Function emit 

pub fn emit(
    value: Value,
    format: Format,
    pretty: bool,
    null_as: Option<&str>,
) -> Result<String>

Converts the merged IR into format and serializes it.

null_as substitutes a string for every null rather than failing on one. It is honoured in the TOML arm and nowhere else: JSON can hold a null perfectly well, so there is nothing there for it to rescue and substituting anyway would corrupt a document that was never in trouble.

Both arms can fail, and each fails on what its own format cannot spell: value::to_toml on nulls, integers past i64::MAX and malformed datetimes, value::to_json on infinities and NaNs. The two sets do not overlap, so each format is the escape from the other’s rejection — which is the help the CLI appends, and the reason neither library error names a flag. Both report key paths alone: nothing about the inputs survives the merge for them to name.