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    use super::*;
    use crate::punctuated::Punctuated;
    use proc_macro2::TokenStream;
    use std::iter;

    #[cfg(feature = "parsing")]
    use crate::parse::{Parse, ParseBuffer, ParseStream, Parser, Result};
    #[cfg(feature = "parsing")]
    use crate::punctuated::Pair;

    ast_struct! {
        /// An attribute like `#[repr(transparent)]`.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or `"full"`
        /// feature.*
        ///
        /// <br>
        ///
        /// # Syntax
        ///
        /// Rust has six types of attributes.
        ///
        /// - Outer attributes like `#[repr(transparent)]`. These appear outside or
        ///   in front of the item they describe.
        /// - Inner attributes like `#![feature(proc_macro)]`. These appear inside
        ///   of the item they describe, usually a module.
        /// - Outer doc comments like `/// # Example`.
        /// - Inner doc comments like `//! Please file an issue`.
        /// - Outer block comments `/** # Example */`.
        /// - Inner block comments `/*! Please file an issue */`.
        ///
        /// The `style` field of type `AttrStyle` distinguishes whether an attribute
        /// is outer or inner. Doc comments and block comments are promoted to
        /// attributes, as this is how they are processed by the compiler and by
        /// `macro_rules!` macros.
        ///
        /// The `path` field gives the possibly colon-delimited path against which
        /// the attribute is resolved. It is equal to `"doc"` for desugared doc
        /// comments. The `tokens` field contains the rest of the attribute body as
        /// tokens.
        ///
        /// ```text
        /// #[derive(Copy)]      #[crate::precondition x < 5]
        ///   ^^^^^^~~~~~~         ^^^^^^^^^^^^^^^^^^^ ~~~~~
        ///   path  tokens                 path        tokens
        /// ```
        ///
        /// <br>
        ///
        /// # Parsing from tokens to Attribute
        ///
        /// This type does not implement the [`Parse`] trait and thus cannot be
        /// parsed directly by [`ParseStream::parse`]. Instead use
        /// [`ParseStream::call`] with one of the two parser functions
        /// [`Attribute::parse_outer`] or [`Attribute::parse_inner`] depending on
        /// which you intend to parse.
        ///
        /// [`Parse`]: parse::Parse
        /// [`ParseStream::parse`]: parse::ParseBuffer::parse
        /// [`ParseStream::call`]: parse::ParseBuffer::call
        ///
        /// ```
        /// use syn::{Attribute, Ident, Result, Token};
        /// use syn::parse::{Parse, ParseStream};
        ///
        /// // Parses a unit struct with attributes.
        /// //
        /// //     #[path = "s.tmpl"]
        /// //     struct S;
        /// struct UnitStruct {
        ///     attrs: Vec<Attribute>,
        ///     struct_token: Token![struct],
        ///     name: Ident,
        ///     semi_token: Token![;],
        /// }
        ///
        /// impl Parse for UnitStruct {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         Ok(UnitStruct {
        ///             attrs: input.call(Attribute::parse_outer)?,
        ///             struct_token: input.parse()?,
        ///             name: input.parse()?,
        ///             semi_token: input.parse()?,
        ///         })
        ///     }
        /// }
        /// ```
        ///
        /// <p><br></p>
        ///
        /// # Parsing from Attribute to structured arguments
        ///
        /// The grammar of attributes in Rust is very flexible, which makes the
        /// syntax tree not that useful on its own. In particular, arguments of the
        /// attribute are held in an arbitrary `tokens: TokenStream`. Macros are
        /// expected to check the `path` of the attribute, decide whether they
        /// recognize it, and then parse the remaining tokens according to whatever
        /// grammar they wish to require for that kind of attribute.
        ///
        /// If the attribute you are parsing is expected to conform to the
        /// conventional structured form of attribute, use [`parse_meta()`] to
        /// obtain that structured representation. If the attribute follows some
        /// other grammar of its own, use [`parse_args()`] to parse that into the
        /// expected data structure.
        ///
        /// [`parse_meta()`]: Attribute::parse_meta
        /// [`parse_args()`]: Attribute::parse_args
        ///
        /// <p><br></p>
        ///
        /// # Doc comments
        ///
        /// The compiler transforms doc comments, such as `/// comment` and `/*!
        /// comment */`, into attributes before macros are expanded. Each comment is
        /// expanded into an attribute of the form `#[doc = r"comment"]`.
        ///
        /// As an example, the following `mod` items are expanded identically:
        ///
        /// ```
        /// # use syn::{ItemMod, parse_quote};
        /// let doc: ItemMod = parse_quote! {
        ///     /// Single line doc comments
        ///     /// We write so many!
        ///     /**
        ///      * Multi-line comments...
        ///      * May span many lines
        ///      */
        ///     mod example {
        ///         //! Of course, they can be inner too
        ///         /*! And fit in a single line */
        ///     }
        /// };
        /// let attr: ItemMod = parse_quote! {
        ///     #[doc = r" Single line doc comments"]
        ///     #[doc = r" We write so many!"]
        ///     #[doc = r"
        ///      * Multi-line comments...
        ///      * May span many lines
        ///      "]
        ///     mod example {
        ///         #![doc = r" Of course, they can be inner too"]
        ///         #![doc = r" And fit in a single line "]
        ///     }
        /// };
        /// assert_eq!(doc, attr);
        /// ```
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub struct Attribute {
            pub pound_token: Token![#],
            pub style: AttrStyle,
            pub bracket_token: token::Bracket,
            pub path: Path,
            pub tokens: TokenStream,
        }
    }

    impl Attribute {
        /// Parses the content of the attribute, consisting of the path and tokens,
        /// as a [`Meta`] if possible.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_meta(&self) -> Result<Meta> {
            fn clone_ident_segment(segment: &PathSegment) -> PathSegment {
                PathSegment {
                    ident: segment.ident.clone(),
                    arguments: PathArguments::None,
                }
            }

            let path = Path {
                leading_colon: self
                    .path
                    .leading_colon
                    .as_ref()
                    .map(|colon| Token![::](colon.spans)),
                segments: self
                    .path
                    .segments
                    .pairs()
                    .map(|pair| match pair {
                        Pair::Punctuated(seg, punct) => {
                            Pair::Punctuated(clone_ident_segment(seg), Token![::](punct.spans))
                        }
                        Pair::End(seg) => Pair::End(clone_ident_segment(seg)),
                    })
                    .collect(),
            };

            let parser = |input: ParseStream| parsing::parse_meta_after_path(path, input);
            parse::Parser::parse2(parser, self.tokens.clone())
        }

        /// Parse the arguments to the attribute as a syntax tree.
        ///
        /// This is similar to `syn::parse2::<T>(attr.tokens)` except that:
        ///
        /// - the surrounding delimiters are *not* included in the input to the
        ///   parser; and
        /// - the error message has a more useful span when `tokens` is empty.
        ///
        /// ```text
        /// #[my_attr(value < 5)]
        ///           ^^^^^^^^^ what gets parsed
        /// ```
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_args<T: Parse>(&self) -> Result<T> {
            self.parse_args_with(T::parse)
        }

        /// Parse the arguments to the attribute using the given parser.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_args_with<F: Parser>(&self, parser: F) -> Result<F::Output> {
            let parser = |input: ParseStream| {
                let args = enter_args(self, input)?;
                parse::parse_stream(parser, &args)
            };
            parser.parse2(self.tokens.clone())
        }

        /// Parses zero or more outer attributes from the stream.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_outer(input: ParseStream) -> Result<Vec<Self>> {
            let mut attrs = Vec::new();
            while input.peek(Token![#]) {
                attrs.push(input.call(parsing::single_parse_outer)?);
            }
            Ok(attrs)
        }

        /// Parses zero or more inner attributes from the stream.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_inner(input: ParseStream) -> Result<Vec<Self>> {
            let mut attrs = Vec::new();
            while input.peek(Token![#]) && input.peek2(Token![!]) {
                attrs.push(input.call(parsing::single_parse_inner)?);
            }
            Ok(attrs)
        }
    }

    #[cfg(feature = "parsing")]
    fn expected_parentheses(attr: &Attribute) -> String {
        let style = match attr.style {
            AttrStyle::Outer => "#",
            AttrStyle::Inner(_) => "#!",
        };

        let mut path = String::new();
        for segment in &attr.path.segments {
            if !path.is_empty() || attr.path.leading_colon.is_some() {
                path += "::";
            }
            path += &segment.ident.to_string();
        }

        format!("{}[{}(...)]", style, path)
    }

    #[cfg(feature = "parsing")]
    fn enter_args<'a>(attr: &Attribute, input: ParseStream<'a>) -> Result<ParseBuffer<'a>> {
        if input.is_empty() {
            let expected = expected_parentheses(attr);
            let msg = format!("expected attribute arguments in parentheses: {}", expected);
            return Err(crate::error::new2(
                attr.pound_token.span,
                attr.bracket_token.span,
                msg,
            ));
        } else if input.peek(Token![=]) {
            let expected = expected_parentheses(attr);
            let msg = format!("expected parentheses: {}", expected);
            return Err(input.error(msg));
        };

        let content;
        if input.peek(token::Paren) {
            parenthesized!(content in input);
        } else if input.peek(token::Bracket) {
            bracketed!(content in input);
        } else if input.peek(token::Brace) {
            braced!(content in input);
        } else {
            return Err(input.error("unexpected token in attribute arguments"));
        }

        if input.is_empty() {
            Ok(content)
        } else {
            Err(input.error("unexpected token in attribute arguments"))
        }
    }

    ast_enum! {
        /// Distinguishes between attributes that decorate an item and attributes
        /// that are contained within an item.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or `"full"`
        /// feature.*
        ///
        /// # Outer attributes
        ///
        /// - `#[repr(transparent)]`
        /// - `/// # Example`
        /// - `/** Please file an issue */`
        ///
        /// # Inner attributes
        ///
        /// - `#![feature(proc_macro)]`
        /// - `//! # Example`
        /// - `/*! Please file an issue */`
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub enum AttrStyle {
            Outer,
            Inner(Token![!]),
        }
    }

    ast_enum_of_structs! {
        /// Content of a compile-time structured attribute.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or `"full"`
        /// feature.*
        ///
        /// ## Path
        ///
        /// A meta path is like the `test` in `#[test]`.
        ///
        /// ## List
        ///
        /// A meta list is like the `derive(Copy)` in `#[derive(Copy)]`.
        ///
        /// ## NameValue
        ///
        /// A name-value meta is like the `path = "..."` in `#[path =
        /// "sys/windows.rs"]`.
        ///
        /// # Syntax tree enum
        ///
        /// This type is a [syntax tree enum].
        ///
        /// [syntax tree enum]: Expr#syntax-tree-enums
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub enum Meta {
            Path(Path),

            /// A structured list within an attribute, like `derive(Copy, Clone)`.
            List(MetaList),

            /// A name-value pair within an attribute, like `feature = "nightly"`.
            NameValue(MetaNameValue),
        }
    }

    ast_struct! {
        /// A structured list within an attribute, like `derive(Copy, Clone)`.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or
        /// `"full"` feature.*
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub struct MetaList {
            pub path: Path,
            pub paren_token: token::Paren,
            pub nested: Punctuated<NestedMeta, Token![,]>,
        }
    }

    ast_struct! {
        /// A name-value pair within an attribute, like `feature = "nightly"`.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or
        /// `"full"` feature.*
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub struct MetaNameValue {
            pub path: Path,
            pub eq_token: Token![=],
            pub lit: Lit,
        }
    }

    impl Meta {
        /// Returns the identifier that begins this structured meta item.
        ///
        /// For example this would return the `test` in `#[test]`, the `derive` in
        /// `#[derive(Copy)]`, and the `path` in `#[path = "sys/windows.rs"]`.
        pub fn path(&self) -> &Path {
            match self {
                Meta::Path(path) => path,
                Meta::List(meta) => &meta.path,
                Meta::NameValue(meta) => &meta.path,
            }
        }
    }

    ast_enum_of_structs! {
        /// Element of a compile-time attribute list.
        ///
        /// *This type is available only if Syn is built with the `"derive"` or `"full"`
        /// feature.*
        #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
        pub enum NestedMeta {
            /// A structured meta item, like the `Copy` in `#[derive(Copy)]` which
            /// would be a nested `Meta::Path`.
            Meta(Meta),

            /// A Rust literal, like the `"new_name"` in `#[rename("new_name")]`.
            Lit(Lit),
        }
    }

    /// Conventional argument type associated with an invocation of an attribute
    /// macro.
    ///
    /// For example if we are developing an attribute macro that is intended to be
    /// invoked on function items as follows:
    ///
    /// ```
    /// # const IGNORE: &str = stringify! {
    /// #[my_attribute(path = "/v1/refresh")]
    /// # };
    /// pub fn refresh() {
    ///     /* ... */
    /// }
    /// ```
    ///
    /// The implementation of this macro would want to parse its attribute arguments
    /// as type `AttributeArgs`.
    ///
    /// ```
    /// # extern crate proc_macro;
    /// #
    /// use proc_macro::TokenStream;
    /// use syn::{parse_macro_input, AttributeArgs, ItemFn};
    ///
    /// # const IGNORE: &str = stringify! {
    /// #[proc_macro_attribute]
    /// # };
    /// pub fn my_attribute(args: TokenStream, input: TokenStream) -> TokenStream {
    ///     let args = parse_macro_input!(args as AttributeArgs);
    ///     let input = parse_macro_input!(input as ItemFn);
    ///
    ///     /* ... */
    /// #   "".parse().unwrap()
    /// }
    /// ```
    #[cfg_attr(doc_cfg, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub type AttributeArgs = Vec<NestedMeta>;

    pub trait FilterAttrs<'a> {
        type Ret: Iterator<Item = &'a Attribute>;

        fn outer(self) -> Self::Ret;
        fn inner(self) -> Self::Ret;
    }

    impl<'a, T> FilterAttrs<'a> for T
    where
        T: IntoIterator<Item = &'a Attribute>,
    {
        type Ret = iter::Filter<T::IntoIter, fn(&&Attribute) -> bool>;

        fn outer(self) -> Self::Ret {
            fn is_outer(attr: &&Attribute) -> bool {
                match attr.style {
                    AttrStyle::Outer => true,
                    AttrStyle::Inner(_) => false,
                }
            }
            self.into_iter().filter(is_outer)
        }

        fn inner(self) -> Self::Ret {
            fn is_inner(attr: &&Attribute) -> bool {
                match attr.style {
                    AttrStyle::Inner(_) => true,
                    AttrStyle::Outer => false,
                }
            }
            self.into_iter().filter(is_inner)
        }
    }

    #[cfg(feature = "parsing")]
    pub mod parsing {
        use super::*;
        use crate::ext::IdentExt;
        use crate::parse::{Parse, ParseStream, Result};
        #[cfg(feature = "full")]
        use crate::private;

        pub fn single_parse_inner(input: ParseStream) -> Result<Attribute> {
            let content;
            Ok(Attribute {
                pound_token: input.parse()?,
                style: AttrStyle::Inner(input.parse()?),
                bracket_token: bracketed!(content in input),
                path: content.call(Path::parse_mod_style)?,
                tokens: content.parse()?,
            })
        }

        pub fn single_parse_outer(input: ParseStream) -> Result<Attribute> {
            let content;
            Ok(Attribute {
                pound_token: input.parse()?,
                style: AttrStyle::Outer,
                bracket_token: bracketed!(content in input),
                path: content.call(Path::parse_mod_style)?,
                tokens: content.parse()?,
            })
        }

        #[cfg(feature = "full")]
        impl private {
            pub fn attrs(outer: Vec<Attribute>, inner: Vec<Attribute>) -> Vec<Attribute> {
                let mut attrs = outer;
                attrs.extend(inner);
                attrs
            }
        }

        // Like Path::parse_mod_style but accepts keywords in the path.
        fn parse_meta_path(input: ParseStream) -> Result<Path> {
            Ok(Path {
                leading_colon: input.parse()?,
                segments: {
                    let mut segments = Punctuated::new();
                    while input.peek(Ident::peek_any) {
                        let ident = Ident::parse_any(input)?;
                        segments.push_value(PathSegment::from(ident));
                        if !input.peek(Token![::]) {
                            break;
                        }
                        let punct = input.parse()?;
                        segments.push_punct(punct);
                    }
                    if segments.is_empty() {
                        return Err(input.error("expected path"));
                    } else if segments.trailing_punct() {
                        return Err(input.error("expected path segment"));
                    }
                    segments
                },
            })
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        impl Parse for Meta {
            fn parse(input: ParseStream) -> Result<Self> {
                let path = input.call(parse_meta_path)?;
                parse_meta_after_path(path, input)
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        impl Parse for MetaList {
            fn parse(input: ParseStream) -> Result<Self> {
                let path = input.call(parse_meta_path)?;
                parse_meta_list_after_path(path, input)
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        impl Parse for MetaNameValue {
            fn parse(input: ParseStream) -> Result<Self> {
                let path = input.call(parse_meta_path)?;
                parse_meta_name_value_after_path(path, input)
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        impl Parse for NestedMeta {
            fn parse(input: ParseStream) -> Result<Self> {
                if input.peek(Lit) && !(input.peek(LitBool) && input.peek2(Token![=])) {
                    input.parse().map(NestedMeta::Lit)
                } else if input.peek(Ident::peek_any)
                    || input.peek(Token![::]) && input.peek3(Ident::peek_any)
                {
                    input.parse().map(NestedMeta::Meta)
                } else {
                    Err(input.error("expected identifier or literal"))
                }
            }
        }

        pub fn parse_meta_after_path(path: Path, input: ParseStream) -> Result<Meta> {
            if input.peek(token::Paren) {
                parse_meta_list_after_path(path, input).map(Meta::List)
            } else if input.peek(Token![=]) {
                parse_meta_name_value_after_path(path, input).map(Meta::NameValue)
            } else {
                Ok(Meta::Path(path))
            }
        }

        fn parse_meta_list_after_path(path: Path, input: ParseStream) -> Result<MetaList> {
            let content;
            Ok(MetaList {
                path,
                paren_token: parenthesized!(content in input),
                nested: content.parse_terminated(NestedMeta::parse)?,
            })
        }

        fn parse_meta_name_value_after_path(path: Path, input: ParseStream) -> Result<MetaNameValue> {
            Ok(MetaNameValue {
                path,
                eq_token: input.parse()?,
                lit: input.parse()?,
            })
        }
    }

    #[cfg(feature = "printing")]
    mod printing {
        use super::*;
        use proc_macro2::TokenStream;
        use quote::ToTokens;

        #[cfg_attr(doc_cfg, doc(cfg(feature = "printing")))]
        impl ToTokens for Attribute {
            fn to_tokens(&self, tokens: &mut TokenStream) {
                self.pound_token.to_tokens(tokens);
                if let AttrStyle::Inner(b) = &self.style {
                    b.to_tokens(tokens);
                }
                self.bracket_token.surround(tokens, |tokens| {
                    self.path.to_tokens(tokens);
                    self.tokens.to_tokens(tokens);
                });
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "printing")))]
        impl ToTokens for MetaList {
            fn to_tokens(&self, tokens: &mut TokenStream) {
                self.path.to_tokens(tokens);
                self.paren_token.surround(tokens, |tokens| {
                    self.nested.to_tokens(tokens);
                })
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "printing")))]
        impl ToTokens for MetaNameValue {
            fn to_tokens(&self, tokens: &mut TokenStream) {
                self.path.to_tokens(tokens);
                self.eq_token.to_tokens(tokens);
                self.lit.to_tokens(tokens);
            }
        }
    }