Note that this is done for “existing formally verified C codebases” which is a lot different from typical systems C code which is not formally verified.
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It frustrates me more than it should, I admit, that people always mention Rust when they talk about safety, but never Ada / SPARK. You want formal verification? Use Ada / SPARK. It has been battle-tested. It has been used for critical systems for a really long time now.
(And a compiler being formally verified vs. being able to write formally verified code means two different things.)
For example if someone on GitHub sees that the project is written in Rust, they are automatically going to assume it is safe, incorrectly so. I do not blame them though.