[derive] Test with UnsafeCells (#2463)

gherrit-pr-id: I15a252332006963e6ca345d8bfcbb5adb554f622

Author: joshlf

zerocopy-derive/tests/unsafe_cell.rs

@@ -0,0 +1,33 @@
+// Copyright 2025 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+// See comment in `include.rs` for why we disable the prelude.
+#![no_implicit_prelude]
+#![allow(warnings)]
+
+include!("include.rs");
+
+// Test to make sure that all of our derives are compatible with `UnsafeCell`s.
+//
+// We test both `FromBytes` and `FromZeros`, as the `FromBytes` implied derive
+// of `TryFromBytes` emits a trivial `is_bit_valid` impl - we want to test the
+// non-trivial impl, which deriving `FromZeros` accomplishes.
+
+#[derive(imp::FromBytes, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[repr(C)]
+struct StructFromBytes(imp::UnsafeCell<u8>);
+
+#[derive(imp::FromZeros, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[repr(C)]
+struct StructFromZeros(imp::UnsafeCell<bool>);
+
+#[derive(imp::FromZeros, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[repr(u8)]
+enum EnumFromZeros {
+    A(imp::UnsafeCell<bool>),
+}