In my experience a Rust codebase with no tests warrants more confidence than any practical level of testing seen in sloppy programming languages, at least until

In my experience a Rust codebase with no tests warrants more confidence than any practical level of testing seen in sloppy programming languages, at least until you get to SQLite-style "10x as much test code as main code" projects. A decent type system lets you reach a given level of confidence at much lower cost than doing the same thing with tests. A decent type system plus a handful of judiciously chosen tests lets you reach a higher level than you would ever achieve with tests alone.