Upgrade rust/crates/memoffset to 0.6.4

Test: make
Change-Id: Idebee12bcb675673f291643fb5c4181cd160da94
diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
index fcba615..036fbbe 100644
--- a/.cargo_vcs_info.json
+++ b/.cargo_vcs_info.json
@@ -1,5 +1,5 @@
 {
   "git": {
-    "sha1": "9846cd17f381a5a9b478267ca5c684ab59346283"
+    "sha1": "bbd58f2cbe92d3424d8c3161c46dd5d022c30437"
   }
 }
diff --git a/Cargo.toml b/Cargo.toml
index e273757..5ea1a37 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,7 +12,7 @@
 
 [package]
 name = "memoffset"
-version = "0.6.3"
+version = "0.6.4"
 authors = ["Gilad Naaman <gilad.naaman@gmail.com>"]
 description = "offset_of functionality for Rust structs."
 readme = "README.md"
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
index 9299a64..d3acd0b 100644
--- a/Cargo.toml.orig
+++ b/Cargo.toml.orig
@@ -1,6 +1,6 @@
 [package]
 name = "memoffset"
-version = "0.6.3"
+version = "0.6.4"
 authors = ["Gilad Naaman <gilad.naaman@gmail.com>"]
 description = "offset_of functionality for Rust structs."
 license = "MIT"
diff --git a/METADATA b/METADATA
index 3e6b088..b95edfd 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/memoffset/memoffset-0.6.3.crate"
+    value: "https://static.crates.io/crates/memoffset/memoffset-0.6.4.crate"
   }
-  version: "0.6.3"
+  version: "0.6.4"
   license_type: NOTICE
   last_upgrade_date {
     year: 2021
-    month: 4
-    day: 8
+    month: 6
+    day: 21
   }
 }
diff --git a/README.md b/README.md
index a60f288..52999e3 100644
--- a/README.md
+++ b/README.md
@@ -69,7 +69,5 @@
 
 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.
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 71c00fc..6b913bc 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -5,7 +5,16 @@
       "name": "crossbeam-epoch_device_test_src_lib"
     },
     {
+      "name": "crossbeam-epoch_device_test_tests_loom"
+    },
+    {
       "name": "memoffset_device_test_src_lib"
+    },
+    {
+      "name": "unicode-xid_device_test_src_lib"
+    },
+    {
+      "name": "unicode-xid_device_test_tests_exhaustive_tests"
     }
   ]
 }
diff --git a/src/lib.rs b/src/lib.rs
index 50d3e10..005ea13 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -61,11 +61,10 @@
 #![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,
     )
 )]
 
diff --git a/src/offset_of.rs b/src/offset_of.rs
index a363d30..d376498 100644
--- a/src/offset_of.rs
+++ b/src/offset_of.rs
@@ -255,6 +255,18 @@
 
     #[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() {
         const fn test_fn() -> usize {
             #[repr(C)]