core/slice/iter/
macros.rs

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
//! Macros used by iterators of slice.

/// Convenience & performance macro for consuming the `end_or_len` field, by
/// giving a `(&mut) usize` or `(&mut) NonNull<T>` depending whether `T` is
/// or is not a ZST respectively.
///
/// Internally, this reads the `end` through a pointer-to-`NonNull` so that
/// it'll get the appropriate non-null metadata in the backend without needing
/// to call `assume` manually.
macro_rules! if_zst {
    (mut $this:ident, $len:ident => $zst_body:expr, $end:ident => $other_body:expr,) => {{
        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block

        if T::IS_ZST {
            // SAFETY: for ZSTs, the pointer is storing a provenance-free length,
            // so consuming and updating it as a `usize` is fine.
            let $len = unsafe { &mut *(&raw mut $this.end_or_len).cast::<usize>() };
            $zst_body
        } else {
            // SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
            let $end = unsafe { &mut *(&raw mut $this.end_or_len).cast::<NonNull<T>>() };
            $other_body
        }
    }};
    ($this:ident, $len:ident => $zst_body:expr, $end:ident => $other_body:expr,) => {{
        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block

        if T::IS_ZST {
            let $len = $this.end_or_len.addr();
            $zst_body
        } else {
            // SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
            let $end = unsafe { *(&raw const $this.end_or_len).cast::<NonNull<T>>() };
            $other_body
        }
    }};
}

// Inlining is_empty and len makes a huge performance difference
macro_rules! is_empty {
    ($self: ident) => {
        if_zst!($self,
            len => len == 0,
            end => $self.ptr == end,
        )
    };
}

macro_rules! len {
    ($self: ident) => {{
        if_zst!($self,
            len => len,
            end => {
                // To get rid of some bounds checks (see `position`), we use ptr_sub instead of
                // offset_from (Tested by `codegen/slice-position-bounds-check`.)
                // SAFETY: by the type invariant pointers are aligned and `start <= end`
                unsafe { end.sub_ptr($self.ptr) }
            },
        )
    }};
}

// The shared definition of the `Iter` and `IterMut` iterators
macro_rules! iterator {
    (
        struct $name:ident -> $ptr:ty,
        $elem:ty,
        $raw_mut:tt,
        {$( $mut_:tt )?},
        $into_ref:ident,
        {$($extra:tt)*}
    ) => {
        impl<'a, T> $name<'a, T> {
            /// Returns the last element and moves the end of the iterator backwards by 1.
            ///
            /// # Safety
            ///
            /// The iterator must not be empty
            #[inline]
            unsafe fn next_back_unchecked(&mut self) -> $elem {
                // SAFETY: the caller promised it's not empty, so
                // the offsetting is in-bounds and there's an element to return.
                unsafe { self.pre_dec_end(1).$into_ref() }
            }

            // Helper function for creating a slice from the iterator.
            #[inline(always)]
            fn make_slice(&self) -> &'a [T] {
                // SAFETY: the iterator was created from a slice with pointer
                // `self.ptr` and length `len!(self)`. This guarantees that all
                // the prerequisites for `from_raw_parts` are fulfilled.
                unsafe { from_raw_parts(self.ptr.as_ptr(), len!(self)) }
            }

            // Helper function for moving the start of the iterator forwards by `offset` elements,
            // returning the old start.
            // Unsafe because the offset must not exceed `self.len()`.
            #[inline(always)]
            unsafe fn post_inc_start(&mut self, offset: usize) -> NonNull<T> {
                let old = self.ptr;

                // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                // so this new pointer is inside `self` and thus guaranteed to be non-null.
                unsafe {
                    if_zst!(mut self,
                        // Using the intrinsic directly avoids emitting a UbCheck
                        len => *len = crate::intrinsics::unchecked_sub(*len, offset),
                        _end => self.ptr = self.ptr.add(offset),
                    );
                }
                old
            }

            // Helper function for moving the end of the iterator backwards by `offset` elements,
            // returning the new end.
            // Unsafe because the offset must not exceed `self.len()`.
            #[inline(always)]
            unsafe fn pre_dec_end(&mut self, offset: usize) -> NonNull<T> {
                if_zst!(mut self,
                    // SAFETY: By our precondition, `offset` can be at most the
                    // current length, so the subtraction can never overflow.
                    len => unsafe {
                        // Using the intrinsic directly avoids emitting a UbCheck
                        *len = crate::intrinsics::unchecked_sub(*len, offset);
                        self.ptr
                    },
                    // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                    // which is guaranteed to not overflow an `isize`. Also, the resulting pointer
                    // is in bounds of `slice`, which fulfills the other requirements for `offset`.
                    end => unsafe {
                        *end = end.sub(offset);
                        *end
                    },
                )
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<T> ExactSizeIterator for $name<'_, T> {
            #[inline(always)]
            fn len(&self) -> usize {
                len!(self)
            }

            #[inline(always)]
            fn is_empty(&self) -> bool {
                is_empty!(self)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, T> Iterator for $name<'a, T> {
            type Item = $elem;

            #[inline]
            fn next(&mut self) -> Option<$elem> {
                // could be implemented with slices, but this avoids bounds checks

                // SAFETY: The call to `next_unchecked` is
                // safe since we check if the iterator is empty first.
                unsafe {
                    if is_empty!(self) {
                        None
                    } else {
                        Some(self.next_unchecked())
                    }
                }
            }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) {
                let exact = len!(self);
                (exact, Some(exact))
            }

            #[inline]
            fn count(self) -> usize {
                len!(self)
            }

            #[inline]
            fn nth(&mut self, n: usize) -> Option<$elem> {
                if n >= len!(self) {
                    // This iterator is now empty.
                    if_zst!(mut self,
                        len => *len = 0,
                        end => self.ptr = *end,
                    );
                    return None;
                }
                // SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
                unsafe {
                    self.post_inc_start(n);
                    Some(self.next_unchecked())
                }
            }

            #[inline]
            fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
                let advance = cmp::min(len!(self), n);
                // SAFETY: By construction, `advance` does not exceed `self.len()`.
                unsafe { self.post_inc_start(advance) };
                NonZero::new(n - advance).map_or(Ok(()), Err)
            }

            #[inline]
            fn last(mut self) -> Option<$elem> {
                self.next_back()
            }

            #[inline]
            fn fold<B, F>(self, init: B, mut f: F) -> B
                where
                    F: FnMut(B, Self::Item) -> B,
            {
                // this implementation consists of the following optimizations compared to the
                // default implementation:
                // - do-while loop, as is llvm's preferred loop shape,
                //   see https://releases.llvm.org/16.0.0/docs/LoopTerminology.html#more-canonical-loops
                // - bumps an index instead of a pointer since the latter case inhibits
                //   some optimizations, see #111603
                // - avoids Option wrapping/matching
                if is_empty!(self) {
                    return init;
                }
                let mut acc = init;
                let mut i = 0;
                let len = len!(self);
                loop {
                    // SAFETY: the loop iterates `i in 0..len`, which always is in bounds of
                    // the slice allocation
                    acc = f(acc, unsafe { & $( $mut_ )? *self.ptr.add(i).as_ptr() });
                    // SAFETY: `i` can't overflow since it'll only reach usize::MAX if the
                    // slice had that length, in which case we'll break out of the loop
                    // after the increment
                    i = unsafe { i.unchecked_add(1) };
                    if i == len {
                        break;
                    }
                }
                acc
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn for_each<F>(mut self, mut f: F)
            where
                Self: Sized,
                F: FnMut(Self::Item),
            {
                while let Some(x) = self.next() {
                    f(x);
                }
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn all<F>(&mut self, mut f: F) -> bool
            where
                Self: Sized,
                F: FnMut(Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if !f(x) {
                        return false;
                    }
                }
                true
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn any<F>(&mut self, mut f: F) -> bool
            where
                Self: Sized,
                F: FnMut(Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if f(x) {
                        return true;
                    }
                }
                false
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
            where
                Self: Sized,
                P: FnMut(&Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if predicate(&x) {
                        return Some(x);
                    }
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn find_map<B, F>(&mut self, mut f: F) -> Option<B>
            where
                Self: Sized,
                F: FnMut(Self::Item) -> Option<B>,
            {
                while let Some(x) = self.next() {
                    if let Some(y) = f(x) {
                        return Some(y);
                    }
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile. Also, the `assume` avoids a bounds check.
            #[inline]
            #[rustc_inherit_overflow_checks]
            fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
                Self: Sized,
                P: FnMut(Self::Item) -> bool,
            {
                let n = len!(self);
                let mut i = 0;
                while let Some(x) = self.next() {
                    if predicate(x) {
                        // SAFETY: we are guaranteed to be in bounds by the loop invariant:
                        // when `i >= n`, `self.next()` returns `None` and the loop breaks.
                        unsafe { assert_unchecked(i < n) };
                        return Some(i);
                    }
                    i += 1;
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile. Also, the `assume` avoids a bounds check.
            #[inline]
            fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
                P: FnMut(Self::Item) -> bool,
                Self: Sized + ExactSizeIterator + DoubleEndedIterator
            {
                let n = len!(self);
                let mut i = n;
                while let Some(x) = self.next_back() {
                    i -= 1;
                    if predicate(x) {
                        // SAFETY: `i` must be lower than `n` since it starts at `n`
                        // and is only decreasing.
                        unsafe { assert_unchecked(i < n) };
                        return Some(i);
                    }
                }
                None
            }

            #[inline]
            unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
                // SAFETY: the caller must guarantee that `i` is in bounds of
                // the underlying slice, so `i` cannot overflow an `isize`, and
                // the returned references is guaranteed to refer to an element
                // of the slice and thus guaranteed to be valid.
                //
                // Also note that the caller also guarantees that we're never
                // called with the same index again, and that no other methods
                // that will access this subslice are called, so it is valid
                // for the returned reference to be mutable in the case of
                // `IterMut`
                unsafe { & $( $mut_ )? * self.ptr.as_ptr().add(idx) }
            }

            $($extra)*
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, T> DoubleEndedIterator for $name<'a, T> {
            #[inline]
            fn next_back(&mut self) -> Option<$elem> {
                // could be implemented with slices, but this avoids bounds checks

                // SAFETY: The call to `next_back_unchecked`
                // is safe since we check if the iterator is empty first.
                unsafe {
                    if is_empty!(self) {
                        None
                    } else {
                        Some(self.next_back_unchecked())
                    }
                }
            }

            #[inline]
            fn nth_back(&mut self, n: usize) -> Option<$elem> {
                if n >= len!(self) {
                    // This iterator is now empty.
                    if_zst!(mut self,
                        len => *len = 0,
                        end => *end = self.ptr,
                    );
                    return None;
                }
                // SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
                unsafe {
                    self.pre_dec_end(n);
                    Some(self.next_back_unchecked())
                }
            }

            #[inline]
            fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
                let advance = cmp::min(len!(self), n);
                // SAFETY: By construction, `advance` does not exceed `self.len()`.
                unsafe { self.pre_dec_end(advance) };
                NonZero::new(n - advance).map_or(Ok(()), Err)
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<T> FusedIterator for $name<'_, T> {}

        #[unstable(feature = "trusted_len", issue = "37572")]
        unsafe impl<T> TrustedLen for $name<'_, T> {}

        impl<'a, T> UncheckedIterator for $name<'a, T> {
            #[inline]
            unsafe fn next_unchecked(&mut self) -> $elem {
                // SAFETY: The caller promised there's at least one more item.
                unsafe {
                    self.post_inc_start(1).$into_ref()
                }
            }
        }

        #[stable(feature = "default_iters", since = "1.70.0")]
        impl<T> Default for $name<'_, T> {
            /// Creates an empty slice iterator.
            ///
            /// ```
            #[doc = concat!("# use core::slice::", stringify!($name), ";")]
            #[doc = concat!("let iter: ", stringify!($name<'_, u8>), " = Default::default();")]
            /// assert_eq!(iter.len(), 0);
            /// ```
            fn default() -> Self {
                (& $( $mut_ )? []).into_iter()
            }
        }
    }
}

macro_rules! forward_iterator {
    ($name:ident: $elem:ident, $iter_of:ty) => {
        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, $elem, P> Iterator for $name<'a, $elem, P>
        where
            P: FnMut(&T) -> bool,
        {
            type Item = $iter_of;

            #[inline]
            fn next(&mut self) -> Option<$iter_of> {
                self.inner.next()
            }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) {
                self.inner.size_hint()
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, $elem, P> FusedIterator for $name<'a, $elem, P> where P: FnMut(&T) -> bool {}
    };
}