Rollup merge of #69587 - petrochenkov:reqname, r=Centril

[rust.git] / src / libcore / intrinsics.rs

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 43f8cfc0c473fcadad0e4b9585ec9b47a60cee80..fca2c3d31d946e348d2969325ec861e779e6c088 100644 (file)
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -881,7 +881,8 @@
     /// // clone the vector as we will reuse them later
     /// let v_clone = v_orig.clone();
     ///
-    /// // Using transmute: this is Undefined Behavior, and a bad idea.
+    /// // Using transmute: this relies on the unspecified data layout of `Vec`, which is a
+    /// // bad idea and could cause Undefined Behavior.
     /// // However, it is no-copy.
     /// let v_transmuted = unsafe {
     ///     std::mem::transmute::<Vec<&i32>, Vec<Option<&i32>>>(v_clone)
@@ -897,13 +898,14 @@
     ///
     /// let v_clone = v_orig.clone();
     ///
-    /// // The no-copy, unsafe way, still using transmute, but not UB.
-    /// // This is equivalent to the original, but safer, and reuses the
-    /// // same `Vec` internals. Therefore, the new inner type must have the
-    /// // exact same size, and the same alignment, as the old type.
+    /// // The no-copy, unsafe way, still using transmute, but not relying on the data layout.
+    /// // Like the first approach, this reuses the `Vec` internals.
+    /// // Therefore, the new inner type must have the
+    /// // exact same size, *and the same alignment*, as the old type.
     /// // The same caveats exist for this method as transmute, for
     /// // the original inner type (`&i32`) to the converted inner type
-    /// // (`Option<&i32>`), so read the nomicon pages linked above.
+    /// // (`Option<&i32>`), so read the nomicon pages linked above and also
+    /// // consult the [`from_raw_parts`] documentation.
     /// let v_from_raw = unsafe {
     // FIXME Update this when vec_into_raw_parts is stabilized
     ///     // Ensure the original vector is not dropped.
@@ -914,6 +916,8 @@
     /// };
     /// ```
     ///
+    /// [`from_raw_parts`]: ../../std/vec/struct.Vec.html#method.from_raw_parts
+    ///
     /// Implementing `split_at_mut`:
     ///
     /// ```
@@ -1423,13 +1427,15 @@ pub(crate) fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
 }
 
 /// Checks whether the regions of memory starting at `src` and `dst` of size
-/// `count * size_of::<T>()` overlap.
-fn overlaps<T>(src: *const T, dst: *const T, count: usize) -> bool {
+/// `count * size_of::<T>()` do *not* overlap.
+pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -> bool {
     let src_usize = src as usize;
     let dst_usize = dst as usize;
     let size = mem::size_of::<T>().checked_mul(count).unwrap();
     let diff = if src_usize > dst_usize { src_usize - dst_usize } else { dst_usize - src_usize };
-    size > diff
+    // If the absolute distance between the ptrs is at least as big as the size of the buffer,
+    // they do not overlap.
+    diff >= size
 }
 
 /// Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
@@ -1525,7 +1531,7 @@ pub unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
 
     debug_assert!(is_aligned_and_not_null(src), "attempt to copy from unaligned or null pointer");
     debug_assert!(is_aligned_and_not_null(dst), "attempt to copy to unaligned or null pointer");
-    debug_assert!(!overlaps(src, dst, count), "attempt to copy to overlapping memory");
+    debug_assert!(is_nonoverlapping(src, dst, count), "attempt to copy to overlapping memory");
     copy_nonoverlapping(src, dst, count)
 }