1 //! Free functions to create `&[T]` and `&mut [T]`.
4 use crate::intrinsics::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 pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
89 debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
91 mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
92 "attempt to create slice covering at least half the address space"
94 // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
95 unsafe { &*ptr::slice_from_raw_parts(data, len) }
98 /// Performs the same functionality as [`from_raw_parts`], except that a
99 /// mutable slice is returned.
103 /// Behavior is undefined if any of the following conditions are violated:
105 /// * `data` must be [valid] for both reads and writes for `len * mem::size_of::<T>()` many bytes,
106 /// and it must be properly aligned. This means in particular:
108 /// * The entire memory range of this slice must be contained within a single allocated object!
109 /// Slices can never span across multiple allocated objects.
110 /// * `data` must be non-null and aligned even for zero-length slices. One
111 /// reason for this is that enum layout optimizations may rely on references
112 /// (including slices of any length) being aligned and non-null to distinguish
113 /// them from other data. You can obtain a pointer that is usable as `data`
114 /// for zero-length slices using [`NonNull::dangling()`].
116 /// * `data` must point to `len` consecutive properly initialized values of type `T`.
118 /// * The memory referenced by the returned slice must not be accessed through any other pointer
119 /// (not derived from the return value) for the duration of lifetime `'a`.
120 /// Both read and write accesses are forbidden.
122 /// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
123 /// See the safety documentation of [`pointer::offset`].
125 /// [valid]: ptr#safety
126 /// [`NonNull::dangling()`]: ptr::NonNull::dangling
128 #[stable(feature = "rust1", since = "1.0.0")]
129 pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
130 debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
132 mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
133 "attempt to create slice covering at least half the address space"
135 // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
136 unsafe { &mut *ptr::slice_from_raw_parts_mut(data, len) }
139 /// Converts a reference to T into a slice of length 1 (without copying).
140 #[stable(feature = "from_ref", since = "1.28.0")]
141 pub fn from_ref<T>(s: &T) -> &[T] {
145 /// Converts a reference to T into a slice of length 1 (without copying).
146 #[stable(feature = "from_ref", since = "1.28.0")]
147 pub fn from_mut<T>(s: &mut T) -> &mut [T] {