message: don't assert the same thing twice
diff: @@ -952,15 +952,14 @@ fn phase_runner(binary: &Path, binary_args: env::Args, phase: RunnerPhase) {
  assert!(suffix.starts_with('='));
  // Drop this argument.
  } else if let Some(suffix) = arg.strip_prefix(json_flag) {
  - assert!(suffix.starts_with('='));
  + let suffix = suffix.strip_prefix('=').unwrap();
  // This is how we pass through --color=always. We detect that Cargo is detecting rustc
  // to emit the diagnostic structure that Cargo would consume from rustc to emit colored
  // diagnostics, and ask rustc to emit them.
  // See https://github.com/rust-lang/miri/issues/2037
  // First skip over the leading `=`, then check for diagnostic-rendered-ansi in the
  // comma-separated list
  - if suffix.strip_prefix('=').unwrap().split(',').any(|a| a == "diagnostic-rendered-ansi")
  - {
  + if suffix.split(',').any(|a| a == "diagnostic-rendered-ansi") {
  cmd.arg("--color=always");
  }
  // But aside from remembering that colored output was requested, drop this argument.
  