fix arith_offset not taking the size of the type into account; test for offset

src/terminator/intrinsic.rs

@@ -43,10 +43,11 @@ pub(super) fn call_intrinsic(
 
 
             "arith_offset" => {
+                // FIXME: Switch to non-checked, wrapped version of pointer_offset
+                let offset = self.value_to_primval(arg_vals[1], isize)?.to_i128()? as i64;
                 let ptr = arg_vals[0].read_ptr(&self.memory)?;
-                let offset = self.value_to_primval(arg_vals[1], isize)?.to_i128()?;
-                let new_ptr = ptr.signed_offset(offset as i64);
-                self.write_primval(dest, PrimVal::Ptr(new_ptr), dest_ty)?;
+                let result_ptr = self.pointer_offset(ptr, substs.type_at(0), offset)?;
+                self.write_primval(dest, PrimVal::Ptr(result_ptr), dest_ty)?;
             }
 
             "assume" => {

tests/run-pass/ptr_arith_offset.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let v = [1i16, 2];
+    let x = &v as *const i16;
+    let x = x.wrapping_offset(1);
+    assert_eq!(unsafe { *x }, 2);
+}

tests/run-pass/ptr_offset.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let v = [1i16, 2];
+    let x = &v as *const i16;
+    let x = unsafe { x.offset(1) };
+    assert_eq!(unsafe { *x }, 2);
+}