To understand the constrains we have here, a ballpark figure is that we run a million tests per commit across all platforms (this is based on counting test files, so doesn't account for the fact that some files can contain thousands of separate test functions, but also doesn't account for the fact that we don't run every test job on every push). On average we see around 10 test failures per push due to test instability. So "on average" tests fail about one time in one hundred thousand. Of course, in reality there are some tests that fail much more often than this (one time in a hundred, say) and many that ~never fail.

Whilst test raciness is perhaps the most common problem, we also see intermittent failures due to race conditions in the browser code itself as well as through infrastructure instability. However it's unclear that moving to Rust will make much difference anyhow; Servo still sees intermittent tests so merely eliminating data races in safe code is insufficient to fix this problem.