Function field_html_into 

pub fn field_html_into(
    field: &Field<'_>,
    filling: &Filling<'_>,
    opts: &Emit,
    out: &mut String,
)

One field, written into a buffer the caller already has.

field_html’s streaming form, byte-identical to it. A form is a run of these, so a host building one should hold a single buffer and append each field into it rather than take a String per field and concatenate.