core/slice/
raw.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
//! Free functions to create `&[T]` and `&mut [T]`.

use crate::ops::Range;
use crate::{array, ptr, ub_checks};

/// Forms a slice from a pointer and a length.
///
/// The `len` argument is the number of **elements**, not the number of bytes.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `data` must be non-null, [valid] for reads for `len * mem::size_of::<T>()` many bytes,
///   and it must be properly aligned. This means in particular:
///
///     * The entire memory range of this slice must be contained within a single allocated object!
///       Slices can never span across multiple allocated objects. See [below](#incorrect-usage)
///       for an example incorrectly not taking this into account.
///     * `data` must be non-null and aligned even for zero-length slices or slices of ZSTs. One
///       reason for this is that enum layout optimizations may rely on references
///       (including slices of any length) being aligned and non-null to distinguish
///       them from other data. You can obtain a pointer that is usable as `data`
///       for zero-length slices using [`NonNull::dangling()`].
///
/// * `data` must point to `len` consecutive properly initialized values of type `T`.
///
/// * The memory referenced by the returned slice must not be mutated for the duration
///   of lifetime `'a`, except inside an `UnsafeCell`.
///
/// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`,
///   and adding that size to `data` must not "wrap around" the address space.
///   See the safety documentation of [`pointer::offset`].
///
/// # Caveat
///
/// The lifetime for the returned slice is inferred from its usage. To
/// prevent accidental misuse, it's suggested to tie the lifetime to whichever
/// source lifetime is safe in the context, such as by providing a helper
/// function taking the lifetime of a host value for the slice, or by explicit
/// annotation.
///
/// # Examples
///
/// ```
/// use std::slice;
///
/// // manifest a slice for a single element
/// let x = 42;
/// let ptr = &x as *const _;
/// let slice = unsafe { slice::from_raw_parts(ptr, 1) };
/// assert_eq!(slice[0], 42);
/// ```
///
/// ### Incorrect usage
///
/// The following `join_slices` function is **unsound** ⚠️
///
/// ```rust,no_run
/// use std::slice;
///
/// fn join_slices<'a, T>(fst: &'a [T], snd: &'a [T]) -> &'a [T] {
///     let fst_end = fst.as_ptr().wrapping_add(fst.len());
///     let snd_start = snd.as_ptr();
///     assert_eq!(fst_end, snd_start, "Slices must be contiguous!");
///     unsafe {
///         // The assertion above ensures `fst` and `snd` are contiguous, but they might
///         // still be contained within _different allocated objects_, in which case
///         // creating this slice is undefined behavior.
///         slice::from_raw_parts(fst.as_ptr(), fst.len() + snd.len())
///     }
/// }
///
/// fn main() {
///     // `a` and `b` are different allocated objects...
///     let a = 42;
///     let b = 27;
///     // ... which may nevertheless be laid out contiguously in memory: | a | b |
///     let _ = join_slices(slice::from_ref(&a), slice::from_ref(&b)); // UB
/// }
/// ```
///
/// ### FFI: Handling null pointers
///
/// In languages such as C++, pointers to empty collections are not guaranteed to be non-null.
/// When accepting such pointers, they have to be checked for null-ness to avoid undefined
/// behavior.
///
/// ```
/// use std::slice;
///
/// /// Sum the elements of an FFI slice.
/// ///
/// /// # Safety
/// ///
/// /// If ptr is not NULL, it must be correctly aligned and
/// /// point to `len` initialized items of type `f32`.
/// unsafe extern "C" fn sum_slice(ptr: *const f32, len: usize) -> f32 {
///     let data = if ptr.is_null() {
///         // `len` is assumed to be 0.
///         &[]
///     } else {
///         // SAFETY: see function docstring.
///         unsafe { slice::from_raw_parts(ptr, len) }
///     };
///     data.into_iter().sum()
/// }
///
/// // This could be the result of C++'s std::vector::data():
/// let ptr = std::ptr::null();
/// // And this could be std::vector::size():
/// let len = 0;
/// assert_eq!(unsafe { sum_slice(ptr, len) }, 0.0);
/// ```
///
/// [valid]: ptr#safety
/// [`NonNull::dangling()`]: ptr::NonNull::dangling
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
#[must_use]
#[rustc_diagnostic_item = "slice_from_raw_parts"]
pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
    // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
    unsafe {
        ub_checks::assert_unsafe_precondition!(
            check_language_ub,
            "slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`",
            (
                data: *mut () = data as *mut (),
                size: usize = size_of::<T>(),
                align: usize = align_of::<T>(),
                len: usize = len,
            ) =>
            ub_checks::is_aligned_and_not_null(data, align)
                && ub_checks::is_valid_allocation_size(size, len)
        );
        &*ptr::slice_from_raw_parts(data, len)
    }
}

/// Performs the same functionality as [`from_raw_parts`], except that a
/// mutable slice is returned.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `data` must be non-null, [valid] for both reads and writes for `len * mem::size_of::<T>()` many bytes,
///   and it must be properly aligned. This means in particular:
///
///     * The entire memory range of this slice must be contained within a single allocated object!
///       Slices can never span across multiple allocated objects.
///     * `data` must be non-null and aligned even for zero-length slices or slices of ZSTs. One
///       reason for this is that enum layout optimizations may rely on references
///       (including slices of any length) being aligned and non-null to distinguish
///       them from other data. You can obtain a pointer that is usable as `data`
///       for zero-length slices using [`NonNull::dangling()`].
///
/// * `data` must point to `len` consecutive properly initialized values of type `T`.
///
/// * The memory referenced by the returned slice must not be accessed through any other pointer
///   (not derived from the return value) for the duration of lifetime `'a`.
///   Both read and write accesses are forbidden.
///
/// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`,
///   and adding that size to `data` must not "wrap around" the address space.
///   See the safety documentation of [`pointer::offset`].
///
/// [valid]: ptr#safety
/// [`NonNull::dangling()`]: ptr::NonNull::dangling
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
#[must_use]
#[rustc_diagnostic_item = "slice_from_raw_parts_mut"]
pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
    // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
    unsafe {
        ub_checks::assert_unsafe_precondition!(
            check_language_ub,
            "slice::from_raw_parts_mut requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`",
            (
                data: *mut () = data as *mut (),
                size: usize = size_of::<T>(),
                align: usize = align_of::<T>(),
                len: usize = len,
            ) =>
            ub_checks::is_aligned_and_not_null(data, align)
                && ub_checks::is_valid_allocation_size(size, len)
        );
        &mut *ptr::slice_from_raw_parts_mut(data, len)
    }
}

/// Converts a reference to T into a slice of length 1 (without copying).
#[stable(feature = "from_ref", since = "1.28.0")]
#[rustc_const_stable(feature = "const_slice_from_ref_shared", since = "1.63.0")]
#[must_use]
pub const fn from_ref<T>(s: &T) -> &[T] {
    array::from_ref(s)
}

/// Converts a reference to T into a slice of length 1 (without copying).
#[stable(feature = "from_ref", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_slice_from_ref", issue = "90206")]
#[must_use]
pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
    array::from_mut(s)
}

/// Forms a slice from a pointer range.
///
/// This function is useful for interacting with foreign interfaces which
/// use two pointers to refer to a range of elements in memory, as is
/// common in C++.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * The `start` pointer of the range must be a non-null, [valid] and properly aligned pointer
///   to the first element of a slice.
///
/// * The `end` pointer must be a [valid] and properly aligned pointer to *one past*
///   the last element, such that the offset from the end to the start pointer is
///   the length of the slice.
///
/// * The entire memory range of this slice must be contained within a single allocated object!
///   Slices can never span across multiple allocated objects.
///
/// * The range must contain `N` consecutive properly initialized values of type `T`.
///
/// * The memory referenced by the returned slice must not be mutated for the duration
///   of lifetime `'a`, except inside an `UnsafeCell`.
///
/// * The total length of the range must be no larger than `isize::MAX`,
///   and adding that size to `start` must not "wrap around" the address space.
///   See the safety documentation of [`pointer::offset`].
///
/// Note that a range created from [`slice::as_ptr_range`] fulfills these requirements.
///
/// # Panics
///
/// This function panics if `T` is a Zero-Sized Type (“ZST”).
///
/// # Caveat
///
/// The lifetime for the returned slice is inferred from its usage. To
/// prevent accidental misuse, it's suggested to tie the lifetime to whichever
/// source lifetime is safe in the context, such as by providing a helper
/// function taking the lifetime of a host value for the slice, or by explicit
/// annotation.
///
/// # Examples
///
/// ```
/// #![feature(slice_from_ptr_range)]
///
/// use core::slice;
///
/// let x = [1, 2, 3];
/// let range = x.as_ptr_range();
///
/// unsafe {
///     assert_eq!(slice::from_ptr_range(range), &x);
/// }
/// ```
///
/// [valid]: ptr#safety
#[unstable(feature = "slice_from_ptr_range", issue = "89792")]
#[rustc_const_unstable(feature = "const_slice_from_ptr_range", issue = "89792")]
pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
    // SAFETY: the caller must uphold the safety contract for `from_ptr_range`.
    unsafe { from_raw_parts(range.start, range.end.sub_ptr(range.start)) }
}

/// Forms a mutable slice from a pointer range.
///
/// This is the same functionality as [`from_ptr_range`], except that a
/// mutable slice is returned.
///
/// This function is useful for interacting with foreign interfaces which
/// use two pointers to refer to a range of elements in memory, as is
/// common in C++.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * The `start` pointer of the range must be a non-null, [valid] and properly aligned pointer
///   to the first element of a slice.
///
/// * The `end` pointer must be a [valid] and properly aligned pointer to *one past*
///   the last element, such that the offset from the end to the start pointer is
///   the length of the slice.
///
/// * The entire memory range of this slice must be contained within a single allocated object!
///   Slices can never span across multiple allocated objects.
///
/// * The range must contain `N` consecutive properly initialized values of type `T`.
///
/// * The memory referenced by the returned slice must not be accessed through any other pointer
///   (not derived from the return value) for the duration of lifetime `'a`.
///   Both read and write accesses are forbidden.
///
/// * The total length of the range must be no larger than `isize::MAX`,
///   and adding that size to `start` must not "wrap around" the address space.
///   See the safety documentation of [`pointer::offset`].
///
/// Note that a range created from [`slice::as_mut_ptr_range`] fulfills these requirements.
///
/// # Panics
///
/// This function panics if `T` is a Zero-Sized Type (“ZST”).
///
/// # Caveat
///
/// The lifetime for the returned slice is inferred from its usage. To
/// prevent accidental misuse, it's suggested to tie the lifetime to whichever
/// source lifetime is safe in the context, such as by providing a helper
/// function taking the lifetime of a host value for the slice, or by explicit
/// annotation.
///
/// # Examples
///
/// ```
/// #![feature(slice_from_ptr_range)]
///
/// use core::slice;
///
/// let mut x = [1, 2, 3];
/// let range = x.as_mut_ptr_range();
///
/// unsafe {
///     assert_eq!(slice::from_mut_ptr_range(range), &mut [1, 2, 3]);
/// }
/// ```
///
/// [valid]: ptr#safety
#[unstable(feature = "slice_from_ptr_range", issue = "89792")]
#[rustc_const_unstable(feature = "const_slice_from_mut_ptr_range", issue = "89792")]
pub const unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
    // SAFETY: the caller must uphold the safety contract for `from_mut_ptr_range`.
    unsafe { from_raw_parts_mut(range.start, range.end.sub_ptr(range.start)) }
}