Apropos serious operating system use of Rust, have you seen the work of the Google Android (a Linux fork) team?I am following the Linux Rust experiment with much interest.
The most curious thing for me is where and how they will harness the (comparative to C) great power of Rust.
But so far I see only simple and trivial kernel drivers being done in Rust.
About three years ago Google decided to analyze the CVE for Android bugs, and pick a hot spot (modules in the codebase where the most defects were found over the past several years) to refactor into Rust. That experiment was successful enough that they kept going.
Apparently the team had almost no Rust experience when they started.
Newly reported exploitable defects in Android began falling immediately, and have continued falling, steadily.
Rust in Android: Move fast and fix things.