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    use std::cell::RefCell;

    use proc_macro2::{Delimiter, Span};

    use buffer::Cursor;
    use error::{self, Error};
    use sealed::lookahead::Sealed;
    use span::IntoSpans;
    use token::Token;

    /// Support for checking the next token in a stream to decide how to parse.
    ///
    /// An important advantage over [`ParseStream::peek`] is that here we
    /// automatically construct an appropriate error message based on the token
    /// alternatives that get peeked. If you are producing your own error message,
    /// go ahead and use `ParseStream::peek` instead.
    ///
    /// Use [`ParseStream::lookahead1`] to construct this object.
    ///
    /// [`ParseStream::peek`]: struct.ParseBuffer.html#method.peek
    /// [`ParseStream::lookahead1`]: struct.ParseBuffer.html#method.lookahead1
    ///
    /// # Example
    ///
    /// ```edition2018
    /// use syn::{ConstParam, Ident, Lifetime, LifetimeDef, Result, Token, TypeParam};
    /// use syn::parse::{Parse, ParseStream};
    ///
    /// // A generic parameter, a single one of the comma-separated elements inside
    /// // angle brackets in:
    /// //
    /// //     fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
    /// //
    /// // On invalid input, lookahead gives us a reasonable error message.
    /// //
    /// //     error: expected one of: identifier, lifetime, `const`
    /// //       |
    /// //     5 |     fn f<!Sized>() {}
    /// //       |          ^
    /// enum GenericParam {
    ///     Type(TypeParam),
    ///     Lifetime(LifetimeDef),
    ///     Const(ConstParam),
    /// }
    ///
    /// impl Parse for GenericParam {
    ///     fn parse(input: ParseStream) -> Result<Self> {
    ///         let lookahead = input.lookahead1();
    ///         if lookahead.peek(Ident) {
    ///             input.parse().map(GenericParam::Type)
    ///         } else if lookahead.peek(Lifetime) {
    ///             input.parse().map(GenericParam::Lifetime)
    ///         } else if lookahead.peek(Token![const]) {
    ///             input.parse().map(GenericParam::Const)
    ///         } else {
    ///             Err(lookahead.error())
    ///         }
    ///     }
    /// }
    /// ```
    pub struct Lookahead1<'a> {
        scope: Span,
        cursor: Cursor<'a>,
        comparisons: RefCell<Vec<&'static str>>,
    }

    pub fn new(scope: Span, cursor: Cursor) -> Lookahead1 {
        Lookahead1 {
            scope: scope,
            cursor: cursor,
            comparisons: RefCell::new(Vec::new()),
        }
    }

    fn peek_impl(
        lookahead: &Lookahead1,
        peek: fn(Cursor) -> bool,
        display: fn() -> &'static str,
    ) -> bool {
        if peek(lookahead.cursor) {
            return true;
        }
        lookahead.comparisons.borrow_mut().push(display());
        false
    }

    impl<'a> Lookahead1<'a> {
        /// Looks at the next token in the parse stream to determine whether it
        /// matches the requested type of token.
        ///
        /// # Syntax
        ///
        /// Note that this method does not use turbofish syntax. Pass the peek type
        /// inside of parentheses.
        ///
        /// - `input.peek(Token![struct])`
        /// - `input.peek(Token![==])`
        /// - `input.peek(Ident)`&emsp;*(does not accept keywords)*
        /// - `input.peek(Ident::peek_any)`
        /// - `input.peek(Lifetime)`
        /// - `input.peek(token::Brace)`
        pub fn peek<T: Peek>(&self, token: T) -> bool {
            let _ = token;
            peek_impl(self, T::Token::peek, T::Token::display)
        }

        /// Triggers an error at the current position of the parse stream.
        ///
        /// The error message will identify all of the expected token types that
        /// have been peeked against this lookahead instance.
        pub fn error(self) -> Error {
            let comparisons = self.comparisons.borrow();
            match comparisons.len() {
                0 => {
                    if self.cursor.eof() {
                        Error::new(self.scope, "unexpected end of input")
                    } else {
                        Error::new(self.cursor.span(), "unexpected token")
                    }
                }
                1 => {
                    let message = format!("expected {}", comparisons[0]);
                    error::new_at(self.scope, self.cursor, message)
                }
                2 => {
                    let message = format!("expected {} or {}", comparisons[0], comparisons[1]);
                    error::new_at(self.scope, self.cursor, message)
                }
                _ => {
                    let join = comparisons.join(", ");
                    let message = format!("expected one of: {}", join);
                    error::new_at(self.scope, self.cursor, message)
                }
            }
        }
    }

    /// Types that can be parsed by looking at just one token.
    ///
    /// Use [`ParseStream::peek`] to peek one of these types in a parse stream
    /// without consuming it from the stream.
    ///
    /// This trait is sealed and cannot be implemented for types outside of Syn.
    ///
    /// [`ParseStream::peek`]: struct.ParseBuffer.html#method.peek
    pub trait Peek: Sealed {
        // Not public API.
        #[doc(hidden)]
        type Token: Token;
    }

    impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
        type Token = T;
    }

    pub enum TokenMarker {}

    impl<S> IntoSpans<S> for TokenMarker {
        fn into_spans(self) -> S {
            match self {}
        }
    }

    pub fn is_delimiter(cursor: Cursor, delimiter: Delimiter) -> bool {
        cursor.group(delimiter).is_some()
    }

    impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}

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