message: Test for warning of migrating to unstable edition
diff: @@ -809,6 +809,7 @@ fn prepare_for_unstable() {
  }
  };
  let latest_stable = Edition::LATEST_STABLE;
  + let prev = latest_stable.previous().unwrap();
  let p = project()
  .file(
  "Cargo.toml",
  @@ -828,13 +829,24 @@ fn prepare_for_unstable() {
  // -j1 to make the error more deterministic (otherwise there can be
  // multiple errors since they run in parallel).
  p.cargo("fix --edition --allow-no-vcs -j1")
  - .with_status(101)
  - .with_stderr(&format!("\
  + .with_stderr(&format_args!("\
  [CHECKING] foo [..]
  -[ERROR] cannot migrate src/lib.rs to edition {next}
  +[WARNING] `src/lib.rs` is on the latest edition, but trying to migrate to edition {next}.
  Edition {next} is unstable and not allowed in this release, consider trying the nightly release channel.
  -error: could not compile `foo`
  -", next=next))
  +
  +If you are trying to migrate from the previous edition ({prev}), the
  +process requires following these steps:
  +
  +1. Start with `edition = \"{prev}\"` in `Cargo.toml`
  +2. Run `cargo fix --edition`
  +3. Modify `Cargo.toml` to set `edition = \"{latest_stable}\"`
  +4. Run `cargo build` or `cargo test` to verify the fixes worked
  +
  +More details may be found at
  +https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html
  +
  +[FINISHED] [..]
  +", next=next, latest_stable=latest_stable, prev=prev))
  .run();
  if !is_nightly() {
  