Function legacy_env_adoptions 

pub fn legacy_env_adoptions<I>(vars: I) -> Vec<(OsString, OsString)>
where
    I: IntoIterator<Item = (OsString, OsString)>,

The PV_* names to set from AV_* variables, as (new name, value) pairs. A legacy variable is adopted only when its new name is unset or empty, so an operator who already migrated is never overridden. Empty counts as unset because compose files render PV_X: ${PV_X:-}, and the daemon already reads an empty value as “not set”.