Add a Miri test for 134713

Author: scottmcm

src/tools/miri/tests/pass/stdlib/swap_nonoverlapping_is_untyped.rs

@@ -0,0 +1,30 @@
+use std::mem::{size_of, align_of};
+
+// See <https://github.com/rust-lang/rust/issues/134713>
+
+#[repr(C)]
+struct Foo(usize, u8);
+
+fn main() {
+    let buf1: [usize; 2] = [1000, 2000];
+    let buf2: [usize; 2] = [3000, 4000];
+
+    // Foo and [usize; 2] have the same size and alignment,
+    // so swap_nonoverlapping should treat them the same
+    assert_eq!(size_of::<Foo>(), size_of::<[usize; 2]>());
+    assert_eq!(align_of::<Foo>(), align_of::<[usize; 2]>());
+
+    let mut b1 = buf1;
+    let mut b2 = buf2;
+    // Safety: b1 and b2 are distinct local variables,
+    // with the same size and alignment as Foo.
+    unsafe {
+        std::ptr::swap_nonoverlapping(
+            b1.as_mut_ptr().cast::<Foo>(),
+            b2.as_mut_ptr().cast::<Foo>(),
+            1,
+        );
+    }
+    assert_eq!(b1, buf2);
+    assert_eq!(b2, buf1);
+}