Function rustc_builtin_macros::format::expand_preparsed_format_args

pub fn expand_preparsed_format_args(
    ecx: &mut ExtCtxt<'_>,
    sp: Span,
    efmt: P<Expr>,
    args: Vec<FormatArg>,
    names: FxHashMap<Symbol, usize>,
    append_newline: bool
) -> P<Expr>

Take the various parts of format_args!(efmt, args..., name=names...) and construct the appropriate formatting expression.