Rollup merge of #74459 - canova:const-unreachable-unchecked, r=oli-obk

Make unreachable_unchecked a const fn

This PR makes `std::hint::unreachable_unchecked` a const fn so we can use it inside a const function.
r? @RalfJung
Fixes #53188.

src/test/ui/consts/const_unsafe_unreachable.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_fn)]
+#![feature(const_unreachable_unchecked)]
+
+const unsafe fn foo(x: bool) -> bool {
+    match x {
+        true => true,
+        false => std::hint::unreachable_unchecked(),
+    }
+}
+
+const BAR: bool = unsafe { foo(true) };
+
+fn main() {
+  assert_eq!(BAR, true);
+}

src/test/ui/consts/const_unsafe_unreachable_ub.rs

@@ -0,0 +1,20 @@
+// build-fail
+
+#![feature(const_fn)]
+#![feature(const_unreachable_unchecked)]
+
+const unsafe fn foo(x: bool) -> bool {
+    match x {
+        true => true,
+        false => std::hint::unreachable_unchecked(),
+    }
+}
+
+#[warn(const_err)]
+const BAR: bool = unsafe { foo(false) };
+
+fn main() {
+  assert_eq!(BAR, true);
+  //~^ ERROR E0080
+  //~| ERROR erroneous constant
+}

src/test/ui/consts/const_unsafe_unreachable_ub.stderr

@@ -0,0 +1,44 @@
+warning: any use of this value will cause an error
+  --> $SRC_DIR/libcore/hint.rs:LL:COL
+   |
+LL |     unsafe { intrinsics::unreachable() }
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |              |
+   |              entering unreachable code
+   |              inside `std::hint::unreachable_unchecked` at $SRC_DIR/libcore/hint.rs:LL:COL
+   |              inside `foo` at $DIR/const_unsafe_unreachable_ub.rs:9:18
+   |              inside `BAR` at $DIR/const_unsafe_unreachable_ub.rs:14:28
+   | 
+  ::: $DIR/const_unsafe_unreachable_ub.rs:14:1
+   |
+LL | const BAR: bool = unsafe { foo(false) };
+   | ----------------------------------------
+   |
+note: the lint level is defined here
+  --> $DIR/const_unsafe_unreachable_ub.rs:13:8
+   |
+LL | #[warn(const_err)]
+   |        ^^^^^^^^^
+
+error[E0080]: evaluation of constant expression failed
+  --> $DIR/const_unsafe_unreachable_ub.rs:17:3
+   |
+LL |   assert_eq!(BAR, true);
+   |   ^^^^^^^^^^^---^^^^^^^^
+   |              |
+   |              referenced constant has errors
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: erroneous constant used
+  --> $DIR/const_unsafe_unreachable_ub.rs:17:3
+   |
+LL |   assert_eq!(BAR, true);
+   |   ^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0080`.