“rustc is written in Rust and compiles itself, so somehow rustc is delegating to rustc to figure out what ‘\n’ means, which seems odd, to say the least, and we still haven’t seen the naked 0x0A byte we’re looking for.”
The Rust compiler maps ‘\n’ to ‘\n’. That only works because an earlier rustc already knew the answer. Chase it back and the byte comes from the OCaml compiler that bootstrapped Rust, which spells it ‘\010’. rustc is at 1.81.0, so that byte has been copied forward at least 81 times. No source file in Rust today says what a newline is.