message: mir-opt-level 4 is the new 3
diff: @@ -24,7 +24,7 @@ function run_tests {
  if [ -z "${MIRI_TEST_TARGET+exists}" ]; then
  # Only for host architecture: tests with optimizations (`-O` is what cargo passes, but crank MIR
  # optimizations up all the way).
  - MIRIFLAGS="-O -Zmir-opt-level=3" ./miri test --locked
  + MIRIFLAGS="-O -Zmir-opt-level=4" ./miri test --locked
  fi
  # On Windows, there is always "python", not "python3" or "python2".
  