* ctl-exporter (I started this, but discovered that the CTL stats API is
  unstable, so it can't live in ports.  Instead, I had to do it in C).

  https://github.com/freebsd/freebsd-src/commit/1a7f22d9c211f504f6c48a86401469181a67ec34

* fusefs tests.  Absolutely impossible to do in C.  I considered Rust, but went

  with C++ so they could live in base.  They are too closely coupled to

  fusefs(5) to live out-of-tree.

  https://github.com/freebsd/freebsd-src/tree/main/tests/sys/fs/fusefs

* devd.  Currently C++, but imp suggested a rewrite.

  https://github.com/freebsd/freebsd-src/tree/main/sbin/devd

* zfsd.  Currently C++, but I've long pondered a rewrite.  Using Rust would

  make it more testable.

  https://github.com/freebsd/freebsd-src/tree/main/cddl/usr.sbin/zfsd

* nscd.  Currently C, but confusing and with no test coverage.  I've

  contemplated a rewrite myself, but I don't want to do it in C.

  https://github.com/freebsd/freebsd-src/tree/main/usr.sbin/nscd

* The userland portion of the 802.11ac and Lightning stacks.  scottl suggested

  that these were good candidates for Rust.

* freebsd-kpi-r14-0 .  https://crates.io/crates/freebsd-kpi-r14-0