error[E0133]: call to unsafe function `std::env::set_var` is unsafe and requires unsafe block (Rust 2024)

Preferred: do not mutate the environment at runtime. Pass config explicitly, or set env vars only for child processes:

std::process::Command::new("tool").env("FOO", "bar").spawn()?;

If you must (e.g. at the very start of main before spawning threads or starting a runtime):

// before (2021)
std::env::set_var("FOO", "bar");
// after (2024)
// SAFETY: called before any other threads are spawned
unsafe { std::env::set_var("FOO", "bar") };

Note: #[tokio::main] starts worker threads before your main body runs, so setting vars inside it is not single-threaded. In tests (which run in parallel threads), prefer passing values explicitly. Replace before_exec with pre_exec (also unsafe).