Ugh.  They didn't compile any C to Rust.  They modified the F*-to-C compiler to emit Rust instead.  So they compiled F* to safe Rust.  And they couldn't even do that 100% reliably; some valid F* constructs couldn't be translated into Rust properly.  They could either translate it into Rust code that wouldn't compile, or translate it into similar-looking Rust code that would compile, but would produce incorrect results.
Flagged, this is just a lie of a title.
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