Indicate that memcmp::eq should be used for HMACs

//! assert!(verifier.finish(&signature).unwrap());

//! ```

//!

//! Compute an HMAC (note that `Verifier` cannot be used with HMACs):

//! Compute an HMAC:

//!

//! ```rust

//! use openssl::sign::Signer;

//! use openssl::pkey::PKey;

//! use openssl::hash::MessageDigest;

//! use openssl::memcmp;

//! use openssl::pkey::PKey;

//! use openssl::sign::Signer;

//!

//! // Create a PKey

//! let key = PKey::hmac(b"my secret").unwrap();

//! signer.update(data).unwrap();

//! signer.update(data2).unwrap();

//! let hmac = signer.finish().unwrap();

//!

//! // `Verifier` cannot be used with HMACs; use the `memcmp::eq` function instead

//! //

//! // Do not simply check for equality with `==`!

//! # let target = hmac.clone();

//! assert!(memcmp::eq(&hmac, &target));

//! ```

use ffi;

use std::io::{self, Write};