update feature flags and docs for const offset_of

README.md

@@ -69,7 +69,7 @@ features = ["unstable_const"]
 
 Your crate root: (`lib.rs`/`main.rs`)
 ```rust,ignore
-#![feature(ptr_offset_from, const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref)]
+#![feature(const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref, const_refs_to_cell)]
 ```
 
 If you intend to use `offset_of!` inside a `const fn`, also add the `const_fn` compiler feature.

src/lib.rs

@@ -60,11 +60,10 @@
 #![cfg_attr(
     feature = "unstable_const",
     feature(
-        ptr_offset_from,
-        const_fn,
         const_ptr_offset_from,
         const_maybe_uninit_as_ptr,
         const_raw_ptr_deref,
+        const_refs_to_cell,
     )
 )]
 

src/offset_of.rs

@@ -253,6 +253,18 @@ mod tests {
         assert_eq!([0; offset_of!(Foo, b)].len(), 4);
     }
 
+    #[cfg(feature = "unstable_const")]
+    #[test]
+    fn const_offset_interior_mutable() {
+        #[repr(C)]
+        struct Foo {
+            a: u32,
+            b: core::cell::Cell<u32>,
+        }
+
+        assert_eq!([0; offset_of!(Foo, b)].len(), 4);
+    }
+
     #[cfg(feature = "unstable_const")]
     #[test]
     fn const_fn_offset() {