Tangential point... is it accurate for the project to describe itself as a "pure-Rust implementation of the QUIC protocol" when it uses platform dependent unsafe code?
https://github.com/djc/quinn/blob/ce514af89a73f530e1366e79e5...
Sometimes the standard library doesn't expose functionality you want, like the capability to read/write the ECN bits of your UDP packets, so you have to bind some syscalls yourself. The capability to do this is one of Rust's core strengths, and it does not compromise a program's rustiness any moreso than calling a wrapper around write to print "Hello, world!" does.
(And speaking as an outsider, arguments about purity seem like the last thing Rust needs.)
For example, if you want to run on musl with a statically linked libc, then a library which is just a wrapper around a C shared library which depends on glibc is a non starter.