1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
//! 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>`]: struct.Punctuated.html
//!
//! ```text
//! a_function_call(arg1, arg2, arg3);
//!                 ~~~~^ ~~~~^ ~~~~
//! ```

#[cfg(feature = "extra-traits")]
use std::fmt::{self, Debug};
#[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;

#[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
#[cfg_attr(feature = "extra-traits", derive(Eq, PartialEq, Hash))]
#[cfg_attr(feature = "clone-impls", derive(Clone))]
pub struct Punctuated<T, P> {
    inner: Vec<(T, P)>,
    last: Option<Box<T>>,
}

impl<T, P> Punctuated<T, P> {
    /// Creates an empty punctuated sequence.
    pub fn new() -> Punctuated<T, P> {
        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(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(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());
        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());
        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, d)| Pair::Punctuated(t, d))
        }
    }

    /// 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());

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

    /// 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 if Syn is built with the `"parsing"`
    /// feature.*
    #[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 if Syn is built with the `"parsing"`
    /// feature.*
    #[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 if Syn is built with the `"parsing"`
    /// feature.*
    #[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 if Syn is built with the `"parsing"`
    /// feature.*
    #[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 = "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());
        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
#[derive(Clone)]
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()
    }
}

/// An iterator over owned values of type `T`.
///
/// Refer to the [module documentation] for details about punctuated sequences.
///
/// [module documentation]: self
#[derive(Clone)]
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()
    }
}

/// 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<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<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>,
}

#[cfg(any(feature = "full", feature = "derive"))]
pub(crate) fn empty_punctuated_iter<'a, T>() -> Iter<'a, T> {
    Iter {
        inner: Box::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: 'a, I: 'a> IterTrait<'a, T> for I
where
    I: DoubleEndedIterator<Item = &'a T> + ExactSizeIterator<Item = &'a T> + Clone,
{
    fn clone_box(&self) -> Box<dyn IterTrait<'a, T, Item = &'a T> + 'a> {
        Box::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<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>,
}

#[cfg(any(feature = "full", feature = "derive"))]
pub(crate) fn empty_punctuated_iter_mut<'a, T>() -> IterMut<'a, T> {
    IterMut {
        inner: Box::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: 'a, I: 'a> IterMutTrait<'a, T> for I where
    I: DoubleEndedIterator<Item = &'a mut T> + ExactSizeIterator<Item = &'a mut T>
{
}

/// 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
#[cfg_attr(feature = "clone-impls", derive(Clone))]
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(_, d) => Some(d),
            Pair::End(_) => None,
        }
    }

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

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};

    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())
        }
    }

    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),
            }
        }
    }
}