Function emit 

pub fn emit(
    value: Value,
    format: Format,
    pretty: bool,
    sources: &[(SourceName, Value)],
) -> Result<String>

Converts the merged IR into format and serializes it.

sources is the layer list, used only to name the file behind a null when TOML conversion rejects one. It is the caller’s, not the merge’s: provenance is never threaded through the merge itself. Pass &[] when there is nothing to attribute.