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<P<Expr>>, 
    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.