Rust's stdlib's API is completely safe here. On Windows[1], it uses the GetEnvironmentVariable/SetEnvironmentVariable API, which as you noted doesn't have this

Rust's stdlib's API is completely safe here. On Windows[1], it uses the GetEnvironmentVariable/SetEnvironmentVariable API, which as you noted doesn't have this problem. On Unix[2], it maintains its own RwLock to provide synchronisation. Additionally, Rust's API only gives out copies of the data, it never gives you a pointer to the original. The problem comes when you do FFI on *nix systems, because those foreign functions may start making unsynchronised calls to getenv/setenv. [1] https://github.com/rust-lang/rust/blob/master/library/std/src/sys/windows/os.rs#L296-L315 https://github.com/rust-lang/rust/blob/master/library/std/sr... [2] https://github.com/rust-lang/rust/blob/master/library/std/src/sys/unix/os.rs#L601-L677 https://github.com/rust-lang/rust/blob/master/library/std/sr...