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    // Copyright 2016 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! # Quasiquoter
    //! This file contains the implementation internals of the quasiquoter provided by `quote!`.

    //! This quasiquoter uses macros 2.0 hygiene to reliably access
    //! items from `proc_macro`, to build a `proc_macro::TokenStream`.

    use {Delimiter, Literal, Spacing, Span, Term, Op, Group, TokenStream, TokenTree};

    use syntax::ext::base::{ExtCtxt, ProcMacro};
    use syntax::parse::token;
    use syntax::tokenstream;

    pub struct Quoter;

    pub fn unquote<T: Into<TokenStream> + Clone>(tokens: &T) -> TokenStream {
        tokens.clone().into()
    }

    pub trait Quote {
        fn quote(self) -> TokenStream;
    }

    macro_rules! tt2ts {
        ($e:expr) => (TokenStream::from(TokenTree::from($e)))
    }

    macro_rules! quote_tok {
        (,) => { tt2ts!(Op::new(',', Spacing::Alone)) };
        (.) => { tt2ts!(Op::new('.', Spacing::Alone)) };
        (:) => { tt2ts!(Op::new(':', Spacing::Alone)) };
        (|) => { tt2ts!(Op::new('|', Spacing::Alone)) };
        (::) => {
            [
                TokenTree::from(Op::new(':', Spacing::Joint)),
                TokenTree::from(Op::new(':', Spacing::Alone)),
            ].iter()
                .cloned()
                .map(|mut x| {
                    x.set_span(Span::def_site());
                    x
                })
                .collect::<TokenStream>()
        };
        (!) => { tt2ts!(Op::new('!', Spacing::Alone)) };
        (<) => { tt2ts!(Op::new('<', Spacing::Alone)) };
        (>) => { tt2ts!(Op::new('>', Spacing::Alone)) };
        (_) => { tt2ts!(Op::new('_', Spacing::Alone)) };
        (0) => { tt2ts!(Literal::i8_unsuffixed(0)) };
        (&) => { tt2ts!(Op::new('&', Spacing::Alone)) };
        ($i:ident) => { tt2ts!(Term::new(stringify!($i), Span::def_site())) };
    }

    macro_rules! quote_tree {
        ((unquote $($t:tt)*)) => { $($t)* };
        ((quote $($t:tt)*)) => { ($($t)*).quote() };
        (($($t:tt)*)) => { tt2ts!(Group::new(Delimiter::Parenthesis, quote!($($t)*))) };
        ([$($t:tt)*]) => { tt2ts!(Group::new(Delimiter::Bracket, quote!($($t)*))) };
        ({$($t:tt)*}) => { tt2ts!(Group::new(Delimiter::Brace, quote!($($t)*))) };
        ($t:tt) => { quote_tok!($t) };
    }

    macro_rules! quote {
        () => { TokenStream::empty() };
        ($($t:tt)*) => {
            [$(quote_tree!($t),)*].iter()
                .cloned()
                .flat_map(|x| x.into_iter())
                .collect::<TokenStream>()
        };
    }

    impl ProcMacro for Quoter {
        fn expand<'cx>(&self, cx: &'cx mut ExtCtxt,
                       _: ::syntax_pos::Span,
                       stream: tokenstream::TokenStream)
                       -> tokenstream::TokenStream {
            let mut info = cx.current_expansion.mark.expn_info().unwrap();
            info.callee.allow_internal_unstable = true;
            cx.current_expansion.mark.set_expn_info(info);
            ::__internal::set_sess(cx, || TokenStream(stream).quote().0)
        }
    }

    impl<T: Quote> Quote for Option<T> {
        fn quote(self) -> TokenStream {
            match self {
                Some(t) => quote!(Some((quote t))),
                None => quote!(None),
            }
        }
    }

    impl Quote for TokenStream {
        fn quote(self) -> TokenStream {
            if self.is_empty() {
                return quote!(::TokenStream::empty());
            }
            let mut after_dollar = false;
            let tokens = self.into_iter().filter_map(|tree| {
                if after_dollar {
                    after_dollar = false;
                    match tree {
                        TokenTree::Term(_) => {
                            let tree = TokenStream::from(tree);
                            return Some(quote!(::__internal::unquote(&(unquote tree)),));
                        }
                        TokenTree::Op(ref tt) if tt.op() == '$' => {}
                        _ => panic!("`$` must be followed by an ident or `$` in `quote!`"),
                    }
                } else if let TokenTree::Op(tt) = tree {
                    if tt.op() == '$' {
                        after_dollar = true;
                        return None;
                    }
                }

                Some(quote!(::TokenStream::from((quote tree)),))
            }).flat_map(|t| t.into_iter()).collect::<TokenStream>();

            if after_dollar {
                panic!("unexpected trailing `$` in `quote!`");
            }

            quote!(
                [(unquote tokens)].iter()
                    .cloned()
                    .flat_map(|x| x.into_iter())
                    .collect::<::TokenStream>()
            )
        }
    }

    impl Quote for TokenTree {
        fn quote(self) -> TokenStream {
            match self {
                TokenTree::Op(tt) => quote!(::TokenTree::Op( (quote tt) )),
                TokenTree::Group(tt) => quote!(::TokenTree::Group( (quote tt) )),
                TokenTree::Term(tt) => quote!(::TokenTree::Term( (quote tt) )),
                TokenTree::Literal(tt) => quote!(::TokenTree::Literal( (quote tt) )),
            }
        }
    }

    impl Quote for char {
        fn quote(self) -> TokenStream {
            TokenTree::from(Literal::character(self)).into()
        }
    }

    impl<'a> Quote for &'a str {
        fn quote(self) -> TokenStream {
            TokenTree::from(Literal::string(self)).into()
        }
    }

    impl Quote for u16 {
        fn quote(self) -> TokenStream {
            TokenTree::from(Literal::u16_unsuffixed(self)).into()
        }
    }

    impl Quote for Group {
        fn quote(self) -> TokenStream {
            quote!(::Group::new((quote self.delimiter()), (quote self.stream())))
        }
    }

    impl Quote for Op {
        fn quote(self) -> TokenStream {
            quote!(::Op::new((quote self.op()), (quote self.spacing())))
        }
    }

    impl Quote for Term {
        fn quote(self) -> TokenStream {
            quote!(::Term::new((quote self.as_str()), (quote self.span())))
        }
    }

    impl Quote for Span {
        fn quote(self) -> TokenStream {
            quote!(::Span::def_site())
        }
    }

    macro_rules! literals {
        ($($i:ident),*; $($raw:ident),*) => {
            pub enum LiteralKind {
                $($i,)*
                $($raw(u16),)*
            }

            impl LiteralKind {
                pub fn with_contents_and_suffix(self, contents: Term, suffix: Option<Term>)
                                                -> Literal {
                    let sym = contents.sym;
                    let suffix = suffix.map(|t| t.sym);
                    match self {
                        $(LiteralKind::$i => {
                            Literal {
                                lit: token::Lit::$i(sym),
                                suffix,
                                span: contents.span,
                            }
                        })*
                        $(LiteralKind::$raw(n) => {
                            Literal {
                                lit: token::Lit::$raw(sym, n),
                                suffix,
                                span: contents.span,
                            }
                        })*
                    }
                }
            }

            impl Literal {
                fn kind_contents_and_suffix(self) -> (LiteralKind, Term, Option<Term>) {
                    let (kind, contents) = match self.lit {
                        $(token::Lit::$i(contents) => (LiteralKind::$i, contents),)*
                        $(token::Lit::$raw(contents, n) => (LiteralKind::$raw(n), contents),)*
                    };
                    let suffix = self.suffix.map(|sym| Term::new(&sym.as_str(), self.span()));
                    (kind, Term::new(&contents.as_str(), self.span()), suffix)
                }
            }

            impl Quote for LiteralKind {
                fn quote(self) -> TokenStream {
                    match self {
                        $(LiteralKind::$i => quote! {
                            ::__internal::LiteralKind::$i
                        },)*
                        $(LiteralKind::$raw(n) => quote! {
                            ::__internal::LiteralKind::$raw((quote n))
                        },)*
                    }
                }
            }

            impl Quote for Literal {
                fn quote(self) -> TokenStream {
                    let (kind, contents, suffix) = self.kind_contents_and_suffix();
                    quote! {
                        (quote kind).with_contents_and_suffix((quote contents), (quote suffix))
                    }
                }
            }
        }
    }

    literals!(Byte, Char, Float, Str_, Integer, ByteStr; StrRaw, ByteStrRaw);

    impl Quote for Delimiter {
        fn quote(self) -> TokenStream {
            macro_rules! gen_match {
                ($($i:ident),*) => {
                    match self {
                        $(Delimiter::$i => { quote!(::Delimiter::$i) })*
                    }
                }
            }

            gen_match!(Parenthesis, Brace, Bracket, None)
        }
    }

    impl Quote for Spacing {
        fn quote(self) -> TokenStream {
            macro_rules! gen_match {
                ($($i:ident),*) => {
                    match self {
                        $(Spacing::$i => { quote!(::Spacing::$i) })*
                    }
                }
            }

            gen_match!(Alone, Joint)
        }
    }

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