Fix const stability

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -24,7 +24,7 @@ use std::ops::Deref;
 use super::ops::{self, NonConstOp, Status};
 use super::qualifs::{self, CustomEq, HasMutInterior, NeedsNonConstDrop};
 use super::resolver::FlowSensitiveAnalysis;
-use super::{is_lang_special_const_fn, ConstCx, Qualif};
+use super::{is_lang_panic_fn, is_lang_special_const_fn, ConstCx, Qualif};
 use crate::const_eval::is_unstable_const_fn;
 
 // We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
@@ -910,7 +910,10 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                         }
                     }
 
-                    return;
+                    if is_lang_panic_fn(tcx, callee) {
+                        // run stability check on non-panic special const fns.
+                        return;
+                    }
                 }
 
                 if Some(callee) == tcx.lang_items().exchange_malloc_fn() {

library/core/src/intrinsics.rs

@@ -2258,6 +2258,7 @@ pub unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
     issue = "none",
     reason = "const_eval_select will never be stable"
 )]
+#[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
 #[lang = "const_eval_select"]
 #[rustc_do_not_const_check]
 pub const unsafe fn const_eval_select<ARG, F, G, RET>(
@@ -2278,6 +2279,7 @@ where
     issue = "none",
     reason = "const_eval_select will never be stable"
 )]
+#[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
 #[lang = "const_eval_select_ct"]
 pub const unsafe fn const_eval_select_ct<ARG, F, G, RET>(
     arg: ARG,

src/test/ui/intrinsics/const-eval-select-stability.rs

@@ -0,0 +1,20 @@
+#![feature(staged_api)]
+#![feature(const_eval_select)]
+#![stable(since = "1.0", feature = "ui_test")]
+
+use std::intrinsics::const_eval_select;
+
+fn log() {
+    println!("HEY HEY HEY")
+}
+
+const fn nothing(){}
+
+#[stable(since = "1.0", feature = "hey")]
+#[rustc_const_stable(since = "1.0", feature = "const_hey")]
+pub const unsafe fn hey() {
+    const_eval_select((), nothing, log);
+    //~^ ERROR `const_eval_select` is not yet stable as a const fn
+}
+
+fn main() {}

src/test/ui/intrinsics/const-eval-select-stability.stderr

@@ -0,0 +1,10 @@
+error: `const_eval_select` is not yet stable as a const fn
+  --> $DIR/const-eval-select-stability.rs:16:5
+   |
+LL |     const_eval_select((), nothing, log);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: const-stable functions can only call other const-stable functions
+
+error: aborting due to previous error
+