warning: panic message is not a string literal
  --> $DIR/const_panic_stability.rs:14:12
   |
LL |     panic!({ "foo" });
   |            ^^^^^^^^^
   |
   = 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/edition-guide/rust-2021/panic-macro-consistency.html>
   = note: `#[warn(non_fmt_panics)]` (part of `#[warn(rust_2021_compatibility)]`) on by default
help: add a "{}" format string to `Display` the message
   |
LL |     panic!("{}", { "foo" });
   |            +++++

warning: 1 warning emitted

