From Pinecone's "Inside the Pinecone" article on their transition from Python, C/C++, and Go to Rust, by Edo Liberty (bold emphasis mine):
[W]e reached a tipping point. We decided to move our entire codebase to Rust... . Rust seemed to give us all the capabilities we needed, however, there was still one minor problem - no one on the team knew Rust. ...
We started with a small team of senior engineers and managers learning Rust and developing the skeleton of the DB and dev environment (for others to build on). Then, slowly, others joined in rewriting and contributing different components until we eventually got rid of the old codebase altogether (I still remember the day my original C modules, from the first days of Pinecone, were taken out). Unbeknownst to most Pinecone customers, the new Rust core was deployed in March this year. And in the process of taking over running workloads, we managed not to drop a single API call!
... We all expect[ed] performance and dev processes to improve. Those indeed happened. What we didn’t expect was the extent to which dev velocity increased and operational incidents decreased. Dev velocity ... improved dramatically with Rust. Built-in testing, CI/CD, benchmarking, and an overzealous compiler increased engineers’ confidence in pushing changes, and enabled them to work on the same code sections and contribute simultaneously without breaking the code base. Most impressively though, real time operational events dropped almost to zero overnight after the original release. Sure, there are still surprises here and there but, by and large, the core engine has been shockingly stable and predictable.