Function report_invalid_references 

fn report_invalid_references(
    ecx: &ExtCtxt<'_>,
    invalid_refs: &[(usize, Option<Span>, PositionUsedAs, FormatArgPositionKind)],
    template: &[FormatArgsPiece],
    fmt_span: Span,
    args: &FormatArguments,
    parser: Parser<'_>,
)

Handle invalid references to positional arguments. Output different errors for the case where all arguments are positional and for when there are named arguments or numbered positional arguments in the format string.