Made std::hint::unreachable_unchecked a const fn.

src/libcore/hint.rs

@@ -47,7 +47,8 @@ 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() -> ! {
     intrinsics::unreachable()
 }
 

src/libcore/intrinsics.rs

@@ -922,6 +922,7 @@ 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

@@ -88,6 +88,7 @@
 #![feature(const_ptr_offset_from)]
 #![feature(const_result)]
 #![feature(const_type_name)]
+#![feature(const_unreachable_unchecked)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

src/librustc_mir/interpret/intrinsics.rs

@@ -88,6 +88,7 @@ impl<'mir, 'tcx, 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_code => throw_ub!(Unreachable),
                 sym::abort => M::abort(self)?,
                 // Unsupported diverging intrinsic.
                 _ => return Ok(false),

src/test/ui/consts/const_unsafe_unreachable.rs

@@ -0,0 +1,28 @@
+// run-pass
+
+#![feature(core_intrinsics)]
+#![feature(const_fn)]
+#![feature(const_if_match)]
+#![feature(const_unreachable_unchecked)]
+
+const unsafe fn f_std_hint_unreachable(x: u8) -> u8 {
+    match x {
+        42 => 34,
+        _ => std::hint::unreachable_unchecked(),
+    }
+}
+
+const unsafe fn f_std_intrinsics_unreachable(x: u8) -> u8 {
+    match x {
+        17 => 22,
+        _ => std::intrinsics::unreachable(),
+    }
+}
+
+const FOO:u8 = unsafe { f_std_hint_unreachable(42) };
+const BAR:u8 = unsafe { f_std_intrinsics_unreachable(17) };
+
+fn main() {
+    assert_eq!(FOO, 34);
+    assert_eq!(BAR, 22);
+}

src/test/ui/consts/const_unsafe_unreachable_ub.rs

@@ -0,0 +1,16 @@
+#![feature(const_fn)]
+#![feature(const_if_match)]
+#![feature(const_unreachable_unchecked)]
+
+const unsafe fn foo(x: u8) -> u8 {
+    match x {
+        42 => 34,
+        _ => std::hint::unreachable_unchecked(),
+    }
+}
+
+const BAR:u8 = unsafe { foo(250) };
+
+fn main() {
+    println!("{}", BAR);
+}