Improve comments about const panic handling

Co-authored-by: Ralf Jung <post@ralfj.de>

compiler/rustc_mir/src/const_eval/machine.rs

@@ -31,6 +31,8 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
     ) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
+        // The list of functions we handle here must be in sync with
+        // `is_lang_panic_fn` in `transform/check_consts/mod.rs`.
         let def_id = instance.def_id();
         if Some(def_id) == self.tcx.lang_items().panic_fn()
             || Some(def_id) == self.tcx.lang_items().panic_str()

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -74,6 +74,9 @@ impl ConstCx<'mir, 'tcx> {
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.
 pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    // We can allow calls to these functions because `hook_panic_fn` in
+    // `const_eval/machine.rs` ensures the calls are handled specially.
+    // Keep in sync with what that function handles!
     Some(def_id) == tcx.lang_items().panic_fn()
         || Some(def_id) == tcx.lang_items().panic_str()
         || Some(def_id) == tcx.lang_items().begin_panic_fn()

library/core/src/panicking.rs

@@ -100,8 +100,8 @@ pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if let Some(msg) = fmt.as_str() {
         panic_str(msg);
     } else {
-        // SAFETY: This is only evaluated at compile time, which handles this
+        // SAFETY: This is only evaluated at compile time, which reliably
-        // fine (in case it turns out this branch turns out to be reachable
+        // handles this UB (in case this branch turns out to be reachable
         // somehow).
         unsafe { crate::hint::unreachable_unchecked() };
     }