fn transcribe_metavar_expr<'a>(
    cx: &ExtCtxt<'a>,
    expr: &MetaVarExpr,
    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    marker: &mut Marker,
    repeats: &[(usize, usize)],
    result: &mut Vec<TreeAndSpacing>,
    sp: &DelimSpan
) -> PResult<'a, ()>