Auto merge of #57145 - RalfJung:panic-if-uninhabited, r=alexcrichton

panic when calling MaybeUninhabited::into_inner on uninhabited type

I do this by adding an internal-only intrinsic `panic_if_uninhabited`. I have no idea what I am doing here, just mindlessly copying code around, so please review carefully!

2 files changed, 11 insertions, 0 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 7508257f780..4f5310f5285 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -690,6 +690,11 @@ extern "rust-intrinsic" {
     /// crate it is invoked in.
     pub fn type_id<T: ?Sized + 'static>() -> u64;
 
+    /// A guard for unsafe functions that cannot ever be executed if `T` is uninhabited:
+    /// This will statically either panic, or do nothing.
+    #[cfg(not(stage0))]
+    pub fn panic_if_uninhabited<T>();
+
     /// Creates a value initialized to zero.
     ///
     /// `init` is unsafe because it returns a zeroed-out datum,
diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index c024868714c..8fcbb73d9ce 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -492,6 +492,8 @@ pub const fn needs_drop<T>() -> bool {
 #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::zeroed` instead")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn zeroed<T>() -> T {
+    #[cfg(not(stage0))]
+    intrinsics::panic_if_uninhabited::<T>();
     intrinsics::init()
 }
 
@@ -624,6 +626,8 @@ pub unsafe fn zeroed<T>() -> T {
 #[rustc_deprecated(since = "2.0.0", reason = "use `mem::MaybeUninit::uninitialized` instead")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn uninitialized<T>() -> T {
+    #[cfg(not(stage0))]
+    intrinsics::panic_if_uninhabited::<T>();
     intrinsics::uninit()
 }
 
@@ -1128,6 +1132,8 @@ impl<T> MaybeUninit<T> {
     #[unstable(feature = "maybe_uninit", issue = "53491")]
     #[inline(always)]
     pub unsafe fn into_inner(self) -> T {
+        #[cfg(not(stage0))]
+        intrinsics::panic_if_uninhabited::<T>();
         ManuallyDrop::into_inner(self.value)
     }