I'd say a program or subcommand tha comes with the compiler/interpreter. Go has gofmt, zig has zig fmt. The compiler already parses the language so a formatter

I'd say a program or subcommand tha comes with the compiler/interpreter. Go has gofmt, zig has zig fmt. The compiler already parses the language so a formatter is only a few steps from that. And being part of the compiler means it's always up to date, there's less bikeshedding, all IDEs and text editors get formatter support easily.