      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

    //! Trait implementations for `str`.

    use crate::cmp::Ordering;
    use crate::ops;
    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()
        }
        #[inline]
        fn ne(&self, other: &str) -> bool {
            !(*self).eq(other)
        }
    }

    #[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]
    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")]
    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")]
    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];
            // 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];
            // 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 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")]
    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 {
            let slice = slice as *const [u8];
            let ptr = slice.as_ptr();
            ptr::slice_from_raw_parts(ptr, self.end) as *const str
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            let slice = slice as *mut [u8];
            let ptr = slice.as_mut_ptr();
            ptr::slice_from_raw_parts_mut(ptr, self.end) as *mut str
        }
        #[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")]
    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 slice = slice as *const [u8];
            // 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 = slice.len() - 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];
            // SAFETY: identical to `get_unchecked`.
            let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
            let len = slice.len() - 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, 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")]
    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")]
    unsafe impl SliceIndex<str> for ops::RangeToInclusive<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if self.end == usize::MAX { None } else { (..self.end + 1).get(slice) }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if self.end == usize::MAX { None } else { (..self.end + 1).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.end + 1).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.end + 1).get_unchecked_mut(slice) }
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            if self.end == usize::MAX {
                str_index_overflow_fail();
            }
            (..self.end + 1).index(slice)
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            if self.end == usize::MAX {
                str_index_overflow_fail();
            }
            (..self.end + 1).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;
    /// use std::num::ParseIntError;
    ///
    /// #[derive(Debug, PartialEq)]
    /// struct Point {
    ///     x: i32,
    ///     y: i32
    /// }
    ///
    /// impl FromStr for Point {
    ///     type Err = ParseIntError;
    ///
    ///     fn from_str(s: &str) -> Result<Self, Self::Err> {
    ///         let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
    ///                                  .split(',')
    ///                                  .collect();
    ///
    ///         let x_fromstr = coords[0].parse::<i32>()?;
    ///         let y_fromstr = coords[1].parse::<i32>()?;
    ///
    ///         Ok(Point { x: x_fromstr, y: y_fromstr })
    ///     }
    /// }
    ///
    /// let p = Point::from_str("(1,2)");
    /// assert_eq!(p.unwrap(), Point{ x: 1, y: 2} )
    /// ```
    #[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 implementation of the trait.
        ///
        /// # Examples
        ///
        /// Basic usage with [`i32`][ithirtytwo], a type that implements `FromStr`:
        ///
        /// [ithirtytwo]: ../../std/primitive.i32.html
        ///
        /// ```
        /// 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")]
        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.
        ///
        /// Yields a `Result<bool, ParseBoolError>`, because `s` may or may not
        /// actually be parseable.
        ///
        /// # 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 { _priv: () }),
            }
        }
    }