adjust code for copy_op changes

Author: RalfJung

src/shims/intrinsics.rs

@@ -68,12 +68,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             "volatile_load" => {
                 let [place] = check_arg_count(args)?;
                 let place = this.deref_operand(place)?;
-                this.copy_op(&place.into(), dest)?;
+                this.copy_op(&place.into(), dest, /*allow_transmute*/ false)?;
             }
             "volatile_store" => {
                 let [place, dest] = check_arg_count(args)?;
                 let place = this.deref_operand(place)?;
-                this.copy_op(dest, &place.into())?;
+                this.copy_op(dest, &place.into(), /*allow_transmute*/ false)?;
             }
 
             "write_bytes" | "volatile_set_memory" => {

tests/pass/transmute_fat.rs

@@ -3,6 +3,7 @@
 
 fn main() {
     // If we are careful, we can exploit data layout...
+    // This is a tricky case since we are transmuting a ScalarPair type to a non-ScalarPair type.
     let raw = unsafe { std::mem::transmute::<&[u8], [*const u8; 2]>(&[42]) };
     let ptr: *const u8 = unsafe { std::mem::transmute_copy(&raw) };
     assert_eq!(unsafe { *ptr }, 42);