basic dyn* support for Miri

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -255,7 +255,22 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
         }
     }
 
-    pub fn offset_with_meta(
+    /// Replace the layout of this operand. There's basically no sanity check that this makes sense,
+    /// you better know what you are doing! If this is an immediate, applying the wrong layout can
+    /// not just lead to invalid data, it can actually *shift the data around* since the offsets of
+    /// a ScalarPair are entirely determined by the layout, not the data.
+    pub fn transmute(&self, layout: TyAndLayout<'tcx>) -> Self {
+        assert_eq!(
+            self.layout.size, layout.size,
+            "transmuting with a size change, that doesn't seem right"
+        );
+        OpTy { layout, ..*self }
+    }
+
+    /// Offset the operand in memory (if possible) and change its metadata.
+    ///
+    /// This can go wrong very easily if you give the wrong layout for the new place!
+    pub(super) fn offset_with_meta(
         &self,
         offset: Size,
         meta: MemPlaceMeta<Prov>,
@@ -276,6 +291,9 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
         }
     }
 
+    /// Offset the operand in memory (if possible).
+    ///
+    /// This can go wrong very easily if you give the wrong layout for the new place!
     pub fn offset(
         &self,
         offset: Size,