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    //! # 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 crate::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};

    macro_rules! quote_tt {
        (($($t:tt)*)) => { Group::new(Delimiter::Parenthesis, quote!($($t)*)) };
        ([$($t:tt)*]) => { Group::new(Delimiter::Bracket, quote!($($t)*)) };
        ({$($t:tt)*}) => { Group::new(Delimiter::Brace, quote!($($t)*)) };
        (,) => { Punct::new(',', Spacing::Alone) };
        (.) => { Punct::new('.', Spacing::Alone) };
        (:) => { Punct::new(':', Spacing::Alone) };
        (;) => { Punct::new(';', Spacing::Alone) };
        (!) => { Punct::new('!', Spacing::Alone) };
        (<) => { Punct::new('<', Spacing::Alone) };
        (>) => { Punct::new('>', Spacing::Alone) };
        (&) => { Punct::new('&', Spacing::Alone) };
        (=) => { Punct::new('=', Spacing::Alone) };
        ($i:ident) => { Ident::new(stringify!($i), Span::def_site()) };
    }

    macro_rules! quote_ts {
        ((@ $($t:tt)*)) => { $($t)* };
        (::) => {
            [
                TokenTree::from(Punct::new(':', Spacing::Joint)),
                TokenTree::from(Punct::new(':', Spacing::Alone)),
            ].iter()
                .cloned()
                .map(|mut x| {
                    x.set_span(Span::def_site());
                    x
                })
                .collect::<TokenStream>()
        };
        ($t:tt) => { TokenTree::from(quote_tt!($t)) };
    }

    /// Simpler version of the real `quote!` macro, implemented solely
    /// through `macro_rules`, for bootstrapping the real implementation
    /// (see the `quote` function), which does not have access to the
    /// real `quote!` macro due to the `proc_macro` crate not being
    /// able to depend on itself.
    ///
    /// Note: supported tokens are a subset of the real `quote!`, but
    /// unquoting is different: instead of `$x`, this uses `(@ expr)`.
    macro_rules! quote {
        () => { TokenStream::new() };
        ($($t:tt)*) => {
            [
                $(TokenStream::from(quote_ts!($t)),)*
            ].iter().cloned().collect::<TokenStream>()
        };
    }

    /// Quote a `TokenStream` into a `TokenStream`.
    /// This is the actual implementation of the `quote!()` proc macro.
    ///
    /// It is loaded by the compiler in `register_builtin_macros`.
    #[unstable(feature = "proc_macro_quote", issue = "54722")]
    pub fn quote(stream: TokenStream) -> TokenStream {
        if stream.is_empty() {
            return quote!(crate::TokenStream::new());
        }
        let mut after_dollar = false;
        let tokens = stream
            .into_iter()
            .filter_map(|tree| {
                if after_dollar {
                    after_dollar = false;
                    match tree {
                        TokenTree::Ident(_) => {
                            return Some(quote!(Into::<crate::TokenStream>::into(
                            Clone::clone(&(@ tree))),));
                        }
                        TokenTree::Punct(ref tt) if tt.as_char() == '$' => {}
                        _ => panic!("`$` must be followed by an ident or `$` in `quote!`"),
                    }
                } else if let TokenTree::Punct(ref tt) = tree {
                    if tt.as_char() == '$' {
                        after_dollar = true;
                        return None;
                    }
                }

                Some(quote!(crate::TokenStream::from((@ match tree {
                    TokenTree::Punct(tt) => quote!(crate::TokenTree::Punct(crate::Punct::new(
                        (@ TokenTree::from(Literal::character(tt.as_char()))),
                        (@ match tt.spacing() {
                            Spacing::Alone => quote!(crate::Spacing::Alone),
                            Spacing::Joint => quote!(crate::Spacing::Joint),
                        }),
                    ))),
                    TokenTree::Group(tt) => quote!(crate::TokenTree::Group(crate::Group::new(
                        (@ match tt.delimiter() {
                            Delimiter::Parenthesis => quote!(crate::Delimiter::Parenthesis),
                            Delimiter::Brace => quote!(crate::Delimiter::Brace),
                            Delimiter::Bracket => quote!(crate::Delimiter::Bracket),
                            Delimiter::None => quote!(crate::Delimiter::None),
                        }),
                        (@ quote(tt.stream())),
                    ))),
                    TokenTree::Ident(tt) => quote!(crate::TokenTree::Ident(crate::Ident::new(
                        (@ TokenTree::from(Literal::string(&tt.to_string()))),
                        (@ quote_span(tt.span())),
                    ))),
                    TokenTree::Literal(tt) => quote!(crate::TokenTree::Literal({
                        let mut iter = (@ TokenTree::from(Literal::string(&tt.to_string())))
                            .parse::<crate::TokenStream>()
                            .unwrap()
                            .into_iter();
                        if let (Some(crate::TokenTree::Literal(mut lit)), None) =
                            (iter.next(), iter.next())
                        {
                            lit.set_span((@ quote_span(tt.span())));
                            lit
                        } else {
                            unreachable!()
                        }
                    }))
                })),))
            })
            .collect::<TokenStream>();

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

        quote!([(@ tokens)].iter().cloned().collect::<crate::TokenStream>())
    }

    /// Quote a `Span` into a `TokenStream`.
    /// This is needed to implement a custom quoter.
    #[unstable(feature = "proc_macro_quote", issue = "54722")]
    pub fn quote_span(_: Span) -> TokenStream {
        quote!(crate::Span::def_site())
    }