# punctuated.rs - source
Source of the Rust file `/home/justine/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.107/src/punctuated.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A punctuated sequence of syntax tree nodes separated by punctuation.
    //!
    //! Lots of things in Rust are punctuated sequences.
    //!
    //! - The fields of a struct are `Punctuated<Field, Token![,]>`.
    //! - The segments of a path are `Punctuated<PathSegment, Token![::]>`.
    //! - The bounds on a generic parameter are `Punctuated<TypeParamBound,
    //!   Token![+]>`.
    //! - The arguments to a function call are `Punctuated<Expr, Token![,]>`.
    //!
    //! This module provides a common representation for these punctuated sequences
    //! in the form of the [`Punctuated<T, P>`] type. We store a vector of pairs of
    //! syntax tree node + punctuation, where every node in the sequence is followed
    //! by punctuation except for possibly the final one.
    //!
    //! [`Punctuated<T, P>`]: Punctuated
    //!
    //! ```text
    //! a_function_call(arg1, arg2, arg3);
    //!                 ~~~~^ ~~~~^ ~~~~
    //! ```

    #[cfg(feature = "extra-traits")]
    use std::fmt::{self, Debug};
    #[cfg(feature = "extra-traits")]
    use std::hash::{Hash, Hasher};
    #[cfg(any(feature = "full", feature = "derive"))]
    use std::iter;
    use std::iter::FromIterator;
    use std::ops::{Index, IndexMut};
    use std::option;
    use std::slice;
    use std::vec;

    use crate::drops::{NoDrop, TrivialDrop};
    #[cfg(feature = "parsing")]
    use crate::parse::{Parse, ParseStream, Result};
    #[cfg(feature = "parsing")]
    use crate::token::Token;

    /// A punctuated sequence of syntax tree nodes of type `T` separated by
    /// punctuation of type `P`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct Punctuated<T, P> {
        inner: Vec<(T, P)>,
        last: Option<Box<T>>,
    }

    impl<T, P> Punctuated<T, P> {
        /// Creates an empty punctuated sequence.
        #[cfg(not(syn_no_const_vec_new))]
        pub const fn new() -> Self {
            Punctuated {
                inner: Vec::new(),
                last: None,
            }
        }

        /// Creates an empty punctuated sequence.
        #[cfg(syn_no_const_vec_new)]
        pub fn new() -> Self {
            Punctuated {
                inner: Vec::new(),
                last: None,
            }
        }

        /// Determines whether this punctuated sequence is empty, meaning it
        /// contains no syntax tree nodes or punctuation.
        pub fn is_empty(&self) -> bool {
            self.inner.len() == 0 && self.last.is_none()
        }

        /// Returns the number of syntax tree nodes in this punctuated sequence.
        ///
        /// This is the number of nodes of type `T`, not counting the punctuation of
        /// type `P`.
        pub fn len(&self) -> usize {
            self.inner.len() + if self.last.is_some() { 1 } else { 0 }
        }

        /// Borrows the first element in this sequence.
        pub fn first(&self) -> Option<&T> {
            self.iter().next()
        }

        /// Mutably borrows the first element in this sequence.
        pub fn first_mut(&mut self) -> Option<&mut T> {
            self.iter_mut().next()
        }

        /// Borrows the last element in this sequence.
        pub fn last(&self) -> Option<&T> {
            self.iter().next_back()
        }

        /// Mutably borrows the last element in this sequence.
        pub fn last_mut(&mut self) -> Option<&mut T> {
            self.iter_mut().next_back()
        }

        /// Returns an iterator over borrowed syntax tree nodes of type `&T`.
        pub fn iter(&self) -> Iter<T> {
            Iter {
                inner: Box::new(NoDrop::new(PrivateIter {
                    inner: self.inner.iter(),
                    last: self.last.as_ref().map(Box::as_ref).into_iter(),
                })),
            }
        }

        /// Returns an iterator over mutably borrowed syntax tree nodes of type
        /// `&mut T`.
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut {
                inner: Box::new(NoDrop::new(PrivateIterMut {
                    inner: self.inner.iter_mut(),
                    last: self.last.as_mut().map(Box::as_mut).into_iter(),
                })),
            }
        }

        /// Returns an iterator over the contents of this sequence as borrowed
        /// punctuated pairs.
        pub fn pairs(&self) -> Pairs<T, P> {
            Pairs {
                inner: self.inner.iter(),
                last: self.last.as_ref().map(Box::as_ref).into_iter(),
            }
        }

        /// Returns an iterator over the contents of this sequence as mutably
        /// borrowed punctuated pairs.
        pub fn pairs_mut(&mut self) -> PairsMut<T, P> {
            PairsMut {
                inner: self.inner.iter_mut(),
                last: self.last.as_mut().map(Box::as_mut).into_iter(),
            }
        }

        /// Returns an iterator over the contents of this sequence as owned
        /// punctuated pairs.
        pub fn into_pairs(self) -> IntoPairs<T, P> {
            IntoPairs {
                inner: self.inner.into_iter(),
                last: self.last.map(|t| *t).into_iter(),
            }
        }

        /// Appends a syntax tree node onto the end of this punctuated sequence. The
        /// sequence must previously have a trailing punctuation.
        ///
        /// Use [`push`] instead if the punctuated sequence may or may not already
        /// have trailing punctuation.
        ///
        /// [`push`]: Punctuated::push
        ///
        /// # Panics
        ///
        /// Panics if the sequence does not already have a trailing punctuation when
        /// this method is called.
        pub fn push_value(&mut self, value: T) {
            assert!(
                self.empty_or_trailing(),
                "Punctuated::push_value: cannot push value if Punctuated is missing trailing punctuation",
            );

            self.last = Some(Box::new(value));
        }

        /// Appends a trailing punctuation onto the end of this punctuated sequence.
        /// The sequence must be non-empty and must not already have trailing
        /// punctuation.
        ///
        /// # Panics
        ///
        /// Panics if the sequence is empty or already has a trailing punctuation.
        pub fn push_punct(&mut self, punctuation: P) {
            assert!(
                self.last.is_some(),
                "Punctuated::push_punct: cannot push punctuation if Punctuated is empty or already has trailing punctuation",
            );

            let last = self.last.take().unwrap();
            self.inner.push((*last, punctuation));
        }

        /// Removes the last punctuated pair from this sequence, or `None` if the
        /// sequence is empty.
        pub fn pop(&mut self) -> Option<Pair<T, P>> {
            if self.last.is_some() {
                self.last.take().map(|t| Pair::End(*t))
            } else {
                self.inner.pop().map(|(t, p)| Pair::Punctuated(t, p))
            }
        }

        /// Determines whether this punctuated sequence ends with a trailing
        /// punctuation.
        pub fn trailing_punct(&self) -> bool {
            self.last.is_none() && !self.is_empty()
        }

        /// Returns true if either this `Punctuated` is empty, or it has a trailing
        /// punctuation.
        ///
        /// Equivalent to `punctuated.is_empty() || punctuated.trailing_punct()`.
        pub fn empty_or_trailing(&self) -> bool {
            self.last.is_none()
        }

        /// Appends a syntax tree node onto the end of this punctuated sequence.
        ///
        /// If there is not a trailing punctuation in this sequence when this method
        /// is called, the default value of punctuation type `P` is inserted before
        /// the given value of type `T`.
        pub fn push(&mut self, value: T)
        where
            P: Default,
        {
            if !self.empty_or_trailing() {
                self.push_punct(Default::default());
            }
            self.push_value(value);
        }

        /// Inserts an element at position `index`.
        ///
        /// # Panics
        ///
        /// Panics if `index` is greater than the number of elements previously in
        /// this punctuated sequence.
        pub fn insert(&mut self, index: usize, value: T)
        where
            P: Default,
        {
            assert!(
                index <= self.len(),
                "Punctuated::insert: index out of range",
            );

            if index == self.len() {
                self.push(value);
            } else {
                self.inner.insert(index, (value, Default::default()));
            }
        }

        /// Clears the sequence of all values and punctuation, making it empty.
        pub fn clear(&mut self) {
            self.inner.clear();
            self.last = None;
        }

        /// Parses zero or more occurrences of `T` separated by punctuation of type
        /// `P`, with optional trailing punctuation.
        ///
        /// Parsing continues until the end of this parse stream. The entire content
        /// of this parse stream must consist of `T` and `P`.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_terminated(input: ParseStream) -> Result<Self>
        where
            T: Parse,
            P: Parse,
        {
            Self::parse_terminated_with(input, T::parse)
        }

        /// Parses zero or more occurrences of `T` using the given parse function,
        /// separated by punctuation of type `P`, with optional trailing
        /// punctuation.
        ///
        /// Like [`parse_terminated`], the entire content of this stream is expected
        /// to be parsed.
        ///
        /// [`parse_terminated`]: Punctuated::parse_terminated
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_terminated_with(
            input: ParseStream,
            parser: fn(ParseStream) -> Result<T>,
        ) -> Result<Self>
        where
            P: Parse,
        {
            let mut punctuated = Punctuated::new();

            loop {
                if input.is_empty() {
                    break;
                }
                let value = parser(input)?;
                punctuated.push_value(value);
                if input.is_empty() {
                    break;
                }
                let punct = input.parse()?;
                punctuated.push_punct(punct);
            }

            Ok(punctuated)
        }

        /// Parses one or more occurrences of `T` separated by punctuation of type
        /// `P`, not accepting trailing punctuation.
        ///
        /// Parsing continues as long as punctuation `P` is present at the head of
        /// the stream. This method returns upon parsing a `T` and observing that it
        /// is not followed by a `P`, even if there are remaining tokens in the
        /// stream.
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_separated_nonempty(input: ParseStream) -> Result<Self>
        where
            T: Parse,
            P: Token + Parse,
        {
            Self::parse_separated_nonempty_with(input, T::parse)
        }

        /// Parses one or more occurrences of `T` using the given parse function,
        /// separated by punctuation of type `P`, not accepting trailing
        /// punctuation.
        ///
        /// Like [`parse_separated_nonempty`], may complete early without parsing
        /// the entire content of this stream.
        ///
        /// [`parse_separated_nonempty`]: Punctuated::parse_separated_nonempty
        ///
        /// *This function is available only if Syn is built with the `"parsing"`
        /// feature.*
        #[cfg(feature = "parsing")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
        pub fn parse_separated_nonempty_with(
            input: ParseStream,
            parser: fn(ParseStream) -> Result<T>,
        ) -> Result<Self>
        where
            P: Token + Parse,
        {
            let mut punctuated = Punctuated::new();

            loop {
                let value = parser(input)?;
                punctuated.push_value(value);
                if !P::peek(input.cursor()) {
                    break;
                }
                let punct = input.parse()?;
                punctuated.push_punct(punct);
            }

            Ok(punctuated)
        }
    }

    #[cfg(feature = "clone-impls")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "clone-impls")))]
    impl<T, P> Clone for Punctuated<T, P>
    where
        T: Clone,
        P: Clone,
    {
        fn clone(&self) -> Self {
            Punctuated {
                inner: self.inner.clone(),
                last: self.last.clone(),
            }
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl<T, P> Eq for Punctuated<T, P>
    where
        T: Eq,
        P: Eq,
    {
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl<T, P> PartialEq for Punctuated<T, P>
    where
        T: PartialEq,
        P: PartialEq,
    {
        fn eq(&self, other: &Self) -> bool {
            let Punctuated { inner, last } = self;
            *inner == other.inner && *last == other.last
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl<T, P> Hash for Punctuated<T, P>
    where
        T: Hash,
        P: Hash,
    {
        fn hash<H: Hasher>(&self, state: &mut H) {
            let Punctuated { inner, last } = self;
            inner.hash(state);
            last.hash(state);
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl<T: Debug, P: Debug> Debug for Punctuated<T, P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let mut list = f.debug_list();
            for (t, p) in &self.inner {
                list.entry(t);
                list.entry(p);
            }
            if let Some(last) = &self.last {
                list.entry(last);
            }
            list.finish()
        }
    }

    impl<T, P> FromIterator<T> for Punctuated<T, P>
    where
        P: Default,
    {
        fn from_iter<I: IntoIterator<Item = T>>(i: I) -> Self {
            let mut ret = Punctuated::new();
            ret.extend(i);
            ret
        }
    }

    impl<T, P> Extend<T> for Punctuated<T, P>
    where
        P: Default,
    {
        fn extend<I: IntoIterator<Item = T>>(&mut self, i: I) {
            for value in i {
                self.push(value);
            }
        }
    }

    impl<T, P> FromIterator<Pair<T, P>> for Punctuated<T, P> {
        fn from_iter<I: IntoIterator<Item = Pair<T, P>>>(i: I) -> Self {
            let mut ret = Punctuated::new();
            ret.extend(i);
            ret
        }
    }

    impl<T, P> Extend<Pair<T, P>> for Punctuated<T, P> {
        fn extend<I: IntoIterator<Item = Pair<T, P>>>(&mut self, i: I) {
            assert!(
                self.empty_or_trailing(),
                "Punctuated::extend: Punctuated is not empty or does not have a trailing punctuation",
            );

            let mut nomore = false;
            for pair in i {
                if nomore {
                    panic!("Punctuated extended with items after a Pair::End");
                }
                match pair {
                    Pair::Punctuated(a, b) => self.inner.push((a, b)),
                    Pair::End(a) => {
                        self.last = Some(Box::new(a));
                        nomore = true;
                    }
                }
            }
        }
    }

    impl<T, P> IntoIterator for Punctuated<T, P> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        fn into_iter(self) -> Self::IntoIter {
            let mut elements = Vec::with_capacity(self.len());
            elements.extend(self.inner.into_iter().map(|pair| pair.0));
            elements.extend(self.last.map(|t| *t));

            IntoIter {
                inner: elements.into_iter(),
            }
        }
    }

    impl<'a, T, P> IntoIterator for &'a Punctuated<T, P> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Self::IntoIter {
            Punctuated::iter(self)
        }
    }

    impl<'a, T, P> IntoIterator for &'a mut Punctuated<T, P> {
        type Item = &'a mut T;
        type IntoIter = IterMut<'a, T>;

        fn into_iter(self) -> Self::IntoIter {
            Punctuated::iter_mut(self)
        }
    }

    impl<T, P> Default for Punctuated<T, P> {
        fn default() -> Self {
            Punctuated::new()
        }
    }

    /// An iterator over borrowed pairs of type `Pair<&T, &P>`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct Pairs<'a, T: 'a, P: 'a> {
        inner: slice::Iter<'a, (T, P)>,
        last: option::IntoIter<&'a T>,
    }

    impl<'a, T, P> Iterator for Pairs<'a, T, P> {
        type Item = Pair<&'a T, &'a P>;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner
                .next()
                .map(|(t, p)| Pair::Punctuated(t, p))
                .or_else(|| self.last.next().map(Pair::End))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<'a, T, P> DoubleEndedIterator for Pairs<'a, T, P> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.last
                .next()
                .map(Pair::End)
                .or_else(|| self.inner.next_back().map(|(t, p)| Pair::Punctuated(t, p)))
        }
    }

    impl<'a, T, P> ExactSizeIterator for Pairs<'a, T, P> {
        fn len(&self) -> usize {
            self.inner.len() + self.last.len()
        }
    }

    // No Clone bound on T or P.
    impl<'a, T, P> Clone for Pairs<'a, T, P> {
        fn clone(&self) -> Self {
            Pairs {
                inner: self.inner.clone(),
                last: self.last.clone(),
            }
        }
    }

    /// An iterator over mutably borrowed pairs of type `Pair<&mut T, &mut P>`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct PairsMut<'a, T: 'a, P: 'a> {
        inner: slice::IterMut<'a, (T, P)>,
        last: option::IntoIter<&'a mut T>,
    }

    impl<'a, T, P> Iterator for PairsMut<'a, T, P> {
        type Item = Pair<&'a mut T, &'a mut P>;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner
                .next()
                .map(|(t, p)| Pair::Punctuated(t, p))
                .or_else(|| self.last.next().map(Pair::End))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<'a, T, P> DoubleEndedIterator for PairsMut<'a, T, P> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.last
                .next()
                .map(Pair::End)
                .or_else(|| self.inner.next_back().map(|(t, p)| Pair::Punctuated(t, p)))
        }
    }

    impl<'a, T, P> ExactSizeIterator for PairsMut<'a, T, P> {
        fn len(&self) -> usize {
            self.inner.len() + self.last.len()
        }
    }

    /// An iterator over owned pairs of type `Pair<T, P>`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct IntoPairs<T, P> {
        inner: vec::IntoIter<(T, P)>,
        last: option::IntoIter<T>,
    }

    impl<T, P> Iterator for IntoPairs<T, P> {
        type Item = Pair<T, P>;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner
                .next()
                .map(|(t, p)| Pair::Punctuated(t, p))
                .or_else(|| self.last.next().map(Pair::End))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<T, P> DoubleEndedIterator for IntoPairs<T, P> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.last
                .next()
                .map(Pair::End)
                .or_else(|| self.inner.next_back().map(|(t, p)| Pair::Punctuated(t, p)))
        }
    }

    impl<T, P> ExactSizeIterator for IntoPairs<T, P> {
        fn len(&self) -> usize {
            self.inner.len() + self.last.len()
        }
    }

    impl<T, P> Clone for IntoPairs<T, P>
    where
        T: Clone,
        P: Clone,
    {
        fn clone(&self) -> Self {
            IntoPairs {
                inner: self.inner.clone(),
                last: self.last.clone(),
            }
        }
    }

    /// An iterator over owned values of type `T`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct IntoIter<T> {
        inner: vec::IntoIter<T>,
    }

    impl<T> Iterator for IntoIter<T> {
        type Item = T;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<T> DoubleEndedIterator for IntoIter<T> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.inner.next_back()
        }
    }

    impl<T> ExactSizeIterator for IntoIter<T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    impl<T> Clone for IntoIter<T>
    where
        T: Clone,
    {
        fn clone(&self) -> Self {
            IntoIter {
                inner: self.inner.clone(),
            }
        }
    }

    /// An iterator over borrowed values of type `&T`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct Iter<'a, T: 'a> {
        // The `Item = &'a T` needs to be specified to support rustc 1.31 and older.
        // On modern compilers we would be able to write just IterTrait<'a, T> where
        // Item can be inferred unambiguously from the supertrait.
        inner: Box<NoDrop<dyn IterTrait<'a, T, Item = &'a T> + 'a>>,
    }

    trait IterTrait<'a, T: 'a>:
        DoubleEndedIterator<Item = &'a T> + ExactSizeIterator<Item = &'a T>
    {
        fn clone_box(&self) -> Box<NoDrop<dyn IterTrait<'a, T, Item = &'a T> + 'a>>;
    }

    struct PrivateIter<'a, T: 'a, P: 'a> {
        inner: slice::Iter<'a, (T, P)>,
        last: option::IntoIter<&'a T>,
    }

    impl<'a, T, P> TrivialDrop for PrivateIter<'a, T, P>
    where
        slice::Iter<'a, (T, P)>: TrivialDrop,
        option::IntoIter<&'a T>: TrivialDrop,
    {
    }

    #[cfg(any(feature = "full", feature = "derive"))]
    pub(crate) fn empty_punctuated_iter<'a, T>() -> Iter<'a, T> {
        Iter {
            inner: Box::new(NoDrop::new(iter::empty())),
        }
    }

    // No Clone bound on T.
    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Self {
            Iter {
                inner: self.inner.clone_box(),
            }
        }
    }

    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.inner.next_back()
        }
    }

    impl<'a, T> ExactSizeIterator for Iter<'a, T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    impl<'a, T, P> Iterator for PrivateIter<'a, T, P> {
        type Item = &'a T;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner
                .next()
                .map(|pair| &pair.0)
                .or_else(|| self.last.next())
        }
    }

    impl<'a, T, P> DoubleEndedIterator for PrivateIter<'a, T, P> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.last
                .next()
                .or_else(|| self.inner.next_back().map(|pair| &pair.0))
        }
    }

    impl<'a, T, P> ExactSizeIterator for PrivateIter<'a, T, P> {
        fn len(&self) -> usize {
            self.inner.len() + self.last.len()
        }
    }

    // No Clone bound on T or P.
    impl<'a, T, P> Clone for PrivateIter<'a, T, P> {
        fn clone(&self) -> Self {
            PrivateIter {
                inner: self.inner.clone(),
                last: self.last.clone(),
            }
        }
    }

    impl<'a, T, I> IterTrait<'a, T> for I
    where
        T: 'a,
        I: DoubleEndedIterator<Item = &'a T>
            + ExactSizeIterator<Item = &'a T>
            + Clone
            + TrivialDrop
            + 'a,
    {
        fn clone_box(&self) -> Box<NoDrop<dyn IterTrait<'a, T, Item = &'a T> + 'a>> {
            Box::new(NoDrop::new(self.clone()))
        }
    }

    /// An iterator over mutably borrowed values of type `&mut T`.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub struct IterMut<'a, T: 'a> {
        inner: Box<NoDrop<dyn IterMutTrait<'a, T, Item = &'a mut T> + 'a>>,
    }

    trait IterMutTrait<'a, T: 'a>:
        DoubleEndedIterator<Item = &'a mut T> + ExactSizeIterator<Item = &'a mut T>
    {
    }

    struct PrivateIterMut<'a, T: 'a, P: 'a> {
        inner: slice::IterMut<'a, (T, P)>,
        last: option::IntoIter<&'a mut T>,
    }

    impl<'a, T, P> TrivialDrop for PrivateIterMut<'a, T, P>
    where
        slice::IterMut<'a, (T, P)>: TrivialDrop,
        option::IntoIter<&'a mut T>: TrivialDrop,
    {
    }

    #[cfg(any(feature = "full", feature = "derive"))]
    pub(crate) fn empty_punctuated_iter_mut<'a, T>() -> IterMut<'a, T> {
        IterMut {
            inner: Box::new(NoDrop::new(iter::empty())),
        }
    }

    impl<'a, T> Iterator for IterMut<'a, T> {
        type Item = &'a mut T;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len(), Some(self.len()))
        }
    }

    impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.inner.next_back()
        }
    }

    impl<'a, T> ExactSizeIterator for IterMut<'a, T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    impl<'a, T, P> Iterator for PrivateIterMut<'a, T, P> {
        type Item = &'a mut T;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner
                .next()
                .map(|pair| &mut pair.0)
                .or_else(|| self.last.next())
        }
    }

    impl<'a, T, P> DoubleEndedIterator for PrivateIterMut<'a, T, P> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.last
                .next()
                .or_else(|| self.inner.next_back().map(|pair| &mut pair.0))
        }
    }

    impl<'a, T, P> ExactSizeIterator for PrivateIterMut<'a, T, P> {
        fn len(&self) -> usize {
            self.inner.len() + self.last.len()
        }
    }

    impl<'a, T, I> IterMutTrait<'a, T> for I
    where
        T: 'a,
        I: DoubleEndedIterator<Item = &'a mut T> + ExactSizeIterator<Item = &'a mut T> + 'a,
    {
    }

    /// A single syntax tree node of type `T` followed by its trailing punctuation
    /// of type `P` if any.
    ///
    /// Refer to the [module documentation] for details about punctuated sequences.
    ///
    /// [module documentation]: self
    pub enum Pair<T, P> {
        Punctuated(T, P),
        End(T),
    }

    impl<T, P> Pair<T, P> {
        /// Extracts the syntax tree node from this punctuated pair, discarding the
        /// following punctuation.
        pub fn into_value(self) -> T {
            match self {
                Pair::Punctuated(t, _) | Pair::End(t) => t,
            }
        }

        /// Borrows the syntax tree node from this punctuated pair.
        pub fn value(&self) -> &T {
            match self {
                Pair::Punctuated(t, _) | Pair::End(t) => t,
            }
        }

        /// Mutably borrows the syntax tree node from this punctuated pair.
        pub fn value_mut(&mut self) -> &mut T {
            match self {
                Pair::Punctuated(t, _) | Pair::End(t) => t,
            }
        }

        /// Borrows the punctuation from this punctuated pair, unless this pair is
        /// the final one and there is no trailing punctuation.
        pub fn punct(&self) -> Option<&P> {
            match self {
                Pair::Punctuated(_, p) => Some(p),
                Pair::End(_) => None,
            }
        }

        /// Mutably borrows the punctuation from this punctuated pair, unless the
        /// pair is the final one and there is no trailing punctuation.
        ///
        /// # Example
        ///
        /// ```
        /// # use proc_macro2::Span;
        /// # use syn::punctuated::Punctuated;
        /// # use syn::{parse_quote, Token, TypeParamBound};
        /// #
        /// # let mut punctuated = Punctuated::<TypeParamBound, Token![+]>::new();
        /// # let span = Span::call_site();
        /// #
        /// punctuated.insert(0, parse_quote!('lifetime));
        /// if let Some(punct) = punctuated.pairs_mut().next().unwrap().punct_mut() {
        ///     punct.span = span;
        /// }
        /// ```
        pub fn punct_mut(&mut self) -> Option<&mut P> {
            match self {
                Pair::Punctuated(_, p) => Some(p),
                Pair::End(_) => None,
            }
        }

        /// Creates a punctuated pair out of a syntax tree node and an optional
        /// following punctuation.
        pub fn new(t: T, p: Option<P>) -> Self {
            match p {
                Some(p) => Pair::Punctuated(t, p),
                None => Pair::End(t),
            }
        }

        /// Produces this punctuated pair as a tuple of syntax tree node and
        /// optional following punctuation.
        pub fn into_tuple(self) -> (T, Option<P>) {
            match self {
                Pair::Punctuated(t, p) => (t, Some(p)),
                Pair::End(t) => (t, None),
            }
        }
    }

    #[cfg(feature = "clone-impls")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "clone-impls")))]
    impl<T, P> Clone for Pair<T, P>
    where
        T: Clone,
        P: Clone,
    {
        fn clone(&self) -> Self {
            match self {
                Pair::Punctuated(t, p) => Pair::Punctuated(t.clone(), p.clone()),
                Pair::End(t) => Pair::End(t.clone()),
            }
        }
    }

    impl<T, P> Index<usize> for Punctuated<T, P> {
        type Output = T;

        fn index(&self, index: usize) -> &Self::Output {
            if index == self.len() - 1 {
                match &self.last {
                    Some(t) => t,
                    None => &self.inner[index].0,
                }
            } else {
                &self.inner[index].0
            }
        }
    }

    impl<T, P> IndexMut<usize> for Punctuated<T, P> {
        fn index_mut(&mut self, index: usize) -> &mut Self::Output {
            if index == self.len() - 1 {
                match &mut self.last {
                    Some(t) => t,
                    None => &mut self.inner[index].0,
                }
            } else {
                &mut self.inner[index].0
            }
        }
    }

    #[cfg(feature = "printing")]
    mod printing {
        use super::*;
        use proc_macro2::TokenStream;
        use quote::{ToTokens, TokenStreamExt};

        #[cfg_attr(doc_cfg, doc(cfg(feature = "printing")))]
        impl<T, P> ToTokens for Punctuated<T, P>
        where
            T: ToTokens,
            P: ToTokens,
        {
            fn to_tokens(&self, tokens: &mut TokenStream) {
                tokens.append_all(self.pairs());
            }
        }

        #[cfg_attr(doc_cfg, doc(cfg(feature = "printing")))]
        impl<T, P> ToTokens for Pair<T, P>
        where
            T: ToTokens,
            P: ToTokens,
        {
            fn to_tokens(&self, tokens: &mut TokenStream) {
                match self {
                    Pair::Punctuated(a, b) => {
                        a.to_tokens(tokens);
                        b.to_tokens(tokens);
                    }
                    Pair::End(a) => a.to_tokens(tokens),
                }
            }
        }
    }