Function in_build_directory 

pub fn in_build_directory(exe: &Path) -> bool

Whether this executable is sitting in a cargo build directory.

A unit pointing into one breaks at the next cargo clean, and it breaks at reboot — the moment nobody is watching. Detected by the profile directory cargo actually writes into rather than by looking for a component called target: CARGO_TARGET_DIR renames that one, and the first version of this check was defeated by a target directory called anything else.