The problem with testing is that you need a very large number of tests to cover a moderately complex piece of software: you need to test most branches obviously

The problem with testing is that you need a very large number of tests to cover a moderately complex piece of software: you need to test most branches obviously, but often you need to test combinations of behaviours such that 100% line testing is still not even close to enough testing. The advantage of compile time verification is that you can prove that certain paths and behaviours are impossible and don't need to be tested. This reduces the space of required tests combinatorially. We all already do that: no one bothers testing (for example) that standard library functions work correctly in their own code base. In Rust (and strongly typed languages in general) there are entire classes of test and assertions that aren't needed anymore.