Move free functions to a new module

library/core/src/slice/raw.rs

@@ -0,0 +1,158 @@
+//! Free functions to create `&[T]` and `&mut [T]`.
+
+use crate::intrinsics::is_aligned_and_not_null;
+use crate::mem;
+use crate::ptr;
+
+/// 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 [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. 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`.
+///   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
+/// }
+/// ```
+///
+/// [valid]: ptr#safety
+/// [`NonNull::dangling()`]: ptr::NonNull::dangling
+/// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
+    debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
+    debug_assert!(
+        mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
+        "attempt to create slice covering at least half the address space"
+    );
+    // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
+    unsafe { &*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 [valid] for boths 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. 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`.
+///   See the safety documentation of [`pointer::offset`].
+///
+/// [valid]: ptr#safety
+/// [`NonNull::dangling()`]: ptr::NonNull::dangling
+/// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
+    debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
+    debug_assert!(
+        mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
+        "attempt to create slice covering at least half the address space"
+    );
+    // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
+    unsafe { &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")]
+pub fn from_ref<T>(s: &T) -> &[T] {
+    // SAFETY: a reference is guaranteed to be valid for reads. The returned
+    // reference cannot be mutated as it is an immutable reference.
+    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
+    // Thus the call to `from_raw_parts` is safe.
+    unsafe { from_raw_parts(s, 1) }
+}
+
+/// Converts a reference to T into a slice of length 1 (without copying).
+#[stable(feature = "from_ref", since = "1.28.0")]
+pub fn from_mut<T>(s: &mut T) -> &mut [T] {
+    // SAFETY: a mutable reference is guaranteed to be valid for writes.
+    // The reference cannot be accessed by another pointer as it is an mutable reference.
+    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
+    // Thus the call to `from_raw_parts_mut` is safe.
+    unsafe { from_raw_parts_mut(s, 1) }
+}