Auto merge of #51395 - SimonSapin:repr-transparent, r=SimonSapin

Add #[repr(transparent)] to some libcore types

* `UnsafeCell`
* `Cell`
* `NonZero*`
* `NonNull`
* `Unique`

CC https://github.com/rust-lang/rust/issues/43036

src/libcore/cell.rs

@@ -235,6 +235,7 @@ use ptr;
 ///
 /// See the [module-level documentation](index.html) for more.
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(transparent)]
 pub struct Cell<T> {
     value: UnsafeCell<T>,
 }
@@ -1395,6 +1396,7 @@ impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
 /// ```
 #[lang = "unsafe_cell"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(transparent)]
 pub struct UnsafeCell<T: ?Sized> {
     value: T,
 }

src/libcore/lib.rs

@@ -122,6 +122,7 @@
 #![feature(const_slice_len)]
 #![feature(const_str_as_bytes)]
 #![feature(const_str_len)]
+#![cfg_attr(stage0, feature(repr_transparent))]
 
 #[prelude_import]
 #[allow(unused)]

src/libcore/nonzero.rs

@@ -16,6 +16,7 @@ use ops::CoerceUnsized;
 /// NULL or 0 that might allow certain optimizations.
 #[lang = "non_zero"]
 #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[repr(transparent)]
 pub(crate) struct NonZero<T>(pub(crate) T);
 
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<NonZero<U>> for NonZero<T> {}

src/libcore/num/mod.rs

@@ -48,6 +48,7 @@ macro_rules! nonzero_integers {
             /// ```
             #[stable(feature = "nonzero", since = "1.28.0")]
             #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+            #[repr(transparent)]
             pub struct $Ty(NonZero<$Int>);
 
             impl $Ty {
@@ -123,6 +124,7 @@ nonzero_integers! {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Default, Hash)]
+#[repr(transparent)]
 pub struct Wrapping<T>(#[stable(feature = "rust1", since = "1.0.0")]
                        pub T);
 

src/libcore/ptr.rs

@@ -2665,6 +2665,7 @@ impl<T: ?Sized> PartialOrd for *mut T {
            reason = "use NonNull instead and consider PhantomData<T> \
                      (if you also use #[may_dangle]), Send, and/or Sync")]
 #[doc(hidden)]
+#[repr(transparent)]
 pub struct Unique<T: ?Sized> {
     pointer: NonZero<*const T>,
     // NOTE: this marker has no consequences for variance, but is necessary
@@ -2813,6 +2814,7 @@ impl<'a, T: ?Sized> From<NonNull<T>> for Unique<T> {
 /// such as Box, Rc, Arc, Vec, and LinkedList. This is the case because they
 /// provide a public API that follows the normal shared XOR mutable rules of Rust.
 #[stable(feature = "nonnull", since = "1.25.0")]
+#[repr(transparent)]
 pub struct NonNull<T: ?Sized> {
     pointer: NonZero<*const T>,
 }