Add an intrinsic for ptr::metadata

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -95,6 +95,14 @@ impl<'tcx> CValue<'tcx> {
         CValue(CValueInner::ByValPair(value, extra), layout)
     }
 
+    /// Create an instance of a ZST
+    ///
+    /// The is represented by a dangling pointer of suitable alignment.
+    pub(crate) fn zst(layout: TyAndLayout<'tcx>) -> CValue<'tcx> {
+        assert!(layout.is_zst());
+        CValue::by_ref(crate::Pointer::dangling(layout.align.pref), layout)
+    }
+
     pub(crate) fn layout(&self) -> TyAndLayout<'tcx> {
         self.1
     }