pub fn var<K: AsRef<OsStr> + AsRef<str>>(key: K) -> Result<String, VarError>
Retrieve an environment variable and add it to build dependency info.
Build system executing the compiler will know that the variable was accessed during
compilation, and will be able to rerun the build when the value of that variable changes.
Besides the dependency tracking this function should be equivalent to
env::var from the
standard library, except that the argument must be UTF-8.