test that we properly check dynamic alignment

Author: RalfJung

tests/compile-fail/unaligned_pointers/dyn_alignment.rs

@@ -0,0 +1,19 @@
+// should find the bug even without these
+// compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
+
+#[repr(align(256))]
+#[derive(Debug)]
+struct MuchAlign;
+
+fn main() {
+    let buf = [0u32; 256];
+    // `buf` is sufficiently aligned for `layout.align` on a `dyn Debug`, but not
+    // for the actual alignment required by `MuchAlign`.
+    // We craft a wide reference `&dyn Debug` with the vtable for `MuchAlign`. That should be UB,
+    // as the reference is not aligned to its dynamic alignment requirements.
+    let mut ptr = &MuchAlign as &dyn std::fmt::Debug;
+    // Overwrite the data part of `ptr`.
+    unsafe { (&mut ptr as *mut _ as *mut *const u8).write(&buf as *const _ as *const u8); }
+    // Re-borrow that. This should be UB.
+    let _ptr = &*ptr; //~ ERROR accessing memory with alignment 4, but alignment 256 is required
+}