The problem
When we have a function that interacts with external resources, testing that code might be difficult because side-effects could impact on other tests. Environment variables are global variables stored in the process state. One change in an env variable changes the process state, so this mutation persists along the process lifetime.
In Rust, unit tests are compiled into one binary, which means that all tests run in the same process. That is why we need to be careful when testing code that modifies env vars, because those changes may affect other tests and could produce an undesirable behaviour.
On top of that, Rust tests run in parallel by default, so we need to make sure that there is no more than one thread modifying the same env var at the same time.
Stopping concurrent access to Environment Variables
