Because Rust binaries are so secure, you can expose them over the internet safely. With a little bit of glue, you can automatically recompile the CGI binary every time the rs source is touched, for a dev experience similar to interpreted web scripts.
That doesn’t stand to reason. Safely in that you can confidently avoid things like remote code execution, sure. But that doesn’t necessarily protect you from denial of service attacks; for example, I expect most services built on Hyper to be vulnerable to slowloris attacks <https://en.wikipedia.org/wiki/Slowloris_(computer_security)>, because I believe it still defaults to basically no sane kind of timeout, and doesn’t provide the tools to handle it properly anyway <https://github.com/hyperium/hyper/issues/1628>. If that’s part of your threat model, you should avoid exposing it directly and use a reverse proxy.
Good somewhat related reading, on a technologically simple DoS attack on Rust-based code that was largely fixed by speeding things up, fixing caching and proper use of reverse proxying (via Cloudflare because it was genuinely heavy load): https://fasterthanli.me/articles/i-won-free-load-testing
You can of course still do stupid things inside the rust application that explode your number of concurrent requests, like waiting for slow external resources, databases etc. For a simple static formatter, it should not be the case, it would just repeatedly hit the disk cache and some minimal CPU.
... but with an execution speed that consumes negligible CPU time from the moment the template/source is loaded into memory until it is formatted and put on the wire. Compared to that, a PHP template is dog slow. I don't have any direct experience with Go, small GC allocations for string handling might introduce non-negligible overhead.