panic! always takes a format string

let msg = String::from("broken {value}");

// *** before: ***
panic!(msg);   // the string was taken as it is, braces meant nothing

// *** in version Rust 2021: ***
panic!("{}", msg);            // the first argument must be a format string
std::panic::panic_any(42);    // panicking with any value is a call of its own