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
//! Trait implementations for `str`.

use crate::cmp::Ordering;
use crate::ops;
use crate::panic::debug_assert_nounwind;
use crate::ptr;
use crate::slice::SliceIndex;

use super::ParseBoolError;

/// Implements ordering of strings.
///
/// Strings are ordered  [lexicographically](Ord#lexicographical-comparison) by their byte values. This orders Unicode code
/// points based on their positions in the code charts. This is not necessarily the same as
/// "alphabetical" order, which varies by language and locale. Sorting strings according to
/// culturally-accepted standards requires locale-specific data that is outside the scope of
/// the `str` type.
#[stable(feature = "rust1", since = "1.0.0")]
impl Ord for str {
    #[inline]
    fn cmp(&self, other: &str) -> Ordering {
        self.as_bytes().cmp(other.as_bytes())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl PartialEq for str {
    #[inline]
    fn eq(&self, other: &str) -> bool {
        self.as_bytes() == other.as_bytes()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Eq for str {}

/// Implements comparison operations on strings.
///
/// Strings are compared [lexicographically](Ord#lexicographical-comparison) by their byte values. This compares Unicode code
/// points based on their positions in the code charts. This is not necessarily the same as
/// "alphabetical" order, which varies by language and locale. Comparing strings according to
/// culturally-accepted standards requires locale-specific data that is outside the scope of
/// the `str` type.
#[stable(feature = "rust1", since = "1.0.0")]
impl PartialOrd for str {
    #[inline]
    fn partial_cmp(&self, other: &str) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ops::Index<I> for str
where
    I: SliceIndex<str>,
{
    type Output = I::Output;

    #[inline]
    fn index(&self, index: I) -> &I::Output {
        index.index(self)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ops::IndexMut<I> for str
where
    I: SliceIndex<str>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut I::Output {
        index.index_mut(self)
    }
}

#[inline(never)]
#[cold]
#[track_caller]
const fn str_index_overflow_fail() -> ! {
    panic!("attempted to index str up to maximum usize");
}

/// Implements substring slicing with syntax `&self[..]` or `&mut self[..]`.
///
/// Returns a slice of the whole string, i.e., returns `&self` or `&mut
/// self`. Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`. Unlike
/// other indexing operations, this can never panic.
///
/// This operation is *O*(1).
///
/// Prior to 1.20.0, these indexing operations were still supported by
/// direct implementation of `Index` and `IndexMut`.
///
/// Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`.
#[stable(feature = "str_checked_slicing", since = "1.20.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::RangeFull {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        Some(slice)
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        Some(slice)
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        slice
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        slice
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        slice
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        slice
    }
}

/// Implements substring slicing with syntax `&self[begin .. end]` or `&mut
/// self[begin .. end]`.
///
/// Returns a slice of the given string from the byte range
/// [`begin`, `end`).
///
/// This operation is *O*(1).
///
/// Prior to 1.20.0, these indexing operations were still supported by
/// direct implementation of `Index` and `IndexMut`.
///
/// # Panics
///
/// Panics if `begin` or `end` does not point to the starting byte offset of
/// a character (as defined by `is_char_boundary`), if `begin > end`, or if
/// `end > len`.
///
/// # Examples
///
/// ```
/// let s = "Löwe 老虎 Léopard";
/// assert_eq!(&s[0 .. 1], "L");
///
/// assert_eq!(&s[1 .. 9], "öwe 老");
///
/// // these will panic:
/// // byte 2 lies within `ö`:
/// // &s[2 ..3];
///
/// // byte 8 lies within `老`
/// // &s[1 .. 8];
///
/// // byte 100 is outside the string
/// // &s[3 .. 100];
/// ```
#[stable(feature = "str_checked_slicing", since = "1.20.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::Range<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            // We also checked char boundaries, so this is valid UTF-8.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary.
            // We know the pointer is unique because we got it from `slice`.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let slice = slice as *const [u8];

        debug_assert_nounwind!(
            // We'd like to check that the bounds are on char boundaries,
            // but there's not really a way to do so without reading
            // behind the pointer, which has aliasing implications.
            // It's also not possible to move this check up to
            // `str::get_unchecked` without adding a special function
            // to `SliceIndex` just for this.
            self.end >= self.start && self.end <= slice.len(),
            "str::get_unchecked requires that the range is within the string slice",
        );

        // SAFETY: the caller guarantees that `self` is in bounds of `slice`
        // which satisfies all the conditions for `add`.
        let ptr = unsafe { slice.as_ptr().add(self.start) };
        let len = self.end - self.start;
        ptr::slice_from_raw_parts(ptr, len) as *const str
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let slice = slice as *mut [u8];

        debug_assert_nounwind!(
            self.end >= self.start && self.end <= slice.len(),
            "str::get_unchecked_mut requires that the range is within the string slice",
        );

        // SAFETY: see comments for `get_unchecked`.
        let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
        let len = self.end - self.start;
        ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, self.end);
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        // is_char_boundary checks that the index is in [0, .len()]
        // cannot reuse `get` as above, because of NLL trouble
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, self.end)
        }
    }
}

/// Implements substring slicing for arbitrary bounds.
///
/// Returns a slice of the given string bounded by the byte indices
/// provided by each bound.
///
/// This operation is *O*(1).
///
/// # Panics
///
/// Panics if `begin` or `end` (if it exists and once adjusted for
/// inclusion/exclusion) does not point to the starting byte offset of
/// a character (as defined by `is_char_boundary`), if `begin > end`, or if
/// `end > len`.
#[stable(feature = "slice_index_str_with_ops_bound_pair", since = "1.73.0")]
unsafe impl SliceIndex<str> for (ops::Bound<usize>, ops::Bound<usize>) {
    type Output = str;

    #[inline]
    fn get(self, slice: &str) -> Option<&str> {
        crate::slice::index::into_range(slice.len(), self)?.get(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut str> {
        crate::slice::index::into_range(slice.len(), self)?.get_mut(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const str {
        let len = (slice as *const [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { crate::slice::index::into_range_unchecked(len, self).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut str {
        let len = (slice as *mut [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { crate::slice::index::into_range_unchecked(len, self).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &str) -> &str {
        crate::slice::index::into_slice_range(slice.len(), self).index(slice)
    }

    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut str {
        crate::slice::index::into_slice_range(slice.len(), self).index_mut(slice)
    }
}

/// Implements substring slicing with syntax `&self[.. end]` or `&mut
/// self[.. end]`.
///
/// Returns a slice of the given string from the byte range \[0, `end`).
/// Equivalent to `&self[0 .. end]` or `&mut self[0 .. end]`.
///
/// This operation is *O*(1).
///
/// Prior to 1.20.0, these indexing operations were still supported by
/// direct implementation of `Index` and `IndexMut`.
///
/// # Panics
///
/// Panics if `end` does not point to the starting byte offset of a
/// character (as defined by `is_char_boundary`), or if `end > len`.
#[stable(feature = "str_checked_slicing", since = "1.20.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (0..self.end).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..self.end).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let end = self.end;
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, 0, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, 0, self.end)
        }
    }
}

/// Implements substring slicing with syntax `&self[begin ..]` or `&mut
/// self[begin ..]`.
///
/// Returns a slice of the given string from the byte range \[`begin`, `len`).
/// Equivalent to `&self[begin .. len]` or `&mut self[begin .. len]`.
///
/// This operation is *O*(1).
///
/// Prior to 1.20.0, these indexing operations were still supported by
/// direct implementation of `Index` and `IndexMut`.
///
/// # Panics
///
/// Panics if `begin` does not point to the starting byte offset of
/// a character (as defined by `is_char_boundary`), or if `begin > len`.
#[stable(feature = "str_checked_slicing", since = "1.20.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::RangeFrom<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let len = (slice as *const [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (self.start..len).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let len = (slice as *mut [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (self.start..len).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, slice.len());
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, slice.len())
        }
    }
}

/// Implements substring slicing with syntax `&self[begin ..= end]` or `&mut
/// self[begin ..= end]`.
///
/// Returns a slice of the given string from the byte range
/// [`begin`, `end`]. Equivalent to `&self [begin .. end + 1]` or `&mut
/// self[begin .. end + 1]`, except if `end` has the maximum value for
/// `usize`.
///
/// This operation is *O*(1).
///
/// # Panics
///
/// Panics if `begin` does not point to the starting byte offset of
/// a character (as defined by `is_char_boundary`), if `end` does not point
/// to the ending byte offset of a character (`end + 1` is either a starting
/// byte offset or equal to `len`), if `begin > end`, or if `end >= len`.
#[stable(feature = "inclusive_range", since = "1.26.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::RangeInclusive<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if *self.end() == usize::MAX { None } else { self.into_slice_range().get(slice) }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if *self.end() == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
        unsafe { self.into_slice_range().get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
        unsafe { self.into_slice_range().get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        if *self.end() == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index(slice)
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if *self.end() == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index_mut(slice)
    }
}

/// Implements substring slicing with syntax `&self[..= end]` or `&mut
/// self[..= end]`.
///
/// Returns a slice of the given string from the byte range \[0, `end`\].
/// Equivalent to `&self [0 .. end + 1]`, except if `end` has the maximum
/// value for `usize`.
///
/// This operation is *O*(1).
///
/// # Panics
///
/// Panics if `end` does not point to the ending byte offset of a character
/// (`end + 1` is either a starting byte offset as defined by
/// `is_char_boundary`, or equal to `len`), or if `end >= len`.
#[stable(feature = "inclusive_range", since = "1.26.0")]
#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
unsafe impl SliceIndex<str> for ops::RangeToInclusive<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        (0..=self.end).get(slice)
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        (0..=self.end).get_mut(slice)
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
        unsafe { (0..=self.end).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..=self.end).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        (0..=self.end).index(slice)
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        (0..=self.end).index_mut(slice)
    }
}

/// Parse a value from a string
///
/// `FromStr`'s [`from_str`] method is often used implicitly, through
/// [`str`]'s [`parse`] method. See [`parse`]'s documentation for examples.
///
/// [`from_str`]: FromStr::from_str
/// [`parse`]: str::parse
///
/// `FromStr` does not have a lifetime parameter, and so you can only parse types
/// that do not contain a lifetime parameter themselves. In other words, you can
/// parse an `i32` with `FromStr`, but not a `&i32`. You can parse a struct that
/// contains an `i32`, but not one that contains an `&i32`.
///
/// # Examples
///
/// Basic implementation of `FromStr` on an example `Point` type:
///
/// ```
/// use std::str::FromStr;
///
/// #[derive(Debug, PartialEq)]
/// struct Point {
///     x: i32,
///     y: i32
/// }
///
/// #[derive(Debug, PartialEq, Eq)]
/// struct ParsePointError;
///
/// impl FromStr for Point {
///     type Err = ParsePointError;
///
///     fn from_str(s: &str) -> Result<Self, Self::Err> {
///         let (x, y) = s
///             .strip_prefix('(')
///             .and_then(|s| s.strip_suffix(')'))
///             .and_then(|s| s.split_once(','))
///             .ok_or(ParsePointError)?;
///
///         let x_fromstr = x.parse::<i32>().map_err(|_| ParsePointError)?;
///         let y_fromstr = y.parse::<i32>().map_err(|_| ParsePointError)?;
///
///         Ok(Point { x: x_fromstr, y: y_fromstr })
///     }
/// }
///
/// let expected = Ok(Point { x: 1, y: 2 });
/// // Explicit call
/// assert_eq!(Point::from_str("(1,2)"), expected);
/// // Implicit calls, through parse
/// assert_eq!("(1,2)".parse(), expected);
/// assert_eq!("(1,2)".parse::<Point>(), expected);
/// // Invalid input string
/// assert!(Point::from_str("(1 2)").is_err());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub trait FromStr: Sized {
    /// The associated error which can be returned from parsing.
    #[stable(feature = "rust1", since = "1.0.0")]
    type Err;

    /// Parses a string `s` to return a value of this type.
    ///
    /// If parsing succeeds, return the value inside [`Ok`], otherwise
    /// when the string is ill-formatted return an error specific to the
    /// inside [`Err`]. The error type is specific to the implementation of the trait.
    ///
    /// # Examples
    ///
    /// Basic usage with [`i32`], a type that implements `FromStr`:
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// let s = "5";
    /// let x = i32::from_str(s).unwrap();
    ///
    /// assert_eq!(5, x);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_diagnostic_item = "from_str_method"]
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl FromStr for bool {
    type Err = ParseBoolError;

    /// Parse a `bool` from a string.
    ///
    /// The only accepted values are `"true"` and `"false"`. Any other input
    /// will return an error.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// assert_eq!(FromStr::from_str("true"), Ok(true));
    /// assert_eq!(FromStr::from_str("false"), Ok(false));
    /// assert!(<bool as FromStr>::from_str("not even a boolean").is_err());
    /// ```
    ///
    /// Note, in many cases, the `.parse()` method on `str` is more proper.
    ///
    /// ```
    /// assert_eq!("true".parse(), Ok(true));
    /// assert_eq!("false".parse(), Ok(false));
    /// assert!("not even a boolean".parse::<bool>().is_err());
    /// ```
    #[inline]
    fn from_str(s: &str) -> Result<bool, ParseBoolError> {
        match s {
            "true" => Ok(true),
            "false" => Ok(false),
            _ => Err(ParseBoolError),
        }
    }
}