Function classify_storage_auth_source 

pub fn classify_storage_auth_source<F, S>(
    provider: StorageProvider,
    explicit: &HashMap<String, String, S>,
    env_lookup: &F,
) -> AuthSource
where
    F: Fn(&str) -> Option<String>,
    S: BuildHasher,

Classify a provider’s effective credential source without loading credential values.

Explicit non-empty options take precedence over ambient sources. The environment lookup is intentionally borrowed by callback so short-lived values remain owned by the downstream provider’s refreshable credential chain.