Permit const assertions in stdlib

src/test/ui/consts/const-eval/const_panic_stability.e2018.stderr

@@ -0,0 +1,16 @@
+warning: panic message is not a string literal
+  --> $DIR/const_panic_stability.rs:14:12
+   |
+LL |     panic!({ "foo" });
+   |            ^^^^^^^^^
+   |
+   = note: `#[warn(non_fmt_panics)]` on by default
+   = note: this usage of panic!() is deprecated; it will be a hard error in Rust 2021
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
+help: add a "{}" format string to Display the message
+   |
+LL |     panic!("{}", { "foo" });
+   |            +++++
+
+warning: 1 warning emitted
+

src/test/ui/consts/const-eval/const_panic_stability.e2021.stderr

@@ -0,0 +1,13 @@
+error: format argument must be a string literal
+  --> $DIR/const_panic_stability.rs:14:12
+   |
+LL |     panic!({ "foo" });
+   |            ^^^^^^^^^
+   |
+help: you might be missing a string literal to format with
+   |
+LL |     panic!("{}", { "foo" });
+   |            +++++
+
+error: aborting due to previous error
+

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

@@ -0,0 +1,17 @@
+// revisions: e2018 e2021
+//[e2018] edition:2018
+//[e2021] edition:2021
+//[e2018] check-pass
+#![crate_type = "lib"]
+#![stable(feature = "foo", since = "1.0.0")]
+#![feature(staged_api)]
+
+#[stable(feature = "foo", since = "1.0.0")]
+#[rustc_const_stable(feature = "foo", since = "1.0.0")]
+const fn foo() {
+    assert!(false);
+    assert!(false, "foo");
+    panic!({ "foo" });
+    //[e2018]~^ WARNING panic message is not a string literal
+    //[e2021]~^^ ERROR format argument must be a string literal
+}