Fix offset_of assertions for fields of types with interior mutability.

Implementation of `memoffset::offset_of` (from the `memoffset` crate)
ends up taking a reference to a struct.  In `const` contexts (such as
the context of the assertion) this runs into
rust-lang/rust#80384.  This CL works around
this by opting the generated code into
`#![feature(const_refs_to_cell)]`.

After this CL bindings generated by `cc_bindings_from_rs` will require
a "nightly" version of the Rust compiler.  This is unfortunate, but
this dependency already exists on `rs_bindings_from_cc` side (e.g.
requiring `impl_trait_in_assoc_type` and/or `type_alias_impl_trait`
unstable features).

This CL unblocks implementing `Drop` support.  `Drop` support adds
bindings for additional types, some of which run into this bug.

PiperOrigin-RevId: 546337289
Change-Id: I1684b30a1ac096cc5115aabbe6e5c6504286947c

cc_bindings_from_rs/bindings.rs

@@ -101,6 +101,12 @@ pub fn generate_bindings(input: &Input) -> Result<Output> {
         // bindings need to relax the `improper_ctypes_definitions` warning
         // for `char` (and possibly for other built-in types in the future).
         #![allow(improper_ctypes_definitions)] __NEWLINE__
+
+        // Workaround for b/290271595
+        //
+        // TODO(/~https://github.com/rust-lang/rust/issues/80384): Remove once the feature is
+        // stabilized.
+        #![feature(const_refs_to_cell)] __NEWLINE__
         __NEWLINE__
 
         #rs_body

cc_bindings_from_rs/cc_bindings_from_rs.rs

@@ -362,6 +362,7 @@ inline void public_function() {
 // test_crate
 
 #![allow(improper_ctypes_definitions)]
+#![feature(const_refs_to_cell)]
 
 #[no_mangle]
 extern "C" fn __crubit_thunk__ANY_IDENTIFIER_CHARACTERS()

cc_bindings_from_rs/test/structs/structs.rs

@@ -259,3 +259,33 @@ pub mod nested_ptr_type_mutability_qualifiers {
         }
     }
 }
+
+/// This is a regression test for b/290271595 - it verifies that Rust-side
+/// `offset_of` assertions compile okay for bindings of types that use interior
+/// mutability.  Before the bug was fixed, the test below would result in:
+///
+/// ```
+/// error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
+///   --> .../cc_bindings_from_rs/test/structs/structs_cc_api_impl.rs:254:23
+///     |
+/// 254 | const _: () = assert!(memoffset::offset_of!(::structs::...::SomeStruct, field) == 0);
+///     |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+///     |
+///     = note: see issue #80384 </~https://github.com/rust-lang/rust/issues/80384> for more
+///       information
+///     = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
+///     = note: this error originates in the macro `_memoffset__let_base_ptr` which comes from the
+///       expansion of the macro `memoffset::offset_of` (in Nightly builds, run with -Z
+///       macro-backtrace for more info)
+/// ```
+pub mod interior_mutability {
+    use std::cell::UnsafeCell;
+
+    #[derive(Debug, Default)]
+    pub struct SomeStruct {
+        /// `pub` to make sure that `assert!(memoffset::offset_of!(...) == ...)`
+        /// is generated. (Such assertions are skipped for private
+        /// fields.)
+        pub field: UnsafeCell<i32>,
+    }
+}