1 //! Free functions to create `&[T]` and `&mut [T]`.
4 use crate::intrinsics::{assert_unsafe_precondition, is_aligned_and_not_null};
8 /// Forms a slice from a pointer and a length.
10 /// The `len` argument is the number of **elements**, not the number of bytes.
14 /// Behavior is undefined if any of the following conditions are violated:
16 /// * `data` must be [valid] for reads for `len * mem::size_of::<T>()` many bytes,
17 /// and it must be properly aligned. This means in particular:
19 /// * The entire memory range of this slice must be contained within a single allocated object!
20 /// Slices can never span across multiple allocated objects. See [below](#incorrect-usage)
21 /// for an example incorrectly not taking this into account.
22 /// * `data` must be non-null and aligned even for zero-length slices. One
23 /// reason for this is that enum layout optimizations may rely on references
24 /// (including slices of any length) being aligned and non-null to distinguish
25 /// them from other data. You can obtain a pointer that is usable as `data`
26 /// for zero-length slices using [`NonNull::dangling()`].
28 /// * `data` must point to `len` consecutive properly initialized values of type `T`.
30 /// * The memory referenced by the returned slice must not be mutated for the duration
31 /// of lifetime `'a`, except inside an `UnsafeCell`.
33 /// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
34 /// See the safety documentation of [`pointer::offset`].
38 /// The lifetime for the returned slice is inferred from its usage. To
39 /// prevent accidental misuse, it's suggested to tie the lifetime to whichever
40 /// source lifetime is safe in the context, such as by providing a helper
41 /// function taking the lifetime of a host value for the slice, or by explicit
49 /// // manifest a slice for a single element
51 /// let ptr = &x as *const _;
52 /// let slice = unsafe { slice::from_raw_parts(ptr, 1) };
53 /// assert_eq!(slice[0], 42);
56 /// ### Incorrect usage
58 /// The following `join_slices` function is **unsound** ⚠️
63 /// fn join_slices<'a, T>(fst: &'a [T], snd: &'a [T]) -> &'a [T] {
64 /// let fst_end = fst.as_ptr().wrapping_add(fst.len());
65 /// let snd_start = snd.as_ptr();
66 /// assert_eq!(fst_end, snd_start, "Slices must be contiguous!");
68 /// // The assertion above ensures `fst` and `snd` are contiguous, but they might
69 /// // still be contained within _different allocated objects_, in which case
70 /// // creating this slice is undefined behavior.
71 /// slice::from_raw_parts(fst.as_ptr(), fst.len() + snd.len())
76 /// // `a` and `b` are different allocated objects...
79 /// // ... which may nevertheless be laid out contiguously in memory: | a | b |
80 /// let _ = join_slices(slice::from_ref(&a), slice::from_ref(&b)); // UB
84 /// [valid]: ptr#safety
85 /// [`NonNull::dangling()`]: ptr::NonNull::dangling
87 #[stable(feature = "rust1", since = "1.0.0")]
88 #[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
90 pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
91 // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
93 assert_unsafe_precondition!([T](data: *const T, len: usize) =>
94 is_aligned_and_not_null(data)
95 && crate::mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize
97 &*ptr::slice_from_raw_parts(data, len)
101 /// Performs the same functionality as [`from_raw_parts`], except that a
102 /// mutable slice is returned.
106 /// Behavior is undefined if any of the following conditions are violated:
108 /// * `data` must be [valid] for both reads and writes for `len * mem::size_of::<T>()` many bytes,
109 /// and it must be properly aligned. This means in particular:
111 /// * The entire memory range of this slice must be contained within a single allocated object!
112 /// Slices can never span across multiple allocated objects.
113 /// * `data` must be non-null and aligned even for zero-length slices. One
114 /// reason for this is that enum layout optimizations may rely on references
115 /// (including slices of any length) being aligned and non-null to distinguish
116 /// them from other data. You can obtain a pointer that is usable as `data`
117 /// for zero-length slices using [`NonNull::dangling()`].
119 /// * `data` must point to `len` consecutive properly initialized values of type `T`.
121 /// * The memory referenced by the returned slice must not be accessed through any other pointer
122 /// (not derived from the return value) for the duration of lifetime `'a`.
123 /// Both read and write accesses are forbidden.
125 /// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
126 /// See the safety documentation of [`pointer::offset`].
128 /// [valid]: ptr#safety
129 /// [`NonNull::dangling()`]: ptr::NonNull::dangling
131 #[stable(feature = "rust1", since = "1.0.0")]
132 #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
134 pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
135 // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
137 assert_unsafe_precondition!([T](data: *mut T, len: usize) =>
138 is_aligned_and_not_null(data)
139 && crate::mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize
141 &mut *ptr::slice_from_raw_parts_mut(data, len)
145 /// Converts a reference to T into a slice of length 1 (without copying).
146 #[stable(feature = "from_ref", since = "1.28.0")]
147 #[rustc_const_stable(feature = "const_slice_from_ref_shared", since = "1.63.0")]
149 pub const fn from_ref<T>(s: &T) -> &[T] {
153 /// Converts a reference to T into a slice of length 1 (without copying).
154 #[stable(feature = "from_ref", since = "1.28.0")]
155 #[rustc_const_unstable(feature = "const_slice_from_ref", issue = "90206")]
157 pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
161 /// Forms a slice from a pointer range.
163 /// This function is useful for interacting with foreign interfaces which
164 /// use two pointers to refer to a range of elements in memory, as is
169 /// Behavior is undefined if any of the following conditions are violated:
171 /// * The `start` pointer of the range must be a [valid] and properly aligned pointer
172 /// to the first element of a slice.
174 /// * The `end` pointer must be a [valid] and properly aligned pointer to *one past*
175 /// the last element, such that the offset from the end to the start pointer is
176 /// the length of the slice.
178 /// * The range must contain `N` consecutive properly initialized values of type `T`:
180 /// * The entire memory range of this slice must be contained within a single allocated object!
181 /// Slices can never span across multiple allocated objects.
183 /// * The memory referenced by the returned slice must not be mutated for the duration
184 /// of lifetime `'a`, except inside an `UnsafeCell`.
186 /// * The total length of the range must be no larger than `isize::MAX`.
187 /// See the safety documentation of [`pointer::offset`].
189 /// Note that a range created from [`slice::as_ptr_range`] fulfills these requirements.
193 /// This function panics if `T` is a Zero-Sized Type (“ZST”).
197 /// The lifetime for the returned slice is inferred from its usage. To
198 /// prevent accidental misuse, it's suggested to tie the lifetime to whichever
199 /// source lifetime is safe in the context, such as by providing a helper
200 /// function taking the lifetime of a host value for the slice, or by explicit
206 /// #![feature(slice_from_ptr_range)]
210 /// let x = [1, 2, 3];
211 /// let range = x.as_ptr_range();
214 /// assert_eq!(slice::from_ptr_range(range), &x);
218 /// [valid]: ptr#safety
219 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
220 #[rustc_const_unstable(feature = "const_slice_from_ptr_range", issue = "89792")]
221 pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
222 // SAFETY: the caller must uphold the safety contract for `from_ptr_range`.
223 unsafe { from_raw_parts(range.start, range.end.sub_ptr(range.start)) }
226 /// Forms a mutable slice from a pointer range.
228 /// This is the same functionality as [`from_ptr_range`], except that a
229 /// mutable slice is returned.
231 /// This function is useful for interacting with foreign interfaces which
232 /// use two pointers to refer to a range of elements in memory, as is
237 /// Behavior is undefined if any of the following conditions are violated:
239 /// * The `start` pointer of the range must be a [valid] and properly aligned pointer
240 /// to the first element of a slice.
242 /// * The `end` pointer must be a [valid] and properly aligned pointer to *one past*
243 /// the last element, such that the offset from the end to the start pointer is
244 /// the length of the slice.
246 /// * The range must contain `N` consecutive properly initialized values of type `T`:
248 /// * The entire memory range of this slice must be contained within a single allocated object!
249 /// Slices can never span across multiple allocated objects.
251 /// * The memory referenced by the returned slice must not be accessed through any other pointer
252 /// (not derived from the return value) for the duration of lifetime `'a`.
253 /// Both read and write accesses are forbidden.
255 /// * The total length of the range must be no larger than `isize::MAX`.
256 /// See the safety documentation of [`pointer::offset`].
258 /// Note that a range created from [`slice::as_mut_ptr_range`] fulfills these requirements.
262 /// This function panics if `T` is a Zero-Sized Type (“ZST”).
266 /// The lifetime for the returned slice is inferred from its usage. To
267 /// prevent accidental misuse, it's suggested to tie the lifetime to whichever
268 /// source lifetime is safe in the context, such as by providing a helper
269 /// function taking the lifetime of a host value for the slice, or by explicit
275 /// #![feature(slice_from_ptr_range)]
279 /// let mut x = [1, 2, 3];
280 /// let range = x.as_mut_ptr_range();
283 /// assert_eq!(slice::from_mut_ptr_range(range), &mut [1, 2, 3]);
287 /// [valid]: ptr#safety
288 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
289 #[rustc_const_unstable(feature = "const_slice_from_mut_ptr_range", issue = "89792")]
290 pub const unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
291 // SAFETY: the caller must uphold the safety contract for `from_mut_ptr_range`.
292 unsafe { from_raw_parts_mut(range.start, range.end.sub_ptr(range.start)) }