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    #[cfg(feature = "parsing")]
    use crate::buffer::Cursor;
    use crate::thread::ThreadBound;
    use proc_macro2::{
        Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
    };
    #[cfg(feature = "printing")]
    use quote::ToTokens;
    use std::fmt::{self, Debug, Display};
    use std::iter::FromIterator;
    use std::slice;
    use std::vec;

    /// The result of a Syn parser.
    pub type Result<T> = std::result::Result<T, Error>;

    /// Error returned when a Syn parser cannot parse the input tokens.
    ///
    /// # Error reporting in proc macros
    ///
    /// The correct way to report errors back to the compiler from a procedural
    /// macro is by emitting an appropriately spanned invocation of
    /// [`compile_error!`] in the generated code. This produces a better diagnostic
    /// message than simply panicking the macro.
    ///
    /// [`compile_error!`]: std::compile_error!
    ///
    /// When parsing macro input, the [`parse_macro_input!`] macro handles the
    /// conversion to `compile_error!` automatically.
    ///
    /// [`parse_macro_input!`]: crate::parse_macro_input!
    ///
    /// ```
    /// # 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_attr(args: TokenStream, input: TokenStream) -> TokenStream {
    ///     let args = parse_macro_input!(args as AttributeArgs);
    ///     let input = parse_macro_input!(input as ItemFn);
    ///
    ///     /* ... */
    ///     # TokenStream::new()
    /// }
    /// ```
    ///
    /// For errors that arise later than the initial parsing stage, the
    /// [`.to_compile_error()`] or [`.into_compile_error()`] methods can be used to
    /// perform an explicit conversion to `compile_error!`.
    ///
    /// [`.to_compile_error()`]: Error::to_compile_error
    /// [`.into_compile_error()`]: Error::into_compile_error
    ///
    /// ```
    /// # extern crate proc_macro;
    /// #
    /// # use proc_macro::TokenStream;
    /// # use syn::{parse_macro_input, DeriveInput};
    /// #
    /// # const IGNORE: &str = stringify! {
    /// #[proc_macro_derive(MyDerive)]
    /// # };
    /// pub fn my_derive(input: TokenStream) -> TokenStream {
    ///     let input = parse_macro_input!(input as DeriveInput);
    ///
    ///     // fn(DeriveInput) -> syn::Result<proc_macro2::TokenStream>
    ///     expand::my_derive(input)
    ///         .unwrap_or_else(syn::Error::into_compile_error)
    ///         .into()
    /// }
    /// #
    /// # mod expand {
    /// #     use proc_macro2::TokenStream;
    /// #     use syn::{DeriveInput, Result};
    /// #
    /// #     pub fn my_derive(input: DeriveInput) -> Result<TokenStream> {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// ```
    pub struct Error {
        messages: Vec<ErrorMessage>,
    }

    struct ErrorMessage {
        // Span is implemented as an index into a thread-local interner to keep the
        // size small. It is not safe to access from a different thread. We want
        // errors to be Send and Sync to play nicely with the Failure crate, so pin
        // the span we're given to its original thread and assume it is
        // Span::call_site if accessed from any other thread.
        start_span: ThreadBound<Span>,
        end_span: ThreadBound<Span>,
        message: String,
    }

    #[cfg(test)]
    struct _Test
    where
        Error: Send + Sync;

    impl Error {
        /// Usually the [`ParseStream::error`] method will be used instead, which
        /// automatically uses the correct span from the current position of the
        /// parse stream.
        ///
        /// Use `Error::new` when the error needs to be triggered on some span other
        /// than where the parse stream is currently positioned.
        ///
        /// [`ParseStream::error`]: crate::parse::ParseBuffer::error
        ///
        /// # Example
        ///
        /// ```
        /// use syn::{Error, Ident, LitStr, Result, Token};
        /// use syn::parse::ParseStream;
        ///
        /// // Parses input that looks like `name = "string"` where the key must be
        /// // the identifier `name` and the value may be any string literal.
        /// // Returns the string literal.
        /// fn parse_name(input: ParseStream) -> Result<LitStr> {
        ///     let name_token: Ident = input.parse()?;
        ///     if name_token != "name" {
        ///         // Trigger an error not on the current position of the stream,
        ///         // but on the position of the unexpected identifier.
        ///         return Err(Error::new(name_token.span(), "expected `name`"));
        ///     }
        ///     input.parse::<Token![=]>()?;
        ///     let s: LitStr = input.parse()?;
        ///     Ok(s)
        /// }
        /// ```
        pub fn new<T: Display>(span: Span, message: T) -> Self {
            return new(span, message.to_string());

            fn new(span: Span, message: String) -> Error {
                Error {
                    messages: vec![ErrorMessage {
                        start_span: ThreadBound::new(span),
                        end_span: ThreadBound::new(span),
                        message,
                    }],
                }
            }
        }

        /// Creates an error with the specified message spanning the given syntax
        /// tree node.
        ///
        /// Unlike the `Error::new` constructor, this constructor takes an argument
        /// `tokens` which is a syntax tree node. This allows the resulting `Error`
        /// to attempt to span all tokens inside of `tokens`. While you would
        /// typically be able to use the `Spanned` trait with the above `Error::new`
        /// constructor, implementation limitations today mean that
        /// `Error::new_spanned` may provide a higher-quality error message on
        /// stable Rust.
        ///
        /// When in doubt it's recommended to stick to `Error::new` (or
        /// `ParseStream::error`)!
        #[cfg(feature = "printing")]
        pub fn new_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self {
            return new_spanned(tokens.into_token_stream(), message.to_string());

            fn new_spanned(tokens: TokenStream, message: String) -> Error {
                let mut iter = tokens.into_iter();
                let start = iter.next().map_or_else(Span::call_site, |t| t.span());
                let end = iter.last().map_or(start, |t| t.span());
                Error {
                    messages: vec![ErrorMessage {
                        start_span: ThreadBound::new(start),
                        end_span: ThreadBound::new(end),
                        message,
                    }],
                }
            }
        }

        /// The source location of the error.
        ///
        /// Spans are not thread-safe so this function returns `Span::call_site()`
        /// if called from a different thread than the one on which the `Error` was
        /// originally created.
        pub fn span(&self) -> Span {
            let start = match self.messages[0].start_span.get() {
                Some(span) => *span,
                None => return Span::call_site(),
            };
            let end = match self.messages[0].end_span.get() {
                Some(span) => *span,
                None => return Span::call_site(),
            };
            start.join(end).unwrap_or(start)
        }

        /// Render the error as an invocation of [`compile_error!`].
        ///
        /// The [`parse_macro_input!`] macro provides a convenient way to invoke
        /// this method correctly in a procedural macro.
        ///
        /// [`compile_error!`]: std::compile_error!
        /// [`parse_macro_input!`]: crate::parse_macro_input!
        pub fn to_compile_error(&self) -> TokenStream {
            self.messages
                .iter()
                .map(ErrorMessage::to_compile_error)
                .collect()
        }

        /// Render the error as an invocation of [`compile_error!`].
        ///
        /// [`compile_error!`]: std::compile_error!
        ///
        /// # Example
        ///
        /// ```
        /// # extern crate proc_macro;
        /// #
        /// use proc_macro::TokenStream;
        /// use syn::{parse_macro_input, DeriveInput, Error};
        ///
        /// # const _: &str = stringify! {
        /// #[proc_macro_derive(MyTrait)]
        /// # };
        /// pub fn derive_my_trait(input: TokenStream) -> TokenStream {
        ///     let input = parse_macro_input!(input as DeriveInput);
        ///     my_trait::expand(input)
        ///         .unwrap_or_else(Error::into_compile_error)
        ///         .into()
        /// }
        ///
        /// mod my_trait {
        ///     use proc_macro2::TokenStream;
        ///     use syn::{DeriveInput, Result};
        ///
        ///     pub(crate) fn expand(input: DeriveInput) -> Result<TokenStream> {
        ///         /* ... */
        ///         # unimplemented!()
        ///     }
        /// }
        /// ```
        pub fn into_compile_error(self) -> TokenStream {
            self.to_compile_error()
        }

        /// Add another error message to self such that when `to_compile_error()` is
        /// called, both errors will be emitted together.
        pub fn combine(&mut self, another: Error) {
            self.messages.extend(another.messages);
        }
    }

    impl ErrorMessage {
        fn to_compile_error(&self) -> TokenStream {
            let start = self
                .start_span
                .get()
                .cloned()
                .unwrap_or_else(Span::call_site);
            let end = self.end_span.get().cloned().unwrap_or_else(Span::call_site);

            // compile_error!($message)
            TokenStream::from_iter(vec![
                TokenTree::Ident(Ident::new("compile_error", start)),
                TokenTree::Punct({
                    let mut punct = Punct::new('!', Spacing::Alone);
                    punct.set_span(start);
                    punct
                }),
                TokenTree::Group({
                    let mut group = Group::new(Delimiter::Brace, {
                        TokenStream::from_iter(vec![TokenTree::Literal({
                            let mut string = Literal::string(&self.message);
                            string.set_span(end);
                            string
                        })])
                    });
                    group.set_span(end);
                    group
                }),
            ])
        }
    }

    #[cfg(feature = "parsing")]
    pub fn new_at<T: Display>(scope: Span, cursor: Cursor, message: T) -> Error {
        if cursor.eof() {
            Error::new(scope, format!("unexpected end of input, {}", message))
        } else {
            let span = crate::buffer::open_span_of_group(cursor);
            Error::new(span, message)
        }
    }

    #[cfg(all(feature = "parsing", any(feature = "full", feature = "derive")))]
    pub fn new2<T: Display>(start: Span, end: Span, message: T) -> Error {
        return new2(start, end, message.to_string());

        fn new2(start: Span, end: Span, message: String) -> Error {
            Error {
                messages: vec![ErrorMessage {
                    start_span: ThreadBound::new(start),
                    end_span: ThreadBound::new(end),
                    message,
                }],
            }
        }
    }

    impl Debug for Error {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            if self.messages.len() == 1 {
                formatter
                    .debug_tuple("Error")
                    .field(&self.messages[0])
                    .finish()
            } else {
                formatter
                    .debug_tuple("Error")
                    .field(&self.messages)
                    .finish()
            }
        }
    }

    impl Debug for ErrorMessage {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            Debug::fmt(&self.message, formatter)
        }
    }

    impl Display for Error {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str(&self.messages[0].message)
        }
    }

    impl Clone for Error {
        fn clone(&self) -> Self {
            Error {
                messages: self.messages.clone(),
            }
        }
    }

    impl Clone for ErrorMessage {
        fn clone(&self) -> Self {
            let start = self
                .start_span
                .get()
                .cloned()
                .unwrap_or_else(Span::call_site);
            let end = self.end_span.get().cloned().unwrap_or_else(Span::call_site);
            ErrorMessage {
                start_span: ThreadBound::new(start),
                end_span: ThreadBound::new(end),
                message: self.message.clone(),
            }
        }
    }

    impl std::error::Error for Error {}

    impl From<LexError> for Error {
        fn from(err: LexError) -> Self {
            Error::new(err.span(), "lex error")
        }
    }

    impl IntoIterator for Error {
        type Item = Error;
        type IntoIter = IntoIter;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter {
                messages: self.messages.into_iter(),
            }
        }
    }

    pub struct IntoIter {
        messages: vec::IntoIter<ErrorMessage>,
    }

    impl Iterator for IntoIter {
        type Item = Error;

        fn next(&mut self) -> Option<Self::Item> {
            Some(Error {
                messages: vec![self.messages.next()?],
            })
        }
    }

    impl<'a> IntoIterator for &'a Error {
        type Item = Error;
        type IntoIter = Iter<'a>;

        fn into_iter(self) -> Self::IntoIter {
            Iter {
                messages: self.messages.iter(),
            }
        }
    }

    pub struct Iter<'a> {
        messages: slice::Iter<'a, ErrorMessage>,
    }

    impl<'a> Iterator for Iter<'a> {
        type Item = Error;

        fn next(&mut self) -> Option<Self::Item> {
            Some(Error {
                messages: vec![self.messages.next()?.clone()],
            })
        }
    }

    impl Extend<Error> for Error {
        fn extend<T: IntoIterator<Item = Error>>(&mut self, iter: T) {
            for err in iter {
                self.combine(err);
            }
        }
    }