message: enable overflow checks in the standard library
diff: @@ -354,7 +354,8 @@ path = "lib.rs"
  command.env_remove("RUSTC_WRAPPER");
  command.env_remove("RUSTFLAGS");
  // Disable debug assertions in the standard library -- Miri is already slow enough.
  - command.env("RUSTFLAGS", "-Cdebug-assertions=off");
  + // But keep the overflow checks, they are cheap.
  + command.env("RUSTFLAGS", "-Cdebug-assertions=off -Coverflow-checks=on");
  // Finally run it!
  if command.status().expect("failed to run xargo").success().not() {
  show_error(format!("failed to run xargo"));
  