Add repr(no_niche) to UnsafeCell. Fix #68303.

src/libcore/cell.rs

@@ -1475,6 +1475,7 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 #[lang = "unsafe_cell"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[repr(transparent)]
+#[cfg_attr(not(bootstrap), repr(no_niche))] // rust-lang/rust#68303.
 pub struct UnsafeCell<T: ?Sized> {
     value: T,
 }

src/libcore/lib.rs

@@ -138,6 +138,7 @@
 #![feature(const_type_id)]
 #![feature(const_caller_location)]
 #![feature(assoc_int_consts)]
+#![cfg_attr(not(bootstrap), feature(no_niche))] // rust-lang/rust#68303
 
 #[prelude_import]
 #[allow(unused)]

src/test/ui/layout/unsafe-cell-hides-niche.rs

@@ -0,0 +1,32 @@
+// 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).
+
+// run-pass
+
+#![feature(no_niche)]
+
+use std::cell::UnsafeCell;
+use std::mem::size_of;
+use std::num::NonZeroU32 as N32;
+
+struct Wrapper<T>(T);
+
+#[repr(transparent)]
+struct Transparent<T>(T);
+
+#[repr(no_niche)]
+struct NoNiche<T>(T);
+
+fn main() {
+    assert_eq!(size_of::<Option<Wrapper<u32>>>(),     8);
+    assert_eq!(size_of::<Option<Wrapper<N32>>>(),     4);
+    assert_eq!(size_of::<Option<Transparent<u32>>>(), 8);
+    assert_eq!(size_of::<Option<Transparent<N32>>>(), 4);
+    assert_eq!(size_of::<Option<NoNiche<u32>>>(),     8);
+    assert_eq!(size_of::<Option<NoNiche<N32>>>(),     8);
+
+    assert_eq!(size_of::<Option<UnsafeCell<u32>>>(),  8);
+    assert_eq!(size_of::<Option<UnsafeCell<N32>>>(),  8);
+}