Function first_env_var_set 

fn first_env_var_set<L>(
    vars: &[(&'static str, Option<&'static str>)],
    canonical: &'static str,
    lookup: &L,
) -> Result<(&'static str, String), EnvParseError>
where
    L: Fn(&str) -> Option<OsString>,

The value of the first variable of vars that lookup finds, together with the name of that variable, warning about the deprecated ones on the way.

canonical is the variable a user should set, reported when none of them is.

A variable that is set but does not hold valid Unicode fails even when a lower-priority variable holds a usable value, because silently ignoring a variable the user did set would surprise them.