const_panic: Allow panicking in const fn

compiler/rustc_mir/src/transform/qualify_min_const_fn.rs

@@ -407,6 +407,10 @@ fn check_terminator(
         } => {
             let fn_ty = func.ty(body, tcx);
             if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
+                // Panic functions (with one argument) might be const fn.
+                if super::check_consts::is_lang_panic_fn(tcx, fn_def_id) {
+                    return Ok(());
+                }
                 // Allow unstable const if we opt in by using #[allow_internal_unstable]
                 // on function or macro declaration.
                 if !crate::const_eval::is_min_const_fn(tcx, fn_def_id)

src/test/ui/consts/const-eval/const_fn_panic.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![crate_type = "lib"]
+#![feature(const_panic)]
+
+pub const fn always_panic() {
+    panic!("always")
+}
+
+pub const fn assert_truth() {
+    assert_eq!(2 + 2, 4)
+}

src/test/ui/consts/const-eval/const_fn_panic_libcore.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+#![no_std]
+#![crate_type = "lib"]
+#![feature(const_panic)]
+
+pub const fn always_panic() {
+    panic!("always")
+}
+
+pub const fn assert_truth() {
+    assert_eq!(2 + 2, 4)
+}