Expand assert!(expr) to panic() function instead of panic!() macro.

The panic message might contain braces which should never be
interpreted as format placeholders, which panic!() will do in a future
edition.

Author: m-ou-se

core/src/macros/mod.rs

@@ -1217,6 +1217,7 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[macro_export]
     #[rustc_diagnostic_item = "assert_macro"]
+    #[allow_internal_unstable(core_panic)]
     macro_rules! assert {
         ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
         ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};