const-eval: full support for pointer fragments

Author: RalfJung

core/src/ptr/mod.rs

@@ -1345,40 +1345,6 @@ pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
 /// assert_eq!(x, [7, 8, 3, 4]);
 /// assert_eq!(y, [1, 2, 9]);
 /// ```
-///
-/// # Const evaluation limitations
-///
-/// If this function is invoked during const-evaluation, the current implementation has a small (and
-/// rarely relevant) limitation: if `count` is at least 2 and the data pointed to by `x` or `y`
-/// contains a pointer that crosses the boundary of two `T`-sized chunks of memory, the function may
-/// fail to evaluate (similar to a panic during const-evaluation). This behavior may change in the
-/// future.
-///
-/// The limitation is illustrated by the following example:
-///
-/// ```
-/// use std::mem::size_of;
-/// use std::ptr;
-///
-/// const { unsafe {
-///     const PTR_SIZE: usize = size_of::<*const i32>();
-///     let mut data1 = [0u8; PTR_SIZE];
-///     let mut data2 = [0u8; PTR_SIZE];
-///     // Store a pointer in `data1`.
-///     data1.as_mut_ptr().cast::<*const i32>().write_unaligned(&42);
-///     // Swap the contents of `data1` and `data2` by swapping `PTR_SIZE` many `u8`-sized chunks.
-///     // This call will fail, because the pointer in `data1` crosses the boundary
-///     // between several of the 1-byte chunks that are being swapped here.
-///     //ptr::swap_nonoverlapping(data1.as_mut_ptr(), data2.as_mut_ptr(), PTR_SIZE);
-///     // Swap the contents of `data1` and `data2` by swapping a single chunk of size
-///     // `[u8; PTR_SIZE]`. That works, as there is no pointer crossing the boundary between
-///     // two chunks.
-///     ptr::swap_nonoverlapping(&mut data1, &mut data2, 1);
-///     // Read the pointer from `data2` and dereference it.
-///     let ptr = data2.as_ptr().cast::<*const i32>().read_unaligned();
-///     assert!(*ptr == 42);
-/// } }
-/// ```
 #[inline]
 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
 #[rustc_const_stable(feature = "const_swap_nonoverlapping", since = "1.88.0")]
@@ -1407,9 +1373,7 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
     const_eval_select!(
         @capture[T] { x: *mut T, y: *mut T, count: usize }:
         if const {
-            // At compile-time we want to always copy this in chunks of `T`, to ensure that if there
-            // are pointers inside `T` we will copy them in one go rather than trying to copy a part
-            // of a pointer (which would not work).
+            // At compile-time we don't need all the special code below.
             // SAFETY: Same preconditions as this function
             unsafe { swap_nonoverlapping_const(x, y, count) }
         } else {

coretests/tests/ptr.rs

@@ -936,22 +936,18 @@ fn test_const_swap_ptr() {
         assert!(*s1.0.ptr == 666);
         assert!(*s2.0.ptr == 1);
 
-        // Swap them back, again as an array.
+        // Swap them back, byte-for-byte
         unsafe {
             ptr::swap_nonoverlapping(
-                ptr::from_mut(&mut s1).cast::<T>(),
-                ptr::from_mut(&mut s2).cast::<T>(),
-                1,
+                ptr::from_mut(&mut s1).cast::<u8>(),
+                ptr::from_mut(&mut s2).cast::<u8>(),
+                size_of::<A>(),
             );
         }
 
         // Make sure they still work.
         assert!(*s1.0.ptr == 1);
         assert!(*s2.0.ptr == 666);
-
-        // This is where we'd swap again using a `u8` type and a `count` of `size_of::<T>()` if it
-        // were not for the limitation of `swap_nonoverlapping` around pointers crossing multiple
-        // elements.
     };
 }