Function set_var 

pub fn set_var(
    key: impl AsRef<OsStr>,
    value: impl AsRef<OsStr>,
    cap: &impl Has<EnvWrite>,
)

Sets an environment variable. Requires EnvWrite permission.

Safety note

In Rust edition 2024, std::env::set_var is unsafe because it’s not thread-safe. This wrapper encapsulates that unsafety.

Thread safety

std::env::set_var is not thread-safe. Even though Cap<EnvWrite> can be cloned and transferred across threads via make_send(), calling this function concurrently from multiple threads is undefined behavior. The capability system tracks permission, not exclusivity — synchronization is the caller’s responsibility.