Re-enable copy[_nonoverlapping]() runtime checks

This commit re-enables the debug checks for valid usages of the two
functions `copy()` and `copy_nonoverlapping()`. Those checks were com-
mented out in #79684 in order to make the functions const. All that's
been left was a FIXME, that could not be resolved until there is was way
to only do the checks at runtime.
Since #89247 there is such a way: `const_eval_select()`. This commit
uses that new intrinsic in order to either do nothing (at compile time)
or to do the old checks (at runtime).

The change itself is rather small: in order to make the checks usable
with `const_eval_select`, they are moved into a local function (one for
`copy` and one for `copy_nonoverlapping` to keep symmetry).

The change does not break referential transparency, as there is nothing
you can do at compile time, which you cannot do on runtime without get-
ting undefined behavior. The CTFE-engine won't allow missuses. The other
way round is also fine.

library/core/src/intrinsics.rs

@@ -1951,6 +1951,19 @@ pub(crate) fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
     !ptr.is_null() && ptr as usize % mem::align_of::<T>() == 0
 }
 
+/// Checks whether the regions of memory starting at `src` and `dst` of size
+/// `count * size_of::<T>()` do *not* overlap.
+#[cfg(debug_assertions)]
+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 };
+    // 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
 /// and destination must *not* overlap.
 ///
@@ -2042,15 +2055,24 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
         pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
     }
 
-    // FIXME: Perform these checks only at run time
-    /*if cfg!(debug_assertions)
-        && !(is_aligned_and_not_null(src)
-            && is_aligned_and_not_null(dst)
-            && is_nonoverlapping(src, dst, count))
-    {
-        // Not panicking to keep codegen impact smaller.
-        abort();
-    }*/
+    #[cfg(debug_assertions)]
+    fn runtime_check<T>(src: *const T, dst: *mut T, count: usize) {
+        if !is_aligned_and_not_null(src)
+            || !is_aligned_and_not_null(dst)
+            || !is_nonoverlapping(src, dst, count)
+        {
+            // Not panicking to keep codegen impact smaller.
+            abort();
+        }
+    }
+    #[cfg(debug_assertions)]
+    const fn compiletime_check<T>(_src: *const T, _dst: *mut T, _count: usize) {}
+    #[cfg(debug_assertions)]
+    // SAFETY: runtime debug-assertions are a best-effort basis; it's fine to
+    // not do them during compile time
+    unsafe {
+        const_eval_select((src, dst, count), compiletime_check, runtime_check);
+    }
 
     // SAFETY: the safety contract for `copy_nonoverlapping` must be
     // upheld by the caller.
@@ -2127,11 +2149,21 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
         fn copy<T>(src: *const T, dst: *mut T, count: usize);
     }
 
-    // FIXME: Perform these checks only at run time
-    /*if cfg!(debug_assertions) && !(is_aligned_and_not_null(src) && is_aligned_and_not_null(dst)) {
-        // Not panicking to keep codegen impact smaller.
-        abort();
-    }*/
+    #[cfg(debug_assertions)]
+    fn runtime_check<T>(src: *const T, dst: *mut T) {
+        if !is_aligned_and_not_null(src) || !is_aligned_and_not_null(dst) {
+            // Not panicking to keep codegen impact smaller.
+            abort();
+        }
+    }
+    #[cfg(debug_assertions)]
+    const fn compiletime_check<T>(_src: *const T, _dst: *mut T) {}
+    #[cfg(debug_assertions)]
+    // SAFETY: runtime debug-assertions are a best-effort basis; it's fine to
+    // not do them during compile time
+    unsafe {
+        const_eval_select((src, dst), compiletime_check, runtime_check);
+    }
 
     // SAFETY: the safety contract for `copy` must be upheld by the caller.
     unsafe { copy(src, dst, count) }

library/core/src/lib.rs

@@ -105,7 +105,7 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_discriminant)]
-#![cfg_attr(not(bootstrap), feature(const_eval_select))]
+#![feature(const_eval_select)]
 #![feature(const_float_bits_conv)]
 #![feature(const_float_classify)]
 #![feature(const_fmt_arguments_new)]