message: Allow rustc bootstrap to use unstable features even though it's using a beta-cargo
diff: @@ -354,12 +354,18 @@ impl CliUnstable {
  }
  fn channel() -> String {
  - env::var("__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS").unwrap_or_else(|_| {
  + if let Ok(staging) = env::var("RUSTC_BOOTSTRAP") {
  + if staging == "1" {
  + return "dev".to_string();
  + }
  + }
  + if let Ok(override_channel) = env::var("__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS") {
  + return override_channel;
  + }
  ::version()
  .cfg_info
  .map(|c| c.release_channel)
  .unwrap_or_else(|| String::from("dev"))
  - })
  }
  fn nightly_features_allowed() -> bool {
  