Function report_missing_placeholders 

fn report_missing_placeholders(
    ecx: &ExtCtxt<'_>,
    unused: Vec<(Span, bool)>,
    used: &[bool],
    args: &FormatArguments,
    pieces: &[Piece<'_>],
    invalid_refs: &[(usize, Option<Span>, PositionUsedAs, FormatArgPositionKind)],
    detect_foreign_fmt: bool,
    str_style: Option<usize>,
    fmt_str: &str,
    uncooked_fmt_str: &str,
    fmt_span: Span,
)