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
//! Implementation of [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
//!
//! This library uses Rust’s type system to maintain
//! [well-formedness](https://simonsapin.github.io/wtf-8/#well-formed),
//! like the `String` and `&str` types do for UTF-8.
//!
//! Since [WTF-8 must not be used
//! for interchange](https://simonsapin.github.io/wtf-8/#intended-audience),
//! this library deliberately does not provide access to the underlying bytes
//! of WTF-8 strings,
//! nor can it decode WTF-8 from arbitrary bytes.
//! WTF-8 strings can be obtained from UTF-8, UTF-16, or code points.

// this module is imported from @SimonSapin's repo and has tons of dead code on
// unix (it's mostly used on windows), so don't worry about dead code here.
#![allow(dead_code)]

#[cfg(test)]
mod tests;

use core::str::next_code_point;

use crate::borrow::Cow;
use crate::char;
use crate::fmt;
use crate::hash::{Hash, Hasher};
use crate::iter::FromIterator;
use crate::mem;
use crate::ops;
use crate::rc::Rc;
use crate::slice;
use crate::str;
use crate::sync::Arc;
use crate::sys_common::AsInner;

const UTF8_REPLACEMENT_CHARACTER: &str = "\u{FFFD}";

/// A Unicode code point: from U+0000 to U+10FFFF.
///
/// Compares with the `char` type,
/// which represents a Unicode scalar value:
/// a code point that is not a surrogate (U+D800 to U+DFFF).
#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy)]
pub struct CodePoint {
    value: u32,
}

/// Format the code point as `U+` followed by four to six hexadecimal digits.
/// Example: `U+1F4A9`
impl fmt::Debug for CodePoint {
    #[inline]
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(formatter, "U+{:04X}", self.value)
    }
}

impl CodePoint {
    /// Unsafely creates a new `CodePoint` without checking the value.
    ///
    /// Only use when `value` is known to be less than or equal to 0x10FFFF.
    #[inline]
    pub unsafe fn from_u32_unchecked(value: u32) -> CodePoint {
        CodePoint { value }
    }

    /// Creates a new `CodePoint` if the value is a valid code point.
    ///
    /// Returns `None` if `value` is above 0x10FFFF.
    #[inline]
    pub fn from_u32(value: u32) -> Option<CodePoint> {
        match value {
            0..=0x10FFFF => Some(CodePoint { value }),
            _ => None,
        }
    }

    /// Creates a new `CodePoint` from a `char`.
    ///
    /// Since all Unicode scalar values are code points, this always succeeds.
    #[inline]
    pub fn from_char(value: char) -> CodePoint {
        CodePoint { value: value as u32 }
    }

    /// Returns the numeric value of the code point.
    #[inline]
    pub fn to_u32(&self) -> u32 {
        self.value
    }

    /// Optionally returns a Unicode scalar value for the code point.
    ///
    /// Returns `None` if the code point is a surrogate (from U+D800 to U+DFFF).
    #[inline]
    pub fn to_char(&self) -> Option<char> {
        match self.value {
            0xD800..=0xDFFF => None,
            _ => Some(unsafe { char::from_u32_unchecked(self.value) }),
        }
    }

    /// Returns a Unicode scalar value for the code point.
    ///
    /// Returns `'\u{FFFD}'` (the replacement character “�”)
    /// if the code point is a surrogate (from U+D800 to U+DFFF).
    #[inline]
    pub fn to_char_lossy(&self) -> char {
        self.to_char().unwrap_or('\u{FFFD}')
    }
}

/// An owned, growable string of well-formed WTF-8 data.
///
/// Similar to `String`, but can additionally contain surrogate code points
/// if they’re not in a surrogate pair.
#[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
pub struct Wtf8Buf {
    bytes: Vec<u8>,
}

impl ops::Deref for Wtf8Buf {
    type Target = Wtf8;

    fn deref(&self) -> &Wtf8 {
        self.as_slice()
    }
}

impl ops::DerefMut for Wtf8Buf {
    fn deref_mut(&mut self) -> &mut Wtf8 {
        self.as_mut_slice()
    }
}

/// Format the string with double quotes,
/// and surrogates as `\u` followed by four hexadecimal digits.
/// Example: `"a\u{D800}"` for a string with code points [U+0061, U+D800]
impl fmt::Debug for Wtf8Buf {
    #[inline]
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, formatter)
    }
}

impl Wtf8Buf {
    /// Creates a new, empty WTF-8 string.
    #[inline]
    pub fn new() -> Wtf8Buf {
        Wtf8Buf { bytes: Vec::new() }
    }

    /// Creates a new, empty WTF-8 string with pre-allocated capacity for `capacity` bytes.
    #[inline]
    pub fn with_capacity(capacity: usize) -> Wtf8Buf {
        Wtf8Buf { bytes: Vec::with_capacity(capacity) }
    }

    /// Creates a WTF-8 string from a UTF-8 `String`.
    ///
    /// This takes ownership of the `String` and does not copy.
    ///
    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
    #[inline]
    pub fn from_string(string: String) -> Wtf8Buf {
        Wtf8Buf { bytes: string.into_bytes() }
    }

    /// Creates a WTF-8 string from a UTF-8 `&str` slice.
    ///
    /// This copies the content of the slice.
    ///
    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
    #[inline]
    pub fn from_str(str: &str) -> Wtf8Buf {
        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()) }
    }

    pub fn clear(&mut self) {
        self.bytes.clear()
    }

    /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
    ///
    /// This is lossless: calling `.encode_wide()` on the resulting string
    /// will always return the original code units.
    pub fn from_wide(v: &[u16]) -> Wtf8Buf {
        let mut string = Wtf8Buf::with_capacity(v.len());
        for item in char::decode_utf16(v.iter().cloned()) {
            match item {
                Ok(ch) => string.push_char(ch),
                Err(surrogate) => {
                    let surrogate = surrogate.unpaired_surrogate();
                    // Surrogates are known to be in the code point range.
                    let code_point = unsafe { CodePoint::from_u32_unchecked(surrogate as u32) };
                    // Skip the WTF-8 concatenation check,
                    // surrogate pairs are already decoded by decode_utf16
                    string.push_code_point_unchecked(code_point)
                }
            }
        }
        string
    }

    /// Copied from String::push
    /// This does **not** include the WTF-8 concatenation check.
    fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
        let mut bytes = [0; 4];
        let bytes = char::encode_utf8_raw(code_point.value, &mut bytes);
        self.bytes.extend_from_slice(bytes)
    }

    #[inline]
    pub fn as_slice(&self) -> &Wtf8 {
        unsafe { Wtf8::from_bytes_unchecked(&self.bytes) }
    }

    #[inline]
    pub fn as_mut_slice(&mut self) -> &mut Wtf8 {
        unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
    }

    /// Reserves capacity for at least `additional` more bytes to be inserted
    /// in the given `Wtf8Buf`.
    /// The collection may reserve more space to avoid frequent reallocations.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows `usize`.
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        self.bytes.reserve(additional)
    }

    #[inline]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.bytes.reserve_exact(additional)
    }

    #[inline]
    pub fn shrink_to_fit(&mut self) {
        self.bytes.shrink_to_fit()
    }

    #[inline]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.bytes.shrink_to(min_capacity)
    }

    /// Returns the number of bytes that this string buffer can hold without reallocating.
    #[inline]
    pub fn capacity(&self) -> usize {
        self.bytes.capacity()
    }

    /// Append a UTF-8 slice at the end of the string.
    #[inline]
    pub fn push_str(&mut self, other: &str) {
        self.bytes.extend_from_slice(other.as_bytes())
    }

    /// Append a WTF-8 slice at the end of the string.
    ///
    /// This replaces newly paired surrogates at the boundary
    /// with a supplementary code point,
    /// like concatenating ill-formed UTF-16 strings effectively would.
    #[inline]
    pub fn push_wtf8(&mut self, other: &Wtf8) {
        match ((&*self).final_lead_surrogate(), other.initial_trail_surrogate()) {
            // Replace newly paired surrogates by a supplementary code point.
            (Some(lead), Some(trail)) => {
                let len_without_lead_surrogate = self.len() - 3;
                self.bytes.truncate(len_without_lead_surrogate);
                let other_without_trail_surrogate = &other.bytes[3..];
                // 4 bytes for the supplementary code point
                self.bytes.reserve(4 + other_without_trail_surrogate.len());
                self.push_char(decode_surrogate_pair(lead, trail));
                self.bytes.extend_from_slice(other_without_trail_surrogate);
            }
            _ => self.bytes.extend_from_slice(&other.bytes),
        }
    }

    /// Append a Unicode scalar value at the end of the string.
    #[inline]
    pub fn push_char(&mut self, c: char) {
        self.push_code_point_unchecked(CodePoint::from_char(c))
    }

    /// Append a code point at the end of the string.
    ///
    /// This replaces newly paired surrogates at the boundary
    /// with a supplementary code point,
    /// like concatenating ill-formed UTF-16 strings effectively would.
    #[inline]
    pub fn push(&mut self, code_point: CodePoint) {
        if let trail @ 0xDC00..=0xDFFF = code_point.to_u32() {
            if let Some(lead) = (&*self).final_lead_surrogate() {
                let len_without_lead_surrogate = self.len() - 3;
                self.bytes.truncate(len_without_lead_surrogate);
                self.push_char(decode_surrogate_pair(lead, trail as u16));
                return;
            }
        }

        // No newly paired surrogates at the boundary.
        self.push_code_point_unchecked(code_point)
    }

    /// Shortens a string to the specified length.
    ///
    /// # Panics
    ///
    /// Panics if `new_len` > current length,
    /// or if `new_len` is not a code point boundary.
    #[inline]
    pub fn truncate(&mut self, new_len: usize) {
        assert!(is_code_point_boundary(self, new_len));
        self.bytes.truncate(new_len)
    }

    /// Consumes the WTF-8 string and tries to convert it to UTF-8.
    ///
    /// This does not copy the data.
    ///
    /// If the contents are not well-formed UTF-8
    /// (that is, if the string contains surrogates),
    /// the original WTF-8 string is returned instead.
    pub fn into_string(self) -> Result<String, Wtf8Buf> {
        match self.next_surrogate(0) {
            None => Ok(unsafe { String::from_utf8_unchecked(self.bytes) }),
            Some(_) => Err(self),
        }
    }

    /// Consumes the WTF-8 string and converts it lossily to UTF-8.
    ///
    /// This does not copy the data (but may overwrite parts of it in place).
    ///
    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
    pub fn into_string_lossy(mut self) -> String {
        let mut pos = 0;
        loop {
            match self.next_surrogate(pos) {
                Some((surrogate_pos, _)) => {
                    pos = surrogate_pos + 3;
                    self.bytes[surrogate_pos..pos]
                        .copy_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
                }
                None => return unsafe { String::from_utf8_unchecked(self.bytes) },
            }
        }
    }

    /// Converts this `Wtf8Buf` into a boxed `Wtf8`.
    #[inline]
    pub fn into_box(self) -> Box<Wtf8> {
        unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
    }

    /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
    pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
        let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
        Wtf8Buf { bytes: bytes.into_vec() }
    }
}

/// Creates a new WTF-8 string from an iterator of code points.
///
/// This replaces surrogate code point pairs with supplementary code points,
/// like concatenating ill-formed UTF-16 strings effectively would.
impl FromIterator<CodePoint> for Wtf8Buf {
    fn from_iter<T: IntoIterator<Item = CodePoint>>(iter: T) -> Wtf8Buf {
        let mut string = Wtf8Buf::new();
        string.extend(iter);
        string
    }
}

/// Append code points from an iterator to the string.
///
/// This replaces surrogate code point pairs with supplementary code points,
/// like concatenating ill-formed UTF-16 strings effectively would.
impl Extend<CodePoint> for Wtf8Buf {
    fn extend<T: IntoIterator<Item = CodePoint>>(&mut self, iter: T) {
        let iterator = iter.into_iter();
        let (low, _high) = iterator.size_hint();
        // Lower bound of one byte per code point (ASCII only)
        self.bytes.reserve(low);
        iterator.for_each(move |code_point| self.push(code_point));
    }

    #[inline]
    fn extend_one(&mut self, code_point: CodePoint) {
        self.push(code_point);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        // Lower bound of one byte per code point (ASCII only)
        self.bytes.reserve(additional);
    }
}

/// A borrowed slice of well-formed WTF-8 data.
///
/// Similar to `&str`, but can additionally contain surrogate code points
/// if they’re not in a surrogate pair.
#[derive(Eq, Ord, PartialEq, PartialOrd)]
pub struct Wtf8 {
    bytes: [u8],
}

impl AsInner<[u8]> for Wtf8 {
    fn as_inner(&self) -> &[u8] {
        &self.bytes
    }
}

/// Format the slice with double quotes,
/// and surrogates as `\u` followed by four hexadecimal digits.
/// Example: `"a\u{D800}"` for a slice with code points [U+0061, U+D800]
impl fmt::Debug for Wtf8 {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        fn write_str_escaped(f: &mut fmt::Formatter<'_>, s: &str) -> fmt::Result {
            use crate::fmt::Write;
            for c in s.chars().flat_map(|c| c.escape_debug()) {
                f.write_char(c)?
            }
            Ok(())
        }

        formatter.write_str("\"")?;
        let mut pos = 0;
        while let Some((surrogate_pos, surrogate)) = self.next_surrogate(pos) {
            write_str_escaped(formatter, unsafe {
                str::from_utf8_unchecked(&self.bytes[pos..surrogate_pos])
            })?;
            write!(formatter, "\\u{{{:x}}}", surrogate)?;
            pos = surrogate_pos + 3;
        }
        write_str_escaped(formatter, unsafe { str::from_utf8_unchecked(&self.bytes[pos..]) })?;
        formatter.write_str("\"")
    }
}

impl fmt::Display for Wtf8 {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        let wtf8_bytes = &self.bytes;
        let mut pos = 0;
        loop {
            match self.next_surrogate(pos) {
                Some((surrogate_pos, _)) => {
                    formatter.write_str(unsafe {
                        str::from_utf8_unchecked(&wtf8_bytes[pos..surrogate_pos])
                    })?;
                    formatter.write_str(UTF8_REPLACEMENT_CHARACTER)?;
                    pos = surrogate_pos + 3;
                }
                None => {
                    let s = unsafe { str::from_utf8_unchecked(&wtf8_bytes[pos..]) };
                    if pos == 0 { return s.fmt(formatter) } else { return formatter.write_str(s) }
                }
            }
        }
    }
}

impl Wtf8 {
    /// Creates a WTF-8 slice from a UTF-8 `&str` slice.
    ///
    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
    #[inline]
    pub fn from_str(value: &str) -> &Wtf8 {
        unsafe { Wtf8::from_bytes_unchecked(value.as_bytes()) }
    }

    /// Creates a WTF-8 slice from a WTF-8 byte slice.
    ///
    /// Since the byte slice is not checked for valid WTF-8, this functions is
    /// marked unsafe.
    #[inline]
    unsafe fn from_bytes_unchecked(value: &[u8]) -> &Wtf8 {
        mem::transmute(value)
    }

    /// Creates a mutable WTF-8 slice from a mutable WTF-8 byte slice.
    ///
    /// Since the byte slice is not checked for valid WTF-8, this functions is
    /// marked unsafe.
    #[inline]
    unsafe fn from_mut_bytes_unchecked(value: &mut [u8]) -> &mut Wtf8 {
        mem::transmute(value)
    }

    /// Returns the length, in WTF-8 bytes.
    #[inline]
    pub fn len(&self) -> usize {
        self.bytes.len()
    }

    #[inline]
    pub fn is_empty(&self) -> bool {
        self.bytes.is_empty()
    }

    /// Returns the code point at `position` if it is in the ASCII range,
    /// or `b'\xFF' otherwise.
    ///
    /// # Panics
    ///
    /// Panics if `position` is beyond the end of the string.
    #[inline]
    pub fn ascii_byte_at(&self, position: usize) -> u8 {
        match self.bytes[position] {
            ascii_byte @ 0x00..=0x7F => ascii_byte,
            _ => 0xFF,
        }
    }

    /// Returns an iterator for the string’s code points.
    #[inline]
    pub fn code_points(&self) -> Wtf8CodePoints<'_> {
        Wtf8CodePoints { bytes: self.bytes.iter() }
    }

    /// Tries to convert the string to UTF-8 and return a `&str` slice.
    ///
    /// Returns `None` if the string contains surrogates.
    ///
    /// This does not copy the data.
    #[inline]
    pub fn as_str(&self) -> Option<&str> {
        // Well-formed WTF-8 is also well-formed UTF-8
        // if and only if it contains no surrogate.
        match self.next_surrogate(0) {
            None => Some(unsafe { str::from_utf8_unchecked(&self.bytes) }),
            Some(_) => None,
        }
    }

    /// Lossily converts the string to UTF-8.
    /// Returns a UTF-8 `&str` slice if the contents are well-formed in UTF-8.
    ///
    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”).
    ///
    /// This only copies the data if necessary (if it contains any surrogate).
    pub fn to_string_lossy(&self) -> Cow<'_, str> {
        let surrogate_pos = match self.next_surrogate(0) {
            None => return Cow::Borrowed(unsafe { str::from_utf8_unchecked(&self.bytes) }),
            Some((pos, _)) => pos,
        };
        let wtf8_bytes = &self.bytes;
        let mut utf8_bytes = Vec::with_capacity(self.len());
        utf8_bytes.extend_from_slice(&wtf8_bytes[..surrogate_pos]);
        utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
        let mut pos = surrogate_pos + 3;
        loop {
            match self.next_surrogate(pos) {
                Some((surrogate_pos, _)) => {
                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..surrogate_pos]);
                    utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
                    pos = surrogate_pos + 3;
                }
                None => {
                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..]);
                    return Cow::Owned(unsafe { String::from_utf8_unchecked(utf8_bytes) });
                }
            }
        }
    }

    /// Converts the WTF-8 string to potentially ill-formed UTF-16
    /// and return an iterator of 16-bit code units.
    ///
    /// This is lossless:
    /// calling `Wtf8Buf::from_ill_formed_utf16` on the resulting code units
    /// would always return the original WTF-8 string.
    #[inline]
    pub fn encode_wide(&self) -> EncodeWide<'_> {
        EncodeWide { code_points: self.code_points(), extra: 0 }
    }

    #[inline]
    fn next_surrogate(&self, mut pos: usize) -> Option<(usize, u16)> {
        let mut iter = self.bytes[pos..].iter();
        loop {
            let b = *iter.next()?;
            if b < 0x80 {
                pos += 1;
            } else if b < 0xE0 {
                iter.next();
                pos += 2;
            } else if b == 0xED {
                match (iter.next(), iter.next()) {
                    (Some(&b2), Some(&b3)) if b2 >= 0xA0 => {
                        return Some((pos, decode_surrogate(b2, b3)));
                    }
                    _ => pos += 3,
                }
            } else if b < 0xF0 {
                iter.next();
                iter.next();
                pos += 3;
            } else {
                iter.next();
                iter.next();
                iter.next();
                pos += 4;
            }
        }
    }

    #[inline]
    fn final_lead_surrogate(&self) -> Option<u16> {
        match self.bytes {
            [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
            _ => None,
        }
    }

    #[inline]
    fn initial_trail_surrogate(&self) -> Option<u16> {
        match self.bytes {
            [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(b2, b3)),
            _ => None,
        }
    }

    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
        self.bytes.clone_into(&mut buf.bytes)
    }

    /// Boxes this `Wtf8`.
    #[inline]
    pub fn into_box(&self) -> Box<Wtf8> {
        let boxed: Box<[u8]> = self.bytes.into();
        unsafe { mem::transmute(boxed) }
    }

    /// Creates a boxed, empty `Wtf8`.
    pub fn empty_box() -> Box<Wtf8> {
        let boxed: Box<[u8]> = Default::default();
        unsafe { mem::transmute(boxed) }
    }

    #[inline]
    pub fn into_arc(&self) -> Arc<Wtf8> {
        let arc: Arc<[u8]> = Arc::from(&self.bytes);
        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Wtf8) }
    }

    #[inline]
    pub fn into_rc(&self) -> Rc<Wtf8> {
        let rc: Rc<[u8]> = Rc::from(&self.bytes);
        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Wtf8) }
    }

    #[inline]
    pub fn make_ascii_lowercase(&mut self) {
        self.bytes.make_ascii_lowercase()
    }

    #[inline]
    pub fn make_ascii_uppercase(&mut self) {
        self.bytes.make_ascii_uppercase()
    }

    #[inline]
    pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase() }
    }

    #[inline]
    pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase() }
    }

    #[inline]
    pub fn is_ascii(&self) -> bool {
        self.bytes.is_ascii()
    }

    #[inline]
    pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
        self.bytes.eq_ignore_ascii_case(&other.bytes)
    }
}

/// Returns a slice of the given string for the byte range [`begin`..`end`).
///
/// # Panics
///
/// Panics when `begin` and `end` do not point to code point boundaries,
/// or point beyond the end of the string.
impl ops::Index<ops::Range<usize>> for Wtf8 {
    type Output = Wtf8;

    #[inline]
    fn index(&self, range: ops::Range<usize>) -> &Wtf8 {
        // is_code_point_boundary checks that the index is in [0, .len()]
        if range.start <= range.end
            && is_code_point_boundary(self, range.start)
            && is_code_point_boundary(self, range.end)
        {
            unsafe { slice_unchecked(self, range.start, range.end) }
        } else {
            slice_error_fail(self, range.start, range.end)
        }
    }
}

/// Returns a slice of the given string from byte `begin` to its end.
///
/// # Panics
///
/// Panics when `begin` is not at a code point boundary,
/// or is beyond the end of the string.
impl ops::Index<ops::RangeFrom<usize>> for Wtf8 {
    type Output = Wtf8;

    #[inline]
    fn index(&self, range: ops::RangeFrom<usize>) -> &Wtf8 {
        // is_code_point_boundary checks that the index is in [0, .len()]
        if is_code_point_boundary(self, range.start) {
            unsafe { slice_unchecked(self, range.start, self.len()) }
        } else {
            slice_error_fail(self, range.start, self.len())
        }
    }
}

/// Returns a slice of the given string from its beginning to byte `end`.
///
/// # Panics
///
/// Panics when `end` is not at a code point boundary,
/// or is beyond the end of the string.
impl ops::Index<ops::RangeTo<usize>> for Wtf8 {
    type Output = Wtf8;

    #[inline]
    fn index(&self, range: ops::RangeTo<usize>) -> &Wtf8 {
        // is_code_point_boundary checks that the index is in [0, .len()]
        if is_code_point_boundary(self, range.end) {
            unsafe { slice_unchecked(self, 0, range.end) }
        } else {
            slice_error_fail(self, 0, range.end)
        }
    }
}

impl ops::Index<ops::RangeFull> for Wtf8 {
    type Output = Wtf8;

    #[inline]
    fn index(&self, _range: ops::RangeFull) -> &Wtf8 {
        self
    }
}

#[inline]
fn decode_surrogate(second_byte: u8, third_byte: u8) -> u16 {
    // The first byte is assumed to be 0xED
    0xD800 | (second_byte as u16 & 0x3F) << 6 | third_byte as u16 & 0x3F
}

#[inline]
fn decode_surrogate_pair(lead: u16, trail: u16) -> char {
    let code_point = 0x10000 + ((((lead - 0xD800) as u32) << 10) | (trail - 0xDC00) as u32);
    unsafe { char::from_u32_unchecked(code_point) }
}

/// Copied from core::str::StrPrelude::is_char_boundary
#[inline]
pub fn is_code_point_boundary(slice: &Wtf8, index: usize) -> bool {
    if index == slice.len() {
        return true;
    }
    match slice.bytes.get(index) {
        None => false,
        Some(&b) => b < 128 || b >= 192,
    }
}

/// Copied from core::str::raw::slice_unchecked
#[inline]
pub unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
    // memory layout of an &[u8] and &Wtf8 are the same
    Wtf8::from_bytes_unchecked(slice::from_raw_parts(s.bytes.as_ptr().add(begin), end - begin))
}

/// Copied from core::str::raw::slice_error_fail
#[inline(never)]
pub fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
    assert!(begin <= end);
    panic!("index {} and/or {} in `{:?}` do not lie on character boundary", begin, end, s);
}

/// Iterator for the code points of a WTF-8 string.
///
/// Created with the method `.code_points()`.
#[derive(Clone)]
pub struct Wtf8CodePoints<'a> {
    bytes: slice::Iter<'a, u8>,
}

impl<'a> Iterator for Wtf8CodePoints<'a> {
    type Item = CodePoint;

    #[inline]
    fn next(&mut self) -> Option<CodePoint> {
        next_code_point(&mut self.bytes).map(|c| CodePoint { value: c })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.bytes.len();
        (len.saturating_add(3) / 4, Some(len))
    }
}

/// Generates a wide character sequence for potentially ill-formed UTF-16.
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct EncodeWide<'a> {
    code_points: Wtf8CodePoints<'a>,
    extra: u16,
}

// Copied from libunicode/u_str.rs
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for EncodeWide<'a> {
    type Item = u16;

    #[inline]
    fn next(&mut self) -> Option<u16> {
        if self.extra != 0 {
            let tmp = self.extra;
            self.extra = 0;
            return Some(tmp);
        }

        let mut buf = [0; 2];
        self.code_points.next().map(|code_point| {
            let n = char::encode_utf16_raw(code_point.value, &mut buf).len();
            if n == 2 {
                self.extra = buf[1];
            }
            buf[0]
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (low, high) = self.code_points.size_hint();
        // every code point gets either one u16 or two u16,
        // so this iterator is between 1 or 2 times as
        // long as the underlying iterator.
        (low, high.and_then(|n| n.checked_mul(2)))
    }
}

impl Hash for CodePoint {
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.value.hash(state)
    }
}

impl Hash for Wtf8Buf {
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write(&self.bytes);
        0xfeu8.hash(state)
    }
}

impl Hash for Wtf8 {
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write(&self.bytes);
        0xfeu8.hash(state)
    }
}