The last bit of information that GP might be missing is that pub fn main() { let x = 4; // unused println!("{x}"); } would previously

The last bit of information that GP might be missing is that pub fn main() { let x = 4; // unused println!("{x}"); } would previously be a compile time error (because the format! family of macros perform string interpolation at compile time): error: there is no argument named `x` --> <source>:3:15 | 3 | println!("{x}"); | ^^^ So no new existing code will stop compiling, some things that were previously compile errors will now work.