Make unreachable_unchecked a const fn

Author: canova

src/libcore/hint.rs

@@ -45,7 +45,8 @@ use crate::intrinsics;
 /// ```
 #[inline]
 #[stable(feature = "unreachable", since = "1.27.0")]
-pub unsafe fn unreachable_unchecked() -> ! {
+#[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
+pub const unsafe fn unreachable_unchecked() -> ! {
     // SAFETY: the safety contract for `intrinsics::unreachable` must
     // be upheld by the caller.
     unsafe { intrinsics::unreachable() }

src/libcore/intrinsics.rs

@@ -932,6 +932,7 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::hint::unreachable_unchecked`](../../std/hint/fn.unreachable_unchecked.html).
+    #[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
     pub fn unreachable() -> !;
 
     /// Informs the optimizer that a condition is always true.

src/libcore/lib.rs

@@ -91,6 +91,7 @@
 #![feature(const_slice_ptr_len)]
 #![feature(const_type_name)]
 #![feature(const_likely)]
+#![feature(const_unreachable_unchecked)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

src/librustc_mir/interpret/intrinsics.rs

@@ -95,6 +95,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let (dest, ret) = match ret {
             None => match intrinsic_name {
                 sym::transmute => throw_ub_format!("transmuting to uninhabited type"),
+                sym::unreachable => throw_ub!(Unreachable),
                 sym::abort => M::abort(self)?,
                 // Unsupported diverging intrinsic.
                 _ => return Ok(false),