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    //! A stably addressed token buffer supporting efficient traversal based on a
    //! cheaply copyable cursor.
    //!
    //! *This module is available only if Syn is built with the `"parsing"` feature.*

    // This module is heavily commented as it contains most of the unsafe code in
    // Syn, and caution should be used when editing it. The public-facing interface
    // is 100% safe but the implementation is fragile internally.

    #[cfg(all(
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
        feature = "proc-macro"
    ))]
    use crate::proc_macro as pm;
    use crate::Lifetime;
    use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
    use std::cmp::Ordering;
    use std::marker::PhantomData;

    /// Internal type which is used instead of `TokenTree` to represent a token tree
    /// within a `TokenBuffer`.
    enum Entry {
        // Mimicking types from proc-macro.
        // Group entries contain the offset to the matching End entry.
        Group(Group, usize),
        Ident(Ident),
        Punct(Punct),
        Literal(Literal),
        // End entries contain the offset (negative) to the start of the buffer.
        End(isize),
    }

    /// A buffer that can be efficiently traversed multiple times, unlike
    /// `TokenStream` which requires a deep copy in order to traverse more than
    /// once.
    ///
    /// *This type is available only if Syn is built with the `"parsing"` feature.*
    pub struct TokenBuffer {
        // NOTE: Do not implement clone on this - while the current design could be
        // cloned, other designs which could be desirable may not be cloneable.
        entries: Box<[Entry]>,
    }

    impl TokenBuffer {
        fn recursive_new(entries: &mut Vec<Entry>, stream: TokenStream) {
            for tt in stream {
                match tt {
                    TokenTree::Ident(ident) => entries.push(Entry::Ident(ident)),
                    TokenTree::Punct(punct) => entries.push(Entry::Punct(punct)),
                    TokenTree::Literal(literal) => entries.push(Entry::Literal(literal)),
                    TokenTree::Group(group) => {
                        let group_start_index = entries.len();
                        entries.push(Entry::End(0)); // we replace this below
                        Self::recursive_new(entries, group.stream());
                        let group_end_index = entries.len();
                        entries.push(Entry::End(-(group_end_index as isize)));
                        let group_end_offset = group_end_index - group_start_index;
                        entries[group_start_index] = Entry::Group(group, group_end_offset);
                    }
                }
            }
        }

        /// Creates a `TokenBuffer` containing all the tokens from the input
        /// `proc_macro::TokenStream`.
        ///
        /// *This method is available only if Syn is built with both the `"parsing"` and
        /// `"proc-macro"` features.*
        #[cfg(all(
            not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
            feature = "proc-macro"
        ))]
        pub fn new(stream: pm::TokenStream) -> Self {
            Self::new2(stream.into())
        }

        /// Creates a `TokenBuffer` containing all the tokens from the input
        /// `proc_macro2::TokenStream`.
        pub fn new2(stream: TokenStream) -> Self {
            let mut entries = Vec::new();
            Self::recursive_new(&mut entries, stream);
            entries.push(Entry::End(-(entries.len() as isize)));
            Self {
                entries: entries.into_boxed_slice(),
            }
        }

        /// Creates a cursor referencing the first token in the buffer and able to
        /// traverse until the end of the buffer.
        pub fn begin(&self) -> Cursor {
            let ptr = self.entries.as_ptr();
            unsafe { Cursor::create(ptr, ptr.add(self.entries.len() - 1)) }
        }
    }

    /// A cheaply copyable cursor into a `TokenBuffer`.
    ///
    /// This cursor holds a shared reference into the immutable data which is used
    /// internally to represent a `TokenStream`, and can be efficiently manipulated
    /// and copied around.
    ///
    /// An empty `Cursor` can be created directly, or one may create a `TokenBuffer`
    /// object and get a cursor to its first token with `begin()`.
    ///
    /// Two cursors are equal if they have the same location in the same input
    /// stream, and have the same scope.
    ///
    /// *This type is available only if Syn is built with the `"parsing"` feature.*
    pub struct Cursor<'a> {
        // The current entry which the `Cursor` is pointing at.
        ptr: *const Entry,
        // This is the only `Entry::End` object which this cursor is allowed to
        // point at. All other `End` objects are skipped over in `Cursor::create`.
        scope: *const Entry,
        // Cursor is covariant in 'a. This field ensures that our pointers are still
        // valid.
        marker: PhantomData<&'a Entry>,
    }

    impl<'a> Cursor<'a> {
        /// Creates a cursor referencing a static empty TokenStream.
        pub fn empty() -> Self {
            // It's safe in this situation for us to put an `Entry` object in global
            // storage, despite it not actually being safe to send across threads
            // (`Ident` is a reference into a thread-local table). This is because
            // this entry never includes a `Ident` object.
            //
            // This wrapper struct allows us to break the rules and put a `Sync`
            // object in global storage.
            struct UnsafeSyncEntry(Entry);
            unsafe impl Sync for UnsafeSyncEntry {}
            static EMPTY_ENTRY: UnsafeSyncEntry = UnsafeSyncEntry(Entry::End(0));

            Cursor {
                ptr: &EMPTY_ENTRY.0,
                scope: &EMPTY_ENTRY.0,
                marker: PhantomData,
            }
        }

        /// This create method intelligently exits non-explicitly-entered
        /// `None`-delimited scopes when the cursor reaches the end of them,
        /// allowing for them to be treated transparently.
        unsafe fn create(mut ptr: *const Entry, scope: *const Entry) -> Self {
            // NOTE: If we're looking at a `End`, we want to advance the cursor
            // past it, unless `ptr == scope`, which means that we're at the edge of
            // our cursor's scope. We should only have `ptr != scope` at the exit
            // from None-delimited groups entered with `ignore_none`.
            while let Entry::End(_) = *ptr {
                if ptr == scope {
                    break;
                }
                ptr = ptr.add(1);
            }

            Cursor {
                ptr,
                scope,
                marker: PhantomData,
            }
        }

        /// Get the current entry.
        fn entry(self) -> &'a Entry {
            unsafe { &*self.ptr }
        }

        /// Bump the cursor to point at the next token after the current one. This
        /// is undefined behavior if the cursor is currently looking at an
        /// `Entry::End`.
        ///
        /// If the cursor is looking at an `Entry::Group`, the bumped cursor will
        /// point at the first token in the group (with the same scope end).
        unsafe fn bump_ignore_group(self) -> Cursor<'a> {
            Cursor::create(self.ptr.offset(1), self.scope)
        }

        /// While the cursor is looking at a `None`-delimited group, move it to look
        /// at the first token inside instead. If the group is empty, this will move
        /// the cursor past the `None`-delimited group.
        ///
        /// WARNING: This mutates its argument.
        fn ignore_none(&mut self) {
            while let Entry::Group(group, _) = self.entry() {
                if group.delimiter() == Delimiter::None {
                    unsafe { *self = self.bump_ignore_group() };
                } else {
                    break;
                }
            }
        }

        /// Checks whether the cursor is currently pointing at the end of its valid
        /// scope.
        pub fn eof(self) -> bool {
            // We're at eof if we're at the end of our scope.
            self.ptr == self.scope
        }

        /// If the cursor is pointing at a `Group` with the given delimiter, returns
        /// a cursor into that group and one pointing to the next `TokenTree`.
        pub fn group(mut self, delim: Delimiter) -> Option<(Cursor<'a>, Span, Cursor<'a>)> {
            // If we're not trying to enter a none-delimited group, we want to
            // ignore them. We have to make sure to _not_ ignore them when we want
            // to enter them, of course. For obvious reasons.
            if delim != Delimiter::None {
                self.ignore_none();
            }

            if let Entry::Group(group, end_offset) = self.entry() {
                if group.delimiter() == delim {
                    let end_of_group = unsafe { self.ptr.add(*end_offset) };
                    let inside_of_group = unsafe { Cursor::create(self.ptr.add(1), end_of_group) };
                    let after_group = unsafe { Cursor::create(end_of_group, self.scope) };
                    return Some((inside_of_group, group.span(), after_group));
                }
            }

            None
        }

        /// If the cursor is pointing at a `Ident`, returns it along with a cursor
        /// pointing at the next `TokenTree`.
        pub fn ident(mut self) -> Option<(Ident, Cursor<'a>)> {
            self.ignore_none();
            match self.entry() {
                Entry::Ident(ident) => Some((ident.clone(), unsafe { self.bump_ignore_group() })),
                _ => None,
            }
        }

        /// If the cursor is pointing at a `Punct`, returns it along with a cursor
        /// pointing at the next `TokenTree`.
        pub fn punct(mut self) -> Option<(Punct, Cursor<'a>)> {
            self.ignore_none();
            match self.entry() {
                Entry::Punct(punct) if punct.as_char() != '\'' => {
                    Some((punct.clone(), unsafe { self.bump_ignore_group() }))
                }
                _ => None,
            }
        }

        /// If the cursor is pointing at a `Literal`, return it along with a cursor
        /// pointing at the next `TokenTree`.
        pub fn literal(mut self) -> Option<(Literal, Cursor<'a>)> {
            self.ignore_none();
            match self.entry() {
                Entry::Literal(literal) => Some((literal.clone(), unsafe { self.bump_ignore_group() })),
                _ => None,
            }
        }

        /// If the cursor is pointing at a `Lifetime`, returns it along with a
        /// cursor pointing at the next `TokenTree`.
        pub fn lifetime(mut self) -> Option<(Lifetime, Cursor<'a>)> {
            self.ignore_none();
            match self.entry() {
                Entry::Punct(punct) if punct.as_char() == '\'' && punct.spacing() == Spacing::Joint => {
                    let next = unsafe { self.bump_ignore_group() };
                    let (ident, rest) = next.ident()?;
                    let lifetime = Lifetime {
                        apostrophe: punct.span(),
                        ident,
                    };
                    Some((lifetime, rest))
                }
                _ => None,
            }
        }

        /// Copies all remaining tokens visible from this cursor into a
        /// `TokenStream`.
        pub fn token_stream(self) -> TokenStream {
            let mut tts = Vec::new();
            let mut cursor = self;
            while let Some((tt, rest)) = cursor.token_tree() {
                tts.push(tt);
                cursor = rest;
            }
            tts.into_iter().collect()
        }

        /// If the cursor is pointing at a `TokenTree`, returns it along with a
        /// cursor pointing at the next `TokenTree`.
        ///
        /// Returns `None` if the cursor has reached the end of its stream.
        ///
        /// This method does not treat `None`-delimited groups as transparent, and
        /// will return a `Group(None, ..)` if the cursor is looking at one.
        pub fn token_tree(self) -> Option<(TokenTree, Cursor<'a>)> {
            let (tree, len) = match self.entry() {
                Entry::Group(group, end_offset) => (group.clone().into(), *end_offset),
                Entry::Literal(literal) => (literal.clone().into(), 1),
                Entry::Ident(ident) => (ident.clone().into(), 1),
                Entry::Punct(punct) => (punct.clone().into(), 1),
                Entry::End(_) => return None,
            };

            let rest = unsafe { Cursor::create(self.ptr.add(len), self.scope) };
            Some((tree, rest))
        }

        /// Returns the `Span` of the current token, or `Span::call_site()` if this
        /// cursor points to eof.
        pub fn span(self) -> Span {
            match self.entry() {
                Entry::Group(group, _) => group.span(),
                Entry::Literal(literal) => literal.span(),
                Entry::Ident(ident) => ident.span(),
                Entry::Punct(punct) => punct.span(),
                Entry::End(_) => Span::call_site(),
            }
        }

        /// Skip over the next token without cloning it. Returns `None` if this
        /// cursor points to eof.
        ///
        /// This method treats `'lifetimes` as a single token.
        pub(crate) fn skip(self) -> Option<Cursor<'a>> {
            let len = match self.entry() {
                Entry::End(_) => return None,

                // Treat lifetimes as a single tt for the purposes of 'skip'.
                Entry::Punct(punct) if punct.as_char() == '\'' && punct.spacing() == Spacing::Joint => {
                    match unsafe { &*self.ptr.add(1) } {
                        Entry::Ident(_) => 2,
                        _ => 1,
                    }
                }

                Entry::Group(_, end_offset) => *end_offset,
                _ => 1,
            };

            Some(unsafe { Cursor::create(self.ptr.add(len), self.scope) })
        }
    }

    impl<'a> Copy for Cursor<'a> {}

    impl<'a> Clone for Cursor<'a> {
        fn clone(&self) -> Self {
            *self
        }
    }

    impl<'a> Eq for Cursor<'a> {}

    impl<'a> PartialEq for Cursor<'a> {
        fn eq(&self, other: &Self) -> bool {
            self.ptr == other.ptr
        }
    }

    impl<'a> PartialOrd for Cursor<'a> {
        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
            if same_buffer(*self, *other) {
                Some(self.ptr.cmp(&other.ptr))
            } else {
                None
            }
        }
    }

    pub(crate) fn same_scope(a: Cursor, b: Cursor) -> bool {
        a.scope == b.scope
    }

    pub(crate) fn same_buffer(a: Cursor, b: Cursor) -> bool {
        unsafe {
            match (&*a.scope, &*b.scope) {
                (Entry::End(a_offset), Entry::End(b_offset)) => {
                    a.scope.offset(*a_offset) == b.scope.offset(*b_offset)
                }
                _ => unreachable!(),
            }
        }
    }

    #[cfg(any(feature = "full", feature = "derive"))]
    pub(crate) fn cmp_assuming_same_buffer(a: Cursor, b: Cursor) -> Ordering {
        a.ptr.cmp(&b.ptr)
    }

    pub(crate) fn open_span_of_group(cursor: Cursor) -> Span {
        match cursor.entry() {
            Entry::Group(group, _) => group.span_open(),
            _ => cursor.span(),
        }
    }

    pub(crate) fn close_span_of_group(cursor: Cursor) -> Span {
        match cursor.entry() {
            Entry::Group(group, _) => group.span_close(),
            _ => cursor.span(),
        }
    }