1 files changed, 82 insertions, 0 deletions
diff --git a/src/test/ui/layout/unsafe-cell-hides-niche.rs b/src/test/ui/layout/unsafe-cell-hides-niche.rs
new file mode 100644
index 000000000..68bcc3c1a
--- /dev/null
+++ b/src/test/ui/layout/unsafe-cell-hides-niche.rs
@@ -0,0 +1,82 @@
+// For rust-lang/rust#68303: the contents of `UnsafeCell<T>` cannot
+// participate in the niche-optimization for enum discriminants. This
+// test checks that an `Option<UnsafeCell<NonZeroU32>>` has the same
+// size in memory as an `Option<UnsafeCell<u32>>` (namely, 8 bytes).
+
+// check-pass
+// compile-flags: --crate-type=lib
+// only-x86
+
+#![feature(repr_simd)]
+
+use std::cell::{UnsafeCell, RefCell, Cell};
+use std::mem::size_of;
+use std::num::NonZeroU32 as N32;
+use std::sync::{Mutex, RwLock};
+
+struct Wrapper<T>(#[allow(unused_tuple_struct_fields)] T);
+
+#[repr(transparent)]
+struct Transparent<T>(#[allow(unused_tuple_struct_fields)] T);
+
+struct NoNiche<T>(UnsafeCell<T>);
+
+struct Size<const S: usize>;
+
+macro_rules! check_sizes {
+    (check_one_specific_size: $ty:ty, $size:expr) => {
+        const _: Size::<{$size}> = Size::<{size_of::<$ty>()}>;
+    };
+    // Any tests run on `UnsafeCell` must be the same for `Cell`
+    (UnsafeCell<$ty:ty>: $size:expr => $optioned_size:expr) => {
+        check_sizes!(Cell<$ty>: $size => $optioned_size);
+        check_sizes!(@actual_check: UnsafeCell<$ty>: $size => $optioned_size);
+    };
+    ($ty:ty: $size:expr => $optioned_size:expr) => {
+        check_sizes!(@actual_check: $ty: $size => $optioned_size);
+    };
+    // This branch does the actual checking logic, the `@actual_check` prefix is here to distinguish
+    // it from other branches and not accidentally match any.
+    (@actual_check: $ty:ty: $size:expr => $optioned_size:expr) => {
+        check_sizes!(check_one_specific_size: $ty, $size);
+        check_sizes!(check_one_specific_size: Option<$ty>, $optioned_size);
+        check_sizes!(check_no_niche_opt: $size != $optioned_size, $ty);
+    };
+    // only check that there is no niche (size goes up when wrapped in an option),
+    // don't check actual sizes
+    ($ty:ty) => {
+        check_sizes!(check_no_niche_opt: true, $ty);
+    };
+    (check_no_niche_opt: $no_niche_opt:expr, $ty:ty) => {
+        const _: () = if $no_niche_opt { assert!(size_of::<$ty>() < size_of::<Option<$ty>>()); };
+    };
+}
+
+const PTR_SIZE: usize = std::mem::size_of::<*const ()>();
+
+check_sizes!(Wrapper<u32>:     4 => 8);
+check_sizes!(Wrapper<N32>:     4 => 4); // (✓ niche opt)
+check_sizes!(Transparent<u32>: 4 => 8);
+check_sizes!(Transparent<N32>: 4 => 4); // (✓ niche opt)
+check_sizes!(NoNiche<u32>:     4 => 8);
+check_sizes!(NoNiche<N32>:     4 => 8);
+
+check_sizes!(UnsafeCell<u32>:  4 => 8);
+check_sizes!(UnsafeCell<N32>:  4 => 8);
+
+check_sizes!(UnsafeCell<&()>: PTR_SIZE => PTR_SIZE * 2);
+check_sizes!(   RefCell<&()>: PTR_SIZE * 2 => PTR_SIZE * 3);
+
+check_sizes!(RwLock<&()>);
+check_sizes!(Mutex<&()>);
+
+check_sizes!(UnsafeCell<&[i32]>: PTR_SIZE * 2 => PTR_SIZE * 3);
+check_sizes!(UnsafeCell<(&(), &())>: PTR_SIZE * 2 => PTR_SIZE * 3);
+
+trait Trait {}
+check_sizes!(UnsafeCell<&dyn Trait>: PTR_SIZE * 2 => PTR_SIZE * 3);
+
+#[repr(simd)]
+pub struct Vec4<T>([T; 4]);
+
+check_sizes!(UnsafeCell<Vec4<N32>>: 16 => 32);