Function effective_source 

pub fn effective_source(
    config_path: Option<&Path>,
    default: impl FnOnce() -> Option<PathBuf>,
) -> Result<Option<PathBuf>>

Resolve the config source actually consulted. When --config is omitted, the default path is auto-loaded only if present, so report the file that resolve_config will actually read — not just the explicit flag.

Uses try_exists rather than exists: a permission error on the default path must be surfaced, not silently reported as “no config file” — the whole point of validate is telling the operator what the node sees.