Here's my usual reminder: memory safety is necessary, but insufficient for secure software.
In cryptography, constant-time operations are important for countering side-channel attacks. The Marvin attack is a case in point, with the RustCrypto/RSA code impacted.
OpenSSH, a cryptographic peer of OpenSSL, had a recent flaw called Terrapin. There's no direct consequence for this Rustls, but I mention it as the type of thing to be aware of when implementing cryptographic protocols and ciphers.