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    use std;
    use std::fmt::{self, Debug, Display};
    use std::iter::FromIterator;

    use proc_macro2::{
        Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
    };
    #[cfg(feature = "printing")]
    use quote::ToTokens;

    #[cfg(feature = "parsing")]
    use buffer::Cursor;
    #[cfg(all(procmacro2_semver_exempt, feature = "parsing"))]
    use private;
    use thread::ThreadBound;

    /// 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.
    ///
    /// Refer to the [module documentation] for details about parsing in Syn.
    ///
    /// [module documentation]: index.html
    ///
    /// *This type is available if Syn is built with the `"parsing"` feature.*
    pub struct Error {
        // 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`]: struct.ParseBuffer.html#method.error
        ///
        /// # Example
        ///
        /// ```edition2018
        /// 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 {
            Error {
                start_span: ThreadBound::new(span),
                end_span: ThreadBound::new(span),
                message: message.to_string(),
            }
        }

        /// 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 {
            let mut iter = tokens.into_token_stream().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 {
                start_span: ThreadBound::new(start),
                end_span: ThreadBound::new(end),
                message: message.to_string(),
            }
        }

        /// 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.start_span.get() {
                Some(span) => *span,
                None => return Span::call_site(),
            };

            #[cfg(procmacro2_semver_exempt)]
            {
                let end = match self.end_span.get() {
                    Some(span) => *span,
                    None => return Span::call_site(),
                };
                start.join(end).unwrap_or(start)
            }
            #[cfg(not(procmacro2_semver_exempt))]
            {
                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!`]: https://doc.rust-lang.org/std/macro.compile_error.html
        /// [`parse_macro_input!`]: ../macro.parse_macro_input.html
        pub 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 {
            #[cfg(procmacro2_semver_exempt)]
            let span = private::open_span_of_group(cursor);
            #[cfg(not(procmacro2_semver_exempt))]
            let span = cursor.span();
            Error::new(span, message)
        }
    }

    impl Debug for Error {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.debug_tuple("Error").field(&self.message).finish()
        }
    }

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

    impl Clone for Error {
        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);
            Error {
                start_span: ThreadBound::new(start),
                end_span: ThreadBound::new(end),
                message: self.message.clone(),
            }
        }
    }

    impl std::error::Error for Error {
        fn description(&self) -> &str {
            "parse error"
        }
    }

    impl From<LexError> for Error {
        fn from(err: LexError) -> Self {
            Error::new(Span::call_site(), format!("{:?}", err))
        }
    }

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