pub fn one_needle(&self) -> anyhow::Result<&[u8]> {
        anyhow::ensure!(
            self.needles.len() == 1,
            "benchmark only supports one needle, but {} were found",
            self.needles.len(),
        );
        Ok(&self.needles[0])
    }
Return a single needle from this benchmark. If the benchmark has no
needles or more than one, then an error is returned.
benchmarks/shared/lib.rs