj_ref_attrs
            #vis struct #proj_ref_ident #proj_generics #where_clause_ref_fields
        });
        if cx.project_replace.span().is_some() {
            generate.extend(cx.project_replace.ident().is_some(), quote! {
                #proj_own_attrs
                #vis struct #proj_own_ident #orig_generics #where_clause_own_fields
            });
        }

        let proj_mut_body = quote! {
            let Self #proj_pat = self.get_unchecked_mut();
            #proj_ident #proj_body
        };
        let proj_ref_body = quote! {
            let Self #proj_pat = self.get_ref();
            #proj_ref_ident #proj_body
        };
        let proj_own_body = quote! {
            let Self #proj_pat = &mut *__self_ptr;
            #proj_own_body
        };
        generate.extend(false, make_proj_impl(cx, &proj_mut_body, &proj_ref_body, &proj_own_body));

        generate.extend(false, packed_check);
        Ok(())
    }

    fn parse_enum(
        cx: &mut Context<'_>,
        DataEnum { brace_token, variants, .. }: &DataEnum,
        generate: &mut GenerateTokens,
    ) -> Result<()> {
        if let ProjReplace::Unnamed { span } = &cx.project_replace {
            return Err(Error::new(
                *span,
                "`project_replace` argument requires a value when used on enums",
            ));
        }

        // We don't need to check for `#[repr(packed)]`,
        // since it does not apply to enums.

        validate_enum(*brace_token, variants)?;

        let ProjectedVariants {
            proj_variants,
            proj_ref_variants,
            proj_own_variants,
            proj_arms,
            proj_ref_arms,
            proj_own_arms,
        } = visit_variants(cx, variants)?;

        let proj_ident = &cx.proj.mut_ident;
        let proj_ref_ident = &cx.proj.ref_ident;
        let proj_own_ident = &cx.proj.own_ident;
        let vis = &cx.proj.vis;
        let mut orig_generics = cx.orig.generics.clone();
        let orig_where_clause = orig_generics.where_clause.take();
        let proj_generics = &cx.proj.generics;
        let proj_where_clause = &cx.proj.where_clause;

        let (proj_attrs, proj_ref_attrs, proj_own_attrs) = proj_allowed_lints(cx.kind);
        if cx.project {
            generate.extend(true, quote! {
                #proj_attrs
                #vis enum #proj_ident #proj_generics #proj_where_clause {
                    #proj_variants
                }
            });
        }
        if cx.project_ref {
            generate.extend(true, quote! {
                #proj_ref_attrs
                #vis enum #proj_ref_ident #proj_generics #proj_where_clause {
                    #proj_ref_variants
                }
            });
        }
        if cx.project_replace.ident().is_some() {
            generate.extend(true, quote! {
                #proj_own_attrs
                #vis enum #proj_own_ident #orig_generics #orig_where_clause {
                    #proj_own_variants
                }
            });
        }

        let proj_mut_body = quote! {
            match self.get_unchecked_mut() {
                #proj_arms
            }
        };
        let proj_ref_body = quote! {
            match self.get_ref() {
                #proj_ref_arms
            }
        };
        let proj_own_body = quote! {
            match &mut *__self_ptr {
                #proj_own_arms
            }
        };
        generate.extend(false, make_proj_impl(cx, &proj_mut_body, &proj_ref_body, &proj_own_body));

        Ok(())
    }

    fn visit_variants(cx: &mut Context<'_>, variants: &Variants) -> Result<ProjectedVariants> {
        let mut proj_variants = TokenStream::new();
        let mut proj_ref_variants = TokenStream::new();
        let mut proj_own_variants = TokenStream::new();
        let mut proj_arms = TokenStream::new();
        let mut proj_ref_arms = TokenStream::new();
        let mut proj_own_arms = TokenStream::new();

        for Variant { ident, fields, .. } in variants {
            let ProjectedFields {
                proj_pat,
                proj_body,
                proj_fields,
                proj_ref_fields,
                proj_own_fields,
                proj_own_body,
            } = match fields {
                Fields::Named(_) => visit_fields(cx, Some(ident), fields, Delimiter::Brace)?,
                Fields::Unnamed(_) => visit_fields(cx, Some(ident), fields, Delimiter::Parenthesis)?,
                Fields::Unit => ProjectedFields {
                    proj_own_body: proj_own_body(cx, Some(ident), None, &[]),
                    ..ProjectedFields::default()
                },
            };

            let proj_ident = &cx.proj.mut_ident;
            let proj_ref_ident = &cx.proj.ref_ident;
            proj_variants.extend(quote! {
                #ident #proj_fields,
            });
            proj_ref_variants.extend(quote! {
                #ident #proj_ref_fields,
            });
            proj_own_variants.extend(quote! {
                #ident #proj_own_fields,
            });
            proj_arms.extend(quote! {
                Self::#ident #proj_pat => #proj_ident::#ident #proj_body,
            });
            proj_ref_arms.extend(quote! {
                Self::#ident #proj_pat => #proj_ref_ident::#ident #proj_body,
            });
            proj_own_arms.extend(quote! {
                Self::#ident #proj_pat => { #proj_own_body }
            });
        }

        Ok(ProjectedVariants {
            proj_variants,
            proj_ref_variants,
            proj_own_variants,
            proj_arms,
            proj_ref_arms,
            proj_own_arms,
        })
    }

    fn visit_fields(
        cx: &mut Context<'_>,
        variant_ident: Option<&Ident>,
        fields: &Fields,
        delim: Delimiter,
    ) -> Result<ProjectedFields> {
        fn surround(delim: Delimiter, tokens: TokenStream) -> TokenStream {
            Group::new(delim, tokens).into_token_stream()
        }

        let mut proj_pat = TokenStream::new();
        let mut proj_body = TokenStream::new();
        let mut proj_fields = TokenStream::new();
        let mut proj_ref_fields = TokenStream::new();
        let mut proj_own_fields = TokenStream::new();
        let mut proj_move = TokenStream::new();
        let mut pinned_bindings = Vec::with_capacity(fields.len());

        for (i, Field { attrs, vis, ident, colon_token, ty }) in fields.iter().enumerate() {
            let binding = ident.clone().unwrap_or_else(|| format_ident!("_{}", i));
            proj_pat.extend(quote!(#binding,));
            if attrs.position_exact(PIN)?.is_some() {
                let lifetime = &cx.proj.lifetime;
                proj_fields.extend(quote! {
                    #vis #ident #colon_token ::pin_project::__private::Pin<&#lifetime mut (#ty)>,
                });
                proj_ref_fields.extend(quote! {
                    #vis #ident #colon_token ::pin_project::__private::Pin<&#lifetime (#ty)>,
                });
                proj_own_fields.extend(quote! {
                    #vis #ident #colon_token ::pin_project::__private::PhantomData<#ty>,
                });
                proj_body.extend(quote! {
                    #ident #colon_token ::pin_project::__private::Pin::new_unchecked(#binding),
                });
                proj_move.extend(quote! {
                    #ident #colon_token ::pin_project::__private::PhantomData,
                });

                cx.pinned_fields.push(ty.clone());
                pinned_bindings.push(binding);
            } else {
                let lifetime = &cx.proj.lifetime;
                proj_fields.extend(quote! {
                    #vis #ident #colon_token &#lifetime mut (#ty),
                });
                proj_ref_fields.extend(quote! {
                    #vis #ident #colon_token &#lifetime (#ty),
                });
                proj_own_fields.extend(quote! {
                    #vis #ident #colon_token #ty,
                });
                proj_body.extend(quote! {
                    #binding,
                });
                proj_move.extend(quote! {
                    #ident #colon_token ::pin_project::__private::ptr::read(#binding),
                });
            }
        }

        let proj_pat = surround(delim, proj_pat);
        let proj_body = surround(delim, proj_body);
        let proj_fields = surround(delim, proj_fields);
        let proj_ref_fields = surround(delim, proj_ref_fields);
        let proj_own_fields = surround(delim, proj_own_fields);

        let proj_move = Group::new(delim, proj_move);
        let proj_own_body = proj_own_body(cx, variant_ident, Some(proj_move), &pinned_bindings);

        Ok(ProjectedFields {
            proj_pat,
            proj_body,
            proj_own_body,
            proj_fields,
            proj_ref_fields,
            proj_own_fields,
        })
    }

    /// Generates the processing that `project_replace` does for the struct or each variant.
    ///
    /// Note: `pinned_fields` must be in declaration order.
    fn proj_own_body(
        cx: &Context<'_>,
        variant_ident: Option<&Ident>,
        proj_move: Option<Group>,
        pinned_fields: &[Ident],
    ) -> TokenStream {
        let ident = &cx.proj.own_ident;
        let proj_own = match variant_ident {
            Some(variant_ident) => quote!(#ident::#variant_ident),
            None => quote!(#ident),
        };

        // The fields of the struct and the active enum variant are dropped
        // in declaration order.
        // Refs: https://doc.rust-lang.org/reference/destructors.html
        let pinned_fields = pinned_fields.iter().rev();

        quote! {
            // First, extract all the unpinned fields.
            let __result = #proj_own #proj_move;

            // Now create guards to drop all the pinned fields.
            //
            // Due to a compiler bug (https://github.com/rust-lang/rust/issues/47949)
            // this must be in its own scope, or else `__result` will not be dropped
            // if any of the destructors panic.
            {
                #(
                    let __guard = ::pin_project::__private::UnsafeDropInPlaceGuard(#pinned_fields);
                )*
            }

            // Finally, return the result.
            __result
        }
    }

    /// Creates `Unpin` implementation for the original type.
    ///
    /// The kind of `Unpin` impl generated depends on `unpin_impl` field:
    /// * `UnpinImpl::Unsafe` - Implements `Unpin` via `UnsafeUnpin` impl.
    /// * `UnpinImpl::Negative` - Generates `Unpin` impl with bounds that will never be true.
    /// * `UnpinImpl::Default` - Generates `Unpin` impl that requires `Unpin` for all pinned fields.
    fn make_unpin_impl(cx: &Context<'_>) -> TokenStream {
        match cx.unpin_impl {
            UnpinImpl::Unsafe(span) => {
                let mut proj_generics = cx.proj.generics.clone();
                let orig_ident = cx.orig.ident;
                let lifetime = &cx.proj.lifetime;

                // Make the error message highlight `UnsafeUnpin` argument.
                proj_generics.make_where_clause().predicates.push(parse_quote_spanned! { span =>
                    ::pin_project::__private::Wrapper<#lifetime, Self>: ::pin_project::UnsafeUnpin
                });

                let (impl_generics, _, where_clause) = proj_generics.split_for_impl();
                let ty_generics = cx.orig.generics.split_for_impl().1;

                quote_spanned! { span =>
                    impl #impl_generics ::pin_project::__private::Unpin for #orig_ident #ty_generics
                    #where_clause
                    {
                    }
                }
            }
            UnpinImpl::Negative(span) => {
                let mut proj_generics = cx.proj.generics.clone();
                let orig_ident = cx.orig.ident;
                let lifetime = &cx.proj.lifetime;

                proj_generics.make_where_clause().predicates.push(parse_quote! {
                    ::pin_project::__private::Wrapper<
                        #lifetime, ::pin_project::__private::PhantomPinned
                    >: ::pin_project::__private::Unpin
                });

                let (proj_impl_generics, _, proj_where_clause) = proj_generics.split_for_impl();
                let ty_generics = cx.orig.generics.split_for_impl().1;

                // For interoperability with `forbid(unsafe_code)`, `unsafe` token should be
                // call-site span.
                let unsafety = <Token![unsafe]>::default();
                quote_spanned! { span =>
                    impl #proj_impl_generics ::pin_project::__private::Unpin
                        for #orig_ident #ty_generics
                    #proj_where_clause
                    {
                    }

                    // Generate a dummy impl of `UnsafeUnpin`, to ensure that the user cannot implement it.
                    //
                    // To ensure that users don't accidentally write a non-functional `UnsafeUnpin`
                    // impls, we emit one ourselves. If the user ends up writing an `UnsafeUnpin`
                    // impl, they'll get a "conflicting implementations of trait" error when
                    // coherence checks are run.
                    #[doc(hidden)]
                    #unsafety impl #proj_impl_generics ::pin_project::UnsafeUnpin
                        for #orig_ident #ty_generics
                    #proj_where_clause
                    {
                    }
                }
            }
            UnpinImpl::Default => {
                let mut full_where_clause = cx.orig.generics.where_clause.clone().unwrap();

                // Generate a field in our new struct for every
                // pinned field in the original type.
                let fields = cx.pinned_fields.iter().enumerate().map(|(i, ty)| {
                    let field_ident = format_ident!("__field{}", i);
                    quote!(#field_ident: #ty)
                });

                // We could try to determine the subset of type parameters
                // and lifetimes that are actually used by the pinned fields
                // (as opposed to those only used by unpinned fields).
                // However, this would be tricky and error-prone, since
                // it's possible for users to create types that would alias
                // with generic parameters (e.g. 'struct T').
                //
                // Instead, we generate a use of every single type parameter
                // and lifetime used in the original struct. For type parameters,
                // we generate code like this:
                //
                // ```rust
                // struct AlwaysUnpin<T: ?Sized>(PhantomData<T>) {}
                // impl<T: ?Sized> Unpin for AlwaysUnpin<T> {}
                //
                // ...
                // _field: AlwaysUnpin<(A, B, C)>
                // ```
                //
                // This ensures that any unused type parameters
                // don't end up with `Unpin` bounds.
                let lifetime_fields = cx.orig.generics.lifetimes().enumerate().map(
                    |(i, LifetimeDef { lifetime, .. })| {
                        let field_ident = format_ident!("__lifetime{}", i);
                        quote!(#field_ident: &#lifetime ())
                    },
                );

                let orig_ident = cx.orig.ident;
                let struct_ident = format_ident!("__{}", orig_ident);
                let vis = cx.orig.vis;
                let lifetime = &cx.proj.lifetime;
                let type_params = cx.orig.generics.type_params().map(|t| &t.ident);
                let proj_generics = &cx.proj.generics;
                let (proj_impl_generics, proj_ty_generics, _) = proj_generics.split_for_impl();
                let (_, ty_generics, where_clause) = cx.orig.generics.split_for_impl();

                full_where_clause.predicates.push(parse_quote! {
                    #struct_ident #proj_ty_generics: ::pin_project::__private::Unpin
                });

                quote! {
                    // This needs to have the same visibility as the original type,
                    // due to the limitations of the 'public in private' error.
                    //
                    // Our goal is to implement the public trait `Unpin` for
                    // a potentially public user type. Because of this, rust
                    // requires that any types mentioned in the where clause of
                    // our `Unpin` impl also be public. This means that our generated
                    // `__UnpinStruct` type must also be public.
                    // However, we ensure that the user can never actually reference
                    // this 'public' type by creating this type in the inside of `const`.
                    #[allow(missing_debug_implementations)]
                    #vis struct #struct_ident #proj_generics #where_clause {
                        __pin_project_use_generics: ::pin_project::__private::AlwaysUnpin<
                            #lifetime, (#(::pin_project::__private::PhantomData<#type_params>),*)
                        >,

                        #(#fields,)*
                        #(#lifetime_fields,)*
                    }

                    impl #proj_impl_generics ::pin_project::__private::Unpin
                        for #orig_ident #ty_generics
                    #full_where_clause
                    {
                    }

                    // Generate a dummy impl of `UnsafeUnpin`, to ensure that the user cannot implement it.
                    //
                    // To ensure that users don't accidentally write a non-functional `UnsafeUnpin`
                    // impls, we emit one ourselves. If the user ends up writing an `UnsafeUnpin`
                    // impl, they'll get a "conflicting implementations of trait" error when
                    // coherence checks are run.
                    #[doc(hidden)]
                    unsafe impl #proj_impl_generics ::pin_project::UnsafeUnpin
                        for #orig_ident #ty_generics
                    #full_where_clause
                    {
                    }
                }
            }
        }
    }

    /// Creates `Drop` implementation for the original type.
    ///
    /// The kind of `Drop` impl generated depends on `pinned_drop` field:
    /// * `Some` - implements `Drop` via `PinnedDrop` impl.
    /// * `None` - generates code that ensures that `Drop` trait is not implemented,
    ///            instead of generating `Drop` impl.
    fn make_drop_impl(cx: &Context<'_>) -> TokenStream {
        let ident = cx.orig.ident;
        let (impl_generics, ty_generics, where_clause) = cx.orig.generics.split_for_impl();

        if let Some(span) = cx.pinned_drop {
            // For interoperability with `forbid(unsafe_code)`, `unsafe` token should be
            // call-site span.
            let unsafety = <Token![unsafe]>::default();
            quote_spanned! { span =>
                impl #impl_generics ::pin_project::__private::Drop for #ident #ty_generics
                #where_clause
                {
                    fn drop(&mut self) {
                        #unsafety {
                            // Safety - we're in 'drop', so we know that 'self' will
                            // never move again.
                            let __pinned_self = ::pin_project::__private::Pin::new_unchecked(self);
                            // We call `pinned_drop` only once. Since `PinnedDrop::drop`
                            // is an unsafe method and a private API, it is never called again in safe
                            // code *unless the user uses a maliciously crafted macro*.
                            ::pin_project::__private::PinnedDrop::drop(__pinned_self);
                        }
                    }
                }
            }
        } else {
            // If the user does not provide a `PinnedDrop` impl,
            // we need to ensure that they don't provide a `Drop` impl of their
            // own.
            // Based on https://github.com/upsuper/assert-impl/blob/f503255b292ab0ba8d085b657f4065403cfa46eb/src/lib.rs#L80-L87
            //
            // We create a new identifier for each struct, so that the traits
            // for different types do not conflict with each other.
            //
            // Another approach would be to provide an empty Drop impl,
            // which would conflict with a user-provided Drop impl.
            // However, this would trigger the compiler's special handling
            // of Drop types (e.g. fields cannot be moved out of a Drop type).
            // This approach prevents the creation of needless Drop impls,
            // giving users more flexibility.
            let trait_ident = format_ident!("{}MustNotImplDrop", ident);

            quote! {
                // There are two possible cases:
                // 1. The user type does not implement Drop. In this case,
                // the first blanked impl will not apply to it. This code
                // will compile, as there is only one impl of MustNotImplDrop for the user type
                // 2. The user type does impl Drop. This will make the blanket impl applicable,
                // which will then conflict with the explicit MustNotImplDrop impl below.
                // This will result in a compilation error, which is exactly what we want.
                trait #trait_ident {}
                #[allow(clippy::drop_bounds, drop_bounds)]
                impl<T: ::pin_project::__private::Drop> #trait_ident for T {}
                impl #impl_generics #trait_ident for #ident #ty_generics #where_clause {}

                // Generate a dummy impl of `PinnedDrop`, to ensure that the user cannot implement it.
                // Since the user did not pass `PinnedDrop` to `#[pin_project]`, any `PinnedDrop`
                // impl will not actually be called. Unfortunately, we can't detect this situation
                // directly from either the `#[pin_project]` or `#[pinned_drop]` attributes, since
                // we don't know what other attirbutes/impl may exist.
                //
                // To ensure that users don't accidentally write a non-functional `PinnedDrop`
                // impls, we emit one ourselves. If the user ends up writing a `PinnedDrop` impl,
                // they'll get a "conflicting implementations of trait" error when coherence
                // checks are run.
                #[doc(hidden)]
                impl #impl_generics ::pin_project::__private::PinnedDrop for #ident #ty_generics
                #where_clause
                {
                    unsafe fn drop(self: ::pin_project::__private::Pin<&mut Self>) {}
                }
            }
        }
    }

    /// Creates an implementation of the projection methods.
    ///
    /// On structs, both the `project` and `project_ref` methods are always generated,
    /// and the `project_replace` method is only generated if `ProjReplace::span` is `Some`.
    ///
    /// On enums, only methods that the returned projected type is named will be generated.
    fn make_proj_impl(
        cx: &Context<'_>,
        proj_body: &TokenStream,
        proj_ref_body: &TokenStream,
        proj_own_body: &TokenStream,
    ) -> TokenStream {
        let vis = &cx.proj.vis;
        let lifetime = &cx.proj.lifetime;
        let orig_ident = cx.orig.ident;
        let proj_ident = &cx.proj.mut_ident;
        let proj_ref_ident = &cx.proj.ref_ident;
        let proj_own_ident = &cx.proj.own_ident;

        let orig_ty_generics = cx.orig.generics.split_for_impl().1;
        let proj_ty_generics = cx.proj.generics.split_for_impl().1;
        let (impl_generics, ty_generics, where_clause) = cx.orig.generics.split_for_impl();

        let mut project = Some(quote! {
            #vis fn project<#lifetime>(
                self: ::pin_project::__private::Pin<&#lifetime mut Self>,
            ) -> #proj_ident #proj_ty_generics {
                unsafe {
                    #proj_body
                }
            }
        });
        let mut project_ref = Some(quote! {
            #[allow(clippy::missing_const_for_fn)]
            #vis fn project_ref<#lifetime>(
                self: ::pin_project::__private::Pin<&#lifetime Self>,
            ) -> #proj_ref_ident #proj_ty_generics {
                unsafe {
                    #proj_ref_body
                }
            }
        });
        let mut project_replace = cx.project_replace.span().map(|span| {
            // It is enough to only set the span of the signature.
            let sig = quote_spanned! { span =>
                #vis fn project_replace(
                    self: ::pin_project::__private::Pin<&mut Self>,
                    __replacement: Self,
                ) -> #proj_own_ident #orig_ty_generics
            };
            quote! {
                #sig {
                    unsafe {
                        let __self_ptr: *mut Self = self.get_unchecked_mut();

                        // Destructors will run in reverse order, so next create a guard to overwrite
                        // `self` with the replacement value without calling destructors.
                        let __guard = ::pin_project::__private::UnsafeOverwriteGuard {
                            target: __self_ptr,
                            value: ::pin_project::__private::ManuallyDrop::new(__replacement),
                        };

                        #proj_own_body
                    }
                }
            }
        });

        if cx.kind == Enum {
            if !cx.project {
                project = None;
            }
            if !cx.project_ref {
                project_ref = None;
            }
            if cx.project_replace.ident().is_none() {
                project_replace = None;
            }
        }

        quote! {
            impl #impl_generics #orig_ident #ty_generics #where_clause {
                #project
                #project_ref
                #project_replace
            }
        }
    }

    /// Checks that the `[repr(packed)]` attribute is not included.
    ///
    /// This currently does two checks:
    /// * Checks the attributes of structs to ensure there is no `[repr(packed)]`.
    /// * Generates a function that borrows fields without an unsafe block and
    ///   forbidding `safe_packed_borrows` lint.
    fn ensure_not_packed(orig: &OriginalType<'_>, fields: &Fields) -> Result<TokenStream> {
        for meta in orig.attrs.iter().filter_map(|attr| attr.parse_meta().ok()) {
            if let Meta::List(list) = meta {
                if list.path.is_ident("repr") {
                    for repr in list.nested.iter() {
                        match repr {
                            NestedMeta::Meta(Meta::Path(path))
                            | NestedMeta::Meta(Meta::List(MetaList { path, .. }))
                                if path.is_ident("packed") =>
                            {
                                return Err(error!(
                                    repr,
                                    "#[pin_project] attribute may not be used on #[repr(packed)] types"
                                ));
                            }
                            _ => {}
                        }
                    }
                }
            }
        }

        // As proc-macro-derive can't rewrite the structure definition,
        // it's probably no longer necessary, but it keeps it for now.

        // Workaround for https://github.com/taiki-e/pin-project/issues/32
        // Through the tricky use of proc macros, it's possible to bypass
        // the above check for the `repr` attribute.
        // To ensure that it's impossible to use pin projections on a `#[repr(packed)]`
        // struct, we generate code like this:
        //
        // ```rust
        // #[forbid(safe_packed_borrows)]
        // fn assert_not_repr_packed(val: &MyStruct) {
        //     let _field1 = &val.field1;
        //     let _field2 = &val.field2;
        //     ...
        //     let _fieldn = &val.fieldn;
        // }
        // ```
        //
        // Taking a reference to a packed field is unsafe, and applying
        // `#[forbid(safe_packed_borrows)]` makes sure that doing this without
        // an `unsafe` block (which we deliberately do not generate)
        // is a hard error.
        //
        // If the struct ends up having `#[repr(packed)]` applied somehow,
        // this will generate an (unfriendly) error message. Under all reasonable
        // circumstances, we'll detect the `#[repr(packed)]` attribute, and generate
        // a much nicer error above.
        //
        // There is one exception: If the type of a struct field has an alignment of 1
        // (e.g. u8), it is always safe to take a reference to it, even if the struct
        // is `#[repr(packed)]`. If the struct is composed entirely of types of
        // alignment 1, our generated method will not trigger an error if the
        // struct is `#[repr(packed)]`.
        //
        // Fortunately, this should have no observable consequence - `#[repr(packed)]`
        // is essentially a no-op on such a type. Nevertheless, we include a test
        // to ensure that the compiler doesn't ever try to copy the fields on
        // such a struct when trying to drop it - which is reason we prevent
        // `#[repr(packed)]` in the first place.
        //
        // See also https://github.com/taiki-e/pin-project/pull/34.
        let mut field_refs = vec![];
        match fields {
            Fields::Named(FieldsNamed { named, .. }) => {
                for Field { ident, .. } in named {
                    field_refs.push(quote!(&this.#ident));
                }
            }
            Fields::Unnamed(FieldsUnnamed { unnamed, .. }) => {
                for (index, _) in unnamed.iter().enumerate() {
                    let index = Index::from(index);
                    field_refs.push(quote!(&this.#index));
                }
            }
            Fields::Unit => {}
        }

        let (impl_generics, ty_generics, where_clause) = orig.generics.split_for_impl();
        let ident = orig.ident;
        Ok(quote! {
            #[forbid(safe_packed_borrows)]
            fn __assert_not_repr_packed #impl_generics (this: &#ident #ty_generics) #where_clause {
                #(let _ = #field_refs;)*
            }
        })
    }